NBS Measurement Services:

Radiometric

Standards in the

Vacuum Ultraviolet

NBS

Special

Publication

250-3

Jules Z. Klose

J. Mervin Bridges

William R. Ott

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National

Bureau of Standards. The Center provides the Nation with standards and measurement services for ionizing

radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support

to the National Measurement Support Systemfor ionizing radiation; conducts research in radiation related

fields to develop improved radiation measurement methodology; and generates, compiles, and critically

evaluates data to meet major national needs. The Center consists of five Divisions and one Group.

Atomic and Plasma Radiation Division

Carries out basic theoretical and experimental research into the • Atomic Spectroscopy

spectroscopic and radiative properties of atoms and highly ionized • Atomic Radiation Data

species; develops well-defined atomic radiation sources as radiometric • Plasma Radiation

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

Provides the central national basis for the measurement of far ultra- • Far UV Physics

violet, soft x-ray, and electron radiation; develops and disseminates • Electron Physics

radiation standards, measurement services, and data for for these radia- • Photon Physics

tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applications; determines fundamental mechanisms of electron and photon interactions with matter; and develops advanced electron- and photon-based

measurement techiques. The Division consists of the following Groups:

Radiometric Phiysics Division

Provides national measurement standards and support services for ultra- • Spectral Radiometry

violet, visible, and infrared radiation; provides standards dissemination • Spectrophotometry

and measurement quality assurance services; conducts research in optical • Radiometric Measurement Services

radiation, pyrometry, photometry, and quantLD radiometry; and develops

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Develops, operates, and improves major NBS radiation facilities • Accelerator Research

including the electron Linac and race track microtron; develops, • Linac Operations

designs, and builds electronic and mechanical instrunentation for • Electronic Instrumentation

radiation programs and facilities; provides national leadership • Mechanical Instrunentation

in the standardization of nuclear instrirnentat ion; and develops

new competence in radiation sources and instrunentation. The

Division consists of the following Groups:

Ionizing Radiation Division

Provides primary national standards, measurement services, and basic

data for applications of ionizing radiation; develops new methods of

chemical and physical dosimetry; conducts theoretical and experimental

research on the fundamental physical and chemical interactions of

ionizing radiation with matter; provides essential standards and measurement support services to the National Measurement Support

System for Ionizing Radiation; and develops and operates radiation

sources needed to provide primary radiation standards, fields, and

well-characterized beams of radiation for research on radiation

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical

Dosimetry

Neutron Measurements and Research

Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

understanding of physical processes that underlie measurement science.
NBS MEASUREMENT SERVICES:

RADIOMETRIC STANDARDS IN THE

VACUUM ULTRAVIOLET

Jules Z. Klose

J. Mervin Bridges

William R. Ott

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued June 1987
Library of Congress Catalog Card Number: 87-619833

National Bureau of Standards Special Publication 250-3

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-3, 137 pages (June 1987)

CODEN: XNBSAV

Commercial products— materials and instruments— are identified in this document for the sole pur- pose of adequately describing experimental or test procedures. In no event does such identification imply recommendation or endorsement by the National Bureau of Standards of a particular product; nor does it imply that a named material or instrument is necessarily the best available for the purpose

it serves.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1987

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

The calibration and related measurement services of the National Bureau

of Standards are intended to assist the makers and users of precision

measuring instruments in achieving the highest possible levels of

accuracy, quality, and productivity. NBS offers over 300 different

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services

are offered to the public and private organizations alike. They are described in NBS Special Publication (SP) 250, NBS Calibration Services

Users Guide .

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed description of the important features of specific NBS

calibration services. These documents provide a description of the:

(1) specifications for the service; (2) design philosophy and theory;

(3) NBS measurement system; (4) NBS operational procedures; (5)

assessment of measurement uncertainty including random and systematic

errors and an error budget; and (6) internal quality control procedures

used by NBS. These documents will present more detail than can be

given in an NBS calibration report, or than is generally allowed in

articles in scientific journals. In the past NBS has published such

information in a variety of ways. This series will help make this type

of information more readily available to the user.

This document (SP 250-3), NBS Measurement Services: Radiometric

Standards in the Vacuum Ultraviolet, by J. Z. Klose, J. M. Bridges, and

W. R. Ott is the third to be published in this new series of special

publications. Tt covers the calibration of the spectral radiance and

spectral irradiance of ultraviolet-emitting lamps over the wavelength

range of 115 to 330 nm (see test numbers 40010s-40040s in the SP 250

Users Guide). Inquiries concerning the technical content of this

document or the specifications for these services should be directed to

the authors or one of the technical contacts cited in SP 250.

The Center for Radiation Research (CRR) is in the process of publishing

21 documents in this SP 250 series, covering all of the calibration

services offered by CRR. A complete listing of these documents can be

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

be made more useful. Suggestions are also welcome concerning the need

for new calibration services, special tests, and measurement assurance

programs

.

George A. Uriano

Di rector

Measurement Services

i i i

Chris E. Kuyatt

Director

Center for Radiation Research
ABSTRACT

The radiometric calibration program carried out by the vacuum ultraviolet

radiometry group in the Atomic and Plasma Radiation Division of the National

Bureau of Standards is presented in detail. The calibration services are first listed, followed by descriptions of the primary standards, which are the

hydrogen arc and the blackbody line arc, and the secondary standards, which

are the argon mini- and maxi-arcs and the deuterium arc lamp. Next, the calibration methods involving both spectral radiance and irradiance are discussed

along with their uncertainties. Finally, the i ntercompari son of standards as

a method of quality control is described.

Key words: arc (argon); arc (blackbody); arc (hydrogen); irradiance;

lanp (deuterium); radiance; radiometry; standards (radiometric);

ultraviolet; vacuum ultraviolet.

-iv-
TABLE OF CONTENTS

Page

Abstract iv

Li St of Tabl es vi i i

List of Figures ix

I. Introduction 1

II. Radiometric Standards in the Near and Vacuum Ultraviolet ... 3

A. Spectral Irradiance Standard, Argon Mini-Arc

(140-330 nm) 3

B. Spectral Radiance Standard, Argon Mini-Arc

(115-330 nm) 3

C. Spectral Irradiance Standard, Deuterium Arc Lamp

(165-200 nm) 3

D. Special Tests of Radiometric Devices in the Near

and Vacuum Ultraviolet 3

III. Apparatus 6

A. Primary Standards 6

1. The Hydrogen Arc 6 a. Physical Principles 6

b. Description of the Arc 15

2. The Blackbody Line Arc 21

B. Secondary Standards 22

1. Argon Mini -Arc 22

a. Physical Principles 22

b. Description of the Arc 24

c. Spectrum 29

2. Argon Maxi-Arc 32

3. Deuterium Arc Lamp 32

a. Introduction 32

b. Description and Operation of the Lamp 35

C. Measurement Apparatus 38

1. Monochromators 38

2. System Components 40

a. Photomul tipl iers 40

b. Detection Electronics 40

c. Specifications 40

IV. Calibration Methods 43

A. Radiance Calibrations 43

1. Introduction 43

2. Hydrogen Arc 43

-V-
Page

a. Setup 43

b. System Alignment 44

c. Arc Current 44

d. Dark Current and Scattered Light 44

3. Argon Arc 45

4. Blackbody Line Arc 46

5. Tungsten Lamp Standard 47

6. Calibration of Argon Arcs Relative to an NBS Argon Arc 49

B. Irradiance Measurements 50

1. Establishment of an Irradiance Scale 50

2. Argon Arc Calibrations 54

3. Deuterium Lamp Calibration 56

V. Uncertainties 59

A. Argon Mini -Arc Radiance Standard 59

1. Primary Standards 59

2. Arc Current 59

3. Gas Flow 59

4. Pressure 62

5. Alignment 62

6. Reproducibility and Stability 65

7. Gas Impurity 67

^ 8. Summary 67

B. Argon Mini -Arc Irradiance Standard 69

C. Deuterium Lamp Irradiance Standard 75

1. Spectral Irradiance Calibration 75

2. Alignment 75

3. Lamp Current 75

4. Solid Angle 78

5. Variability 78

6. Calibration Checks and Accuracy 79

7. Summary 81

VI. Quality Control Through Intercomparisons 82

A. Introducti on 82

B. The Hydrogen Arc Versus the Gold-Point Blackbody 82

1. Tungsten Strip Lamp 82

]!; 2. Tungsten Strip Lamp with Blackbody Line Arc 82

'

. 3. Argon Mini-Arc and Tungsten-Quartz-Halogen Lamp

with Deuterium Lamp 83

J, C. The Hydrogen Arc Versus the DESY Synchrotron 84

1. Deuterium Lamp 84

VII. Acknowledgments 88

VIII. References 89

-vi -
Page

IX. Appendices A-1

A. The Deuterium Lamp as a Radiometric Standard

Between 115 nm and 167 nm A-1

B. Hollow Cathode Lamps B-1

C. Rare Gas Dimer Lamps C-1

D. Monochromatic Source of Lyman-a Radiation D-1

E. Safety Considerations E-1

F. Calibration Reports F-1-1

1. VUV Spectral Radiance of Argon-Mi ni -Arc F-1-1

2. VUV Spectral Irradiance of Argon-Mini-Arc F-2-1

3. VUV Spectral Irradiance of Deuterium Lamp F-3-1

-vi i-
LIST OF TABLES ^ Page

Table I. ' Maximum Emission Coefficient from a 1-atm

' Hydrogen Plasma in LTE Calculated as a Function

of Wavelength 8

Table II. Spectral Radiance as a Function of Wavelength for

a 50.0-A Argon Mini -arc Light Source with an Arc

Plate Diameter of 4.0 mm 28

Table III. Survey of Emission Lines Appearing in Spectrum of

Mini -arc 31

Table IV. Mini -arc Current Required to Produce a Constant

Spectral Radiance of 2.70 x 10"^ W cm"2 nm"i sr~i

at Various Wavelengths in the 115-300-nm Range .. 60

Table V. Error Budget for the Spectral Radiance

Calibration of the Argon Mini -Arc 68

Table VI. Error Budget for the Spectral Irradiance

Calibration of the Argon Mini -Arc 70

Table Vtl. Error Budget for the Spectral Irradiance

Calibration of the Deuterium Arc Lamp 80

Table VIII. Lyman-a Source Line Irradiances at 50 cm D-3

-vi i i -
LIST OF FIGURES

Page

Figure 1. Comparisons of spectral radiances for far UV sources. The output of the hydrogen arc is given

for the temperature of maximum continuum

emission; the outputs of the other sources are at

typical operating conditions 4

Figure 2. The emission coefficient for a 1-atm hydrogen

plasma in LTE as a function of temperature for

several wavelengths: 250 nm ( ); 200 nm (ooo);

150 nm (---); and 124 nm (-_-) 5

Figure 3. Radial dependence of the continuum intensity and arc temperature for a 2-mm wal 1 -stabi 1 i zed

hydrogen arc operating at 80 A. The dashed lines

define the dimensions of the plasma that is

observed by spectrometers 7

Figure 4, Schematic of the hydrogen wal 1 -stabi 1 i zed arc, A

cutaway top view drawing of one of the arc plates

illustrates the flow of cooling water through

specially machined channels within each plate.

The symbols in the four corners show the

direction of water flow 10

Figure 5. The radial dependence of the hydrogen arc spectral radiance at 160.6 nm for an arc axis

temperature of about 19,500 K. The intense

off-axis radiation is due to Lyman band molecular

emission. The nearly uniform radiation in the

vicinity of the axis contains no molecular

contributions, but is due to the hydrogen

continuum that is being used as a spectral

radiance standard 11

Figure 6. Maximum spectral radiance vs wavelength

calculated for a 5.3-cm length hydrogen arc plasma in LTE at 1 atm pressure. The calculation

is shown as a grid and represents the rms uncertainty due to uncertainties in the arc length (±5%) and the calculated emission

coefficient (between 2% and 13%). The points are

actual measurements of the spectral radiance

calibrated either with a tungsten strip lamp

blackbody limited lines (o), or a low power

hydrogen arc (x) 13

-ix-
Page

Figure 7. A photograph of the hydrogen wal 1 -stabi 1 i zed arc. This particular photograph shows gas ports in

between several of the arc plates. These are not

normally used in hydrogen arc radiometry but are available to admit gas impurities needed for

other applications 14

Figure 8. Exploded view of the major components of the arc:

electrodes (one has been removed), two arc plates, a bakelite spacer and gaskets 16

Figure 9. (a) a schematic illustrating the method of arc plate construction, (b) The assembled arc plate.

(c) A photograph of an arc plate cut away in the

center to expose the water channel 17

Figure 10. An end-on view of the hydrogen arc operated at

100 amperes. The temperature for maximum

continuum emission has been exceeded in the

vicinity of the arc axis for wavelengths in the

visible. This is manifested as a slight contrast

between the brighter outer edge and the darker

center of the plasma 19

Figure 11. A photograph of the argon mini-arc mated to a monochromator 23

Figure 12. A schematic of the argon mini-arc light source .. 25

Figure 13. Spectral radiance as a function of wavelength for

a mini -arc with an arc plate diamter of 4.0 mm operated at two different currents: 50.0 A and

25.0 A 27

Figure 14. A photoelectric scan of the spectrum of the argon

mini -arc between 115 nm and 320 nm, taken with a 0.01 nm spectral resolution and with a solar

blind phototube detector 30

Figure 15. Comparison of the spectral radiance of several UV

primary and transfer standard sources 33

Figure 16. Schematic illustrating the operation of a deuterium lamp. The radiation is measured

through the Suprasil window sealed to the quartz

lamp envelope 34

-X-
Page

Figure 17. Line drawing of the deuterium arc lamp standard

36

Figure 18. Photograph of the deuterium lamp and the

tungsten-quartz-halogen lamp highlighting their

common bipost base 37

Figure 19. Absolute spectral irradiance measured as a function of wavelength at a distance of 50 cm from the field aperture for five different

continuum sources with the indicated power

requirements. The spectrum of the D2 lamp with

MgF2 window below 170 nm, measured for a 1 nm bandpass, is a pseudo-continuum made up of

blended lines. Shown for comparison purposes are spectra of the 250 MeV National Bureau of

Standards Synchrotron Radiation Facility, for a beam current of 10 mA and a field aperture

distance of 19 m, and the tungsten-quartz-halogen

lamp, for the standard 50-cm distance 39

Figure 20. Schematic of the setup for comparing sources of

spectral radiance in a vacuum 42

Figure 21. Schematic of the setup for comparing sources of

spectral radiance in air 48

Figure 22. Schematic of calibration procedure, (a)

Spectroradi ometer is irradiated by the lamp whose

spectral irradiance is to be evaluated; (b)

efficiency is determined on a relative scale by

irradiation with a stopped-down radiance

standard 51

Figure 23. Schematic of the setup for comparing sources of

spectral irradiance in a vacuum 53

Figure 24. Schematic of the setup for comparing sources of

spectral irradiance in air 55

Figure 25. The dependence of the mini -arc spectral radiance

on arc current for several representative

wavelengths 58

Figure 26. The dependence of the mini -arc spectral radiance

on the arc chamber pressure. The arc chamber

pressure, in the absence of an exhaust manifold,

is equal to the local atmospheric pressure. Data

-XI-
Page

are shown for the following wavelengths: 300 nm

(+); 250nm (o) ; 200 nm (x); and 140 nm (.) 61

Figure 27, The dependence of the mini -arc spectral radiance

on transl ational alignment for several field

stops, arc currents, and wavelengths. The zero indication on the translation scale corresponds

to the case where the arc axis is exactly aligned

with the optical axis. Data are shown for the

following conditions: 0.300-mm diam field stop,

50- A current, and wavelengths of 300 nm (•),

250 nm (o), 200 nm ( + ), 140 nm (); 0.300-mm

diam field stop, 20-A current, and 300 nm (a);

0.015-mm2 field stop, 50-A current, and 250 nm

(x) 63

Figure 28. The dependence of the mini -arc spectral radiance

on rotational alignment (pitch and yaw) for

several arc currents and wavelengths. The zero indication on the rotation angle scale

corresponds to the case where the arc axis is

exactly aligned with the optical axis. Data are shown for the following conditions: yaw

variation, 50-A arc current, and wavelengths of

300 nm (•) and 140 nm (a); yaw variation, 20-A

arc current, 300 nm (x); pitch variation, 50-A

arc current, 300 nm (o) 64

Figure 29. Percentage difference in the near UV between

spectral -i rradi ance calibrations using

conventional techniques (essentially using a tungsten-quartz-halogen i rradi ance standard and

integrating sphere) and the method here

introduced for VUV calibrations (essentially

using a mini -arc radiance standard and diffusing

window). The shaded area represents the

uncertainty in the tungsten-lamp calibration.

The error bars represent the imprecision in the

measurements. The measurements were normalized

at 280 nm 71

Figure 30. Typical spectral irradiance of 1000 W

quartz-halogen lamp and a 30-W deuterium lamp .. 73

Figure 31. The dependence of the spectral irradiance upon

pitch and yaw of a deuterium lamp located 50 cm from a l-cm^ aperture. A 2° angular change is

equivalent to a 1.75-cm transl ati onal change ... 74

-xi i-
Page

Figure 32.

Figure 33.

Figure 34,

Figure 35.

Figure 36,

Figure 37,

Figure 38,

Figure 39,

Spectra! irradiance as a function of lamp current

for two representative wavelengths. The values

are normalized to the spectral irradiance at a current of 0.3 A

Spectral irradiance measurements as a function of

distance between the deuterium lamp envelope and

a l-cm^ field stop. Results expected by assuming

the inverse square law and taking into account

the finite size of the field stop and the

position of the radiation center (determined by

least squares fitting) are illustrated by the

solid line. The data are taken at three

representative wavelengths

Results of the i nterl aboratory comparisons of

spectral radiance calibrations of deuterium lamps

between NBS and MPI (No. 6) and between PTB and

MP I (No. 14). The MPI spectra were normalized to

the other data to show only the wavelength

dependent part of the deviations, which is within

±3%

Intensities versus wavelength for 1-mm entrance

and exit slits (solid curve), and the same quantities for 6-mm entrance and exit slits

(dashed curve)

Maximum differences in the intensity ratios over

the range of bandpass ratios versus wavelength .

A portion of the spectrum of a Pt-Cr-Ne hollow

cathode line source

The spectral radiance of an rf -excited krypton

dimer lamp

Intensity of Lyman-a emission and the ratio of

the intensity of Lyman-a to that of the impurity

radiation around 1269 A as a function of oxygen

flow. The intensities were determined by measuring the areas under the spectral distributions of

both Lyman-a and the impurity radiation using a planimeter and adjusting the values according to

the system efficiencies

76

77

86

A2

A3

B-2

C-2

D-2

-xiii-

I. Introduction

The vacuum ultraviolet (VUV) region of the spectrum has become important

in several areas of research and development. These include space-based as- tronomy and astrophysics, thermonuclear fusion research, ultraviolet laser

development, and general atomic physics research. Applications of VUV radiation in chemical, biophysical, and medical fields are widespread. Many applications require a knowledge of not only the wavelength of the radiation involved but also the intensity or flux of radiation. This implies a calibration of some type. The calibration may be based upon a standard source, i.e., one whose output is known, or a standard detector, i.e., one whose response to

a given radiation level is known. Two general cases can be distinguished. In the first case one wishes to determine how much radiation a source such as the sun or a plasma device is emitting at a given wavelength. Usually, the source

is not monochromatic, so a monochromator must be used to select the desired

wavelength. In this case the most direct procedure is to employ a standard

source. The source to be investigated as well as the standard source are set

up in turn so that radiation from each source passes through the same mono- chromator and optical elements. The calibration is performed essentially by a direct substitution of the standard source for the one to be calibrated.

The second case occurs when one wishes to know the flux in a monochromatic beam of radiation, such as the flux emerging from the exit slit of a monochromator. For this determination a standard detector is more appropriate; the flux is determined by simply measuring the signal when the detector

is irradiated with the beam to be calibrated. If one were to attempt to perform the calibration in the first case above using a standard detector or in

the second case using a standard source, one would in both cases need to know

the spectral efficiency of the monochromator used in the measurement. This

would require an additional measurement using a second monochromator and would

introduce additional uncertainties and complexities. Therefore a need exists

for both standard sources and standard detectors.

Standard sources may be divided into primary standards and secondary or transfer standards. Primary standards are ones whose output is known from

basic principles. The primary standards of VUV radiation include plasma

sources, especially the wal 1 -stabi 1 i zed hydrogen and blackbody line arcs, and

electron storage rings emitting synchrotron radiation.

There are storage rings at several laboratories, including NBS, which are used as primary VUV radiation standard sources. They produce continuum

radiation for wavelengths ^ 0.03 nm. Limited access to storage rings,

however, makes it desirable for most workers to have available other standard

VUV sources. The wal 1 -stabi 1 i zed hydrogen and blackbody line arcs were developed to provide alternative primary standard sources. These sources,

however, are also not able to be easily used for calibrations. Hence,

secondary or transfer standards which are relatively easy to apply have been

developed. These include the deuterium lamp and the argon mini-arc. These sources are more readily available and make possible relatively inexpensive

and convenient calibrations. Even for researchers having access to a storage

-1-
ring the secondary standards can be useful in making possible more frequent

calibrations. Also, the secondary standards possess some useful properties

not characteristic of the available primary standards such as, for example,

emission over a relatively large solid angle.

The two principal radiometric quantities which are measured and calibrated are radi ance and i rradi ance . For an object or source which emits radiation, the radiance is the radiant power emitted per area per solid angle:

L = [watts cm~^ sr"^]. If the source emits a continuum, i.e., emits radiation

at all wavelengths near a particular wavelength, the spectral radiance is the

radiance per wavelength interval or bandpass: = [watts cm"^ sr"^ nm"i].

The radiance will in general vary over the source area, the direction, and the

wavelength. The definition assumes that the area, solid angle, and wavelength

band are small enough that the radiance does not vary greatly within these

quantities. Spectral irradiance is the radiant power incident upon a target

per area per wavelength band: Ej, = [watts cm"2 nm"i]. A source of radiation

may serve as a standard source of irradiance by operating it at a given distance from the target area. Some sources may be used as either standard radi- ance or standard irradiance sources. A separate calibration must be performed, however, for each quantity.

The services performed by the Atomic and Plasma Radiation Division of the

National Bureau of Standards include tests and calibrations of portable

secondary VUV standard radiance and irradiance sources. These are usually

deuterium lamps and argon arcs. Also included, however, are special sources

such as rare-gas dimer lamps, which emit continuum radiation over limited

wavelength ranges, and hollow cathode lamps, which eiTiit spectral lines in the

wavelength range from the VUV through the visible. All sources are generally

supplied by customers. The main groups of customers have included those in

the fields of space-based astronomy and solar physics who have used standard

sources to calibrate satellite, rocket, or balloon-borne spectrometers. Other

customers have needed calibrations in the 100-300 nm range for plasma

radiation studies.

-2-
II. NBS Radiometric Standards in the Near and Vacuum Ultraviolet

A. Spectral Irradiance Standard, Argon Mini-Arc (140-330 nm) , $P250 No. 40010S (7.611

An argon mini -arc supplied by the customer is calibrated for spectral

irradiance at 10 nm intervals in the wavelength region 140-330 nm. Absolute

values are obtained by comparing the radiative output with laboratory

standards of both spectral irradiance and spectral radiance. The spectral

irradiance measurement is made at a distance of 50 cm from the field stop.

Uncertainties (2a) are estimated to be less than ±10% in the wavelength region

140-200 nm and within ±6% in the wavelength region 200-330 nm. A measurement

of the spectral transmission of the arc window is included in order that the

calibration be independent of possible window deterioration or damage.

B. Spectral Radiance Standard, Argon Mini-Arc (115-330 nm)

,

SP250 No. 40020S (7.601

The spectral radiance of argon mini-arc radiation sources is determined to

within a 2a uncertainty of less than 6% over the wavelength range 140-330 nm and 11% over the wavelength range 115-140 nm. The calibrated area of the 4 mm diameter radiation source is the central 0.3 mm diameter region. Typical

values of the spectral radiance are: at 250 nm, = 30 mW cm'2 nm~i sr"i

;

and at 150 nm, L;^ = 3 mW cm"^ nm"i sr"i. The transmission of the demountable

lamp window and that of an additional MgF2 window are determined individually

so that the user may check periodically for possible long term variations.

C. Spectral Irradiance Standard, Deuterium Arc Lamp (165-200 nm)

,

SP250 No. 40030S (7.607

The lamp is calibrated at ten wavelengths from 165 to 200 nm at a distance

of 50 cm. Its spectral irradiances are about 0.05 yW cm'2 nm"i at 165 nm,

0.03 pW cm'2 np^-i 170 nm, and 0.05 yW cm"2 nm"i at 200 nm. The approximate

2a uncertainty relative to SI units is estimated to be less than 10%. The

lamp is normally supplied by NBS and requires 300 mA at about 100 V.

D. Special Tests of Radiometric Devices in the Near and Vacuum Ultraviolet,

SP250 No. 40040S (7.6"Ay

Tests of customer supplied radiometric devices not included above are performed at the direction of the customer.

-3-
I

CM

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10'

0) 10'

ifi £

10-I

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10

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Figure 1.

"1 \ 1 1- I2500K

BLACKBODY

BLACKBODY LIMITED LINES

FROM 12 500 K ARC

I cm HYDROGEN ARC

30 WATT

Dg LAMP

/

J L

TUNGSTEN

STRIP

LAMP

100 200

X [nm]

300 400

Comparisons of spectral radiances for far UV sources. The output of the hydrogen arc is given

for the temperature of maximum continuum

emission; the outputs of the other sources are at typical operating conditions.

-4-
5.0

4.0

3.0

(D

^

2.0

UJ 0 2

124 nm

250 nm

o o 200 nm

o O O o o o O O O o O o

° ° o O o o

150 nm

16000

Figure 2.

18000 20000 22000

T( K)

The emission coefficient for a 1-atm hydrogen

plasma in LTE as a function of temperature for

several wavelengths: 250 nm ( ); 200 nm (ooo);

150 nm (- —); and 124 nm ( ).

-5-
III. Apparatus

! A. Primary Standards

' 1. The Hydrogen Arc

a. Physical Principles

A high temperature wal 1 -stabi 1 i zed steady-state hydrogen arc has been

developed as our primary standard of spectral radiance. i This type of arc lends itself to such a use because at sufficiently high temperatures it yields

absolute intensities independent of other radiometric standards or of the

accuracy of any plasma diagnostics. Previous efforts at lower powers were hindered by large uncertainties in plasma diagnostics, a difficulty that has

been overcome in the high temperature arc. Figure 1 illustrates the UV spectrum of several of the more common standard sources, including that part of

the hydrogen arc spectrum that is the subject of this paper.

The method depends upon the phenomenon that the continuum emission co- efficient for a strongly ionized hydrogen plasma which is in or close to the

condition of local thermodynamic equilibrium (LTE) is calculable to within one percent. 2 This follows from the fact that the essential spectroscopic con- stants, i.e., the continuum absorption coefficients and transition probabilities, are exactly known for atomic hydrogen. The continuum intensities

emitted from a typical pure hydrogen wal 1 -stabi 1 i zed arc discharge in the

spectral region above 91.5 nm are optically thin and a function of the electron density and temperature. ^ In the low power hydrogen arc the electron

density and temperature are determined from plasma diagnostics in the visible

using available radiometric standards. ^ These quantities are then used to

calculate the continuum intensities in the VUV. There are known to be

significant uncertainties in the plasma diagnostics, and as a result we have

adopted as our primary standard a higher power hydrogen arc which operates

such that a 2 mm diameter wal 1 -stabi 1 i zed discharge reaches a temperature of

about 20,000 K. For these conditions the continuum emission coefficient as a function of temperature shows a broad maximum which shifts with wavelength as

is shown in Fig. 2. This optimum condition is brought about essentially by

the compensating effects of an increase in the ionization fraction and a de- crease in the total number density as the temperature is increased in a constant 1-atm pressure operating environment. Beyond this maximum5»6 the

ionization is practically complete, and any further increase in arc temperature results only in a gradual decrease in intensity because of the de- crease in the number density. The weak wavelength dependence of this maximum

seen in Fig. 2 occurs because of the change in the energy distribution of the

equilibrated electrons as the electron temperature is varied.

Figure 3 shows the measured radial dependence of the hydrogen continuum

emission coefficient at 190 nn and the corresponding calculated LTE

temperature^ for an arc current of 80 A. Because the maximum in the

coefficient is broad and the absolute magnitude of the peak intensity is not

very sensitive to the electron temperature, the emission characteristics of

the plasma are nearly homogeneous over its central region which extends to

about 0.6 mm from the arc axis. This is especially significant if only a

-6-
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-8-
small sample of the region is observed as indicated in the figure. First, it

means that the alignment precision, which was responsible for much of the

uncertainty in the low power arc method, is not critical here. Second, it means that the arc current necessary to obtain the maximum emission coefficient is also not critical.

Additional advantages of operating the hydrogen arc under such conditions

are: (1) the Lyman band molecular emission, which limited the low power

hydrogen arc to wavelengths above 165 nm, is negligible; (2) the assumption of

LTE appears to be very closely fulfilled as shown by experimental consistency

checks, theoretical validity criteria, and variations in arc current which

produce only very small deviations from LTE;3'8 and (3) the hydrogen arc as it

is described here constitutes a true primary standard of spectral radiance in

that except for the minor uncertainties associated with the theoretical model

for the plasma, knowledge of the absolute continuum intensities does not

depend on any other standards of calibration or sophisticated plasma diagnostics and requires only a measurement of the ambient pressure (»1 atm) and

the arc length. The procedure for applying the arc as a radiometric standard

involves only adjusting the arc current for maximum signal at the wavelength

of interest. The overall system efficiency is given then by the ratio of the

detector response to the hydrogen arc spectral radiance calculated using the

maximum LTE emission coefficient at 1-atm pressure and the actual physical

length of the discharge.

Figure 1 shows the v;avelength dependence of the maximum emission coefficient of the hydrogen arc plasma, and Table I lists the values of the same quantity obtained through a calculation of the hydrogen continuum. 2 The Stark

broadened wing of the Lyman a line at 122 nm has been included in these calculations and, as can be seen from the table, becomes significant at wavelengths

below 140 nm.^ The uncertainty in the calculated continuum emission coefficient is estimated to be about 2%, due mostly to uncertainties in the high density plasma corrections. ^ Combining this uncertainty in quadrature with the

uncertainties in the Stark broadening calculations, the uncertainty in the

total emission coefficient comes out to be 2% above 140 nm, 8% at 130 nm, and

13% at 124 nm. These uncertainties and all those succeeding unless stated

otherwise are taken to be 2 standard deviations (2a), i.e., as having a con- fidence limit of about 95%.

Three factors are of special concern in applying the high power hydrogen

arc standard: the arc length, spatial resolution, and off-axis H2 emission.

Figure 4 is a schematic of the hydrogen arc.i»'*>io The arc is struck between

a set of four tungsten anodes and four tungsten cathodes that are symmetrically located and share the current load equally. At each end of the 2-mm

diameter channel formed by the twenty water-cooled stacked copper plates, the

discharge expands and separates into four smaller arcs terminating at each of

the electrode tips. The length of this inhomogeneous region is made small

compared to the total length of the arc, and the emission coefficient and

temperatures are lower than in the channel. Therefore, its contribution to

the total arc signal obtained in an end-on measurement along the axis of

the discharge is small but not precisely known. The minimum possible

length of the arc is 5.05 cm, the total length of the channel formed by the

twenty arc plates, and the maximum length is 5.5 cm, the total distance

-9-
VACUUM

SHUTTER - MIRROR

Figure 4.

AIR

Schematic of the hydrogen wal 1 -stabi 1 i zed arc. A cutaway top view drawing of one of the arc plates

illustrates the flow of cooling water through

specially machined channels within each plate.

The symbols in the four corners show the

direction of water flow.

-10-
SPATIAL RESOLUTION

X I GAIN

X 10

-1.0 0 5 0 0.5 +L0

R(mm)

Figure 5. The radial dependence of the hydrogen arc spectral radiance at 160.6 nm for an arc axis temperature of about 19,500 K. The intense off-axis radiation is due

to Lyman band molecular emission. The nearly uniform radiation, in the vicinity

of the axis contains no molecular contributions, but is due to the hydrogen

continuum that is being used as a spectral radiance standard.

-11-
between the electrodes. The estimated length of the homogeneous hydrogen arc plasma was thus taken to be 5.3 cm with an uncertainty of 5% (2a).

Spatial resolution is not much of a problem for the high power arc since

the emission characteristics are nearly uniform in the vicinity of the arc axis. The regions of the arc that fall into the cone of observation determined by the f/200 aperture and the 0,30 mm field stop are illustrated in

Fig, 3, Thus, knowledge of the emission coefficient as a function of radial

position allows the calculation of the effect of spatial averaging. The main

result is that the integrated emission coefficient F has a maximum about 1%

lower than e^^^^ from a truly homogeneous plasma with an axis temperature T{e")

slightly higher than T(ep^3x)» plasma is cyl indrical ly symmetric, and

because the area of the differential shells increases as 2TrrAr, the off axis

regions are weighted more than the axis region. Therefore, when one obtains

maximum signal , the current has been adjusted so that the temperature maximum

occurs slightly off-axis as seen in Fig. 3.

Below 165 nm the Lyman bands of H. are emitted from low temperature

off-axis regions. Figure 5 shows a semilog plot of a radial scan of the arc

at a wavelength where one of the stronger features can be observed. The

off-axis line peak is about 100 times larger than the on-axis hydrogen con- tinuum, but clearly if the observation column is restricted to no more than

the central 0,5 mm diameter plasma, H2 lines should not be observed. Molecular emission from the the i nhomogeneous end regions also is a possibility.

However, none is observed since the arc terminates at points that are out

of the line of observation (the cooler plasma regions are off-axis) and because the end-layer on-axis temperature gradient is quite sharp (the length of

the cool plasma is then very short).

In Fig, 6 the calculated spectral radiance of the high power hydrogen arc

is compared with calibrations of that arc made with several other standard

sources,^ The theoretical spectral radiance is the product of the theoretical

maximum hydrogen plasma emission coefficient and the arc length. This quantity is graphed as a shaded area which represents the rms uncertainty due to

the previously described uncertainties in both the calculated emission coefficient and the arc length. The solid and open circles represent the measured

continuum intensities using, respectively, a calibrated tungsten strip lampH

and blackbody limited lines from another arc for spectral radiance normalization. The crosses represent the measured intensities using the low power

hydrogen-arc standard. The error bars attached to the data points represent

uncertainties associated not with the hydrogen arc but with the various standards used to calibrate the spectral radiance. The discontinuity in the wave- length scale is due to the interruption of the hydrogen spectrum by the Balmer

line series of atomic hydrogen, which dominates the high temperature arc spectrum above 360 nm except for a small region around 560 nm between and Hg

,

Inspection of Fig, 6 leads us to conclude that the various standards are con- sistent with one another over the entire range of comparison.

-12-
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-14-
In summary, a high power atmospheric pressure hydrogen arc capable of

operating at temperatures on the order of 20,000 K has been examined theoretically and experimentally and found to be suitable as a primary radiometric standard in the VUV. The experimental investigations have shown

that at such temperatures molecular hydrogen emission below 165 nm is negligible, and the hydrogen plasma continuum emission coefficient can be

measured throughout the 124-360 nm spectral range. By operating the arc at

currents such that the maximum emission coefficient is reached, the uncer- tainties associated with various plasma diagnostics and alignment imprecisions

are minimized to the one-percent range. The results of a comparison with

other available standard sources are consistent with the estimated 2a uncer- tainty of ±6% in the hydrogen arc spectral radiance between 140 nm and 360 nm. This uncertainty is due mainly to the uncertainty in the measurement of the

arc length. Below 140 nm the Lyman a Stark broadened wing becomes significant, and the estimated uncertainty in the hydrogen arc intensities increases

to about ±9% at 130 nm and ±14% at 124 nm due mainly to uncertainties in the

plasma line broadening theory. These results are also consistent with a com- parison standard. Finally, all experimental comparison data confirm that the

hydrogen arc spectral radiance is calculable throughout the 124-360 nm range

without dependence on any other radiometric standard or plasma diagnostic

technique. Therefore, the high power hydrogen arc is a true primary

standard.

b. Description of the Arc

The plasma source for generating the hydrogen continuum is a steady-state

wal 1 -stabi 1 i zed arc operating in a flow system of pure hydrogen at 1 atm

pressure. The general arc design is based on the one developed by Maecker and

Steinberger^^ and is illustrated both in the schematic drawing in Fig. 4 and

the photograph in Fig. 7. The 5.3 cm long discharge is formed between a set

of four tungsten cathodes and four tungsten anodes (only two electrodes from

each set appear in Fig. 4) and is constricted by a stack of 20 copper plates

each with a central bore of 2.0 mm diameter. The thickness of the plates is

limited to 2 mm in order that the arc current, which chooses the path of lowest voltage drop, passes through the resistive plasma rather than through the

plates themselves. The plates must also be very closely spaced in order to

avoid any kinks or asymmetries in the strongly constricted cyl i ndri cal lysymmetric plasma column. As can be seen in Fig, 8, electrical isolation

and uniform spacing between the arc plates are provided by a 0,3 mm thick

bakelite mask surrounding a 0,5 mm thick x 20 mm diameter silicone gasket

whose purpose is to seal the discharge region from the atmosphere. The four

10 mm diameter holes in the plates carry the cooling water to the arc plates

and are likewise sealed by silicone gaskets.

The pile of cascade plates in Fig, 4 is terminated by two large identical

end pieces which contain the cooling water connections, gas ports and

connections, and radial holes for mounting the electrodes. The central

cavities in these end pieces are connected by two insulated pressure

equalization tubes of large cross section through which the arc gas flows.

There is little flow in the arc channel itself which rather depends upon

diffusion from the end sections for its source of gas. The pressure

equalization tubes also serve to minimize instabilities in the gas flows which

-15-
Figure 8. Exploded view of the major components of the arc:

electrodes (one has been removed), two arc plates, a bakelite spacer and gaskets.

-16-
I

(a) A schematic illustrating the method of arc plate construction, (b) The assembled arc plate,

(c) A photograph of an arc plate cut away in the

center to expose the water channels.

-17-
may arise from small pressure fluctuations in the electrode regions due to

occasional shifting of the arc termination points. If they are present, these

instabilities are manifested by a varying arc length. A polycarbonate plastic

extension tube is attached to the end piece on the vacuum side of the arc for

the purposes of insulating the end piece from the vacuum system. It also

allows remote mounting of a UV quality MgFp window with shutter, which doubles

as an observing port for adjustment of the anodes. The window is located

17 cm from the arc because of possible degrading effects caused by heat and

intense UV irradiation. Also, a gas input port directs a flow of gas onto the

window and prevents it from collecting a layer of attenuating tungsten dust

which is evaporated from the anodes at high power levels. The Suprasil window

on the air side of the arc does not require similar precautions since the

cathodes do not heat up as much as the anodes and since the window transmission, which is needed for calibrations in the near UV and visible, can be

conveniently measured by simple removal during arc operation.

As can be seen in Fig. 4, the electrodes, especially insulated and sealed

by o-rings, are positioned as close as possible (about 0.5 mm) to the arc plates in such a way as to form an extension of the channel. Four electrodes

at each end in a cross arrangement are used tn order to subdivide the heat

load and to minimize arc asymmetries in the end zones near the arc termination

points. The electrode cooling is provided by soldering the 48 mm long, 10 mm diameter tungsten rods which serve as the electrode tips to water-cooled

copper tubes.

The arc plates themselves are of fairly complex design. Several variations, designed principally to reduce machining difficulties and increase the

economy of production, have been described in the 1 iterature.^^ »12 Basically,

the problem is to cool the walls efficiently and to avoid the formation of

vapor bubbles and consequent thermal isolation of the heated channel walls

which comes about when there is poor or turbulent contact between the channel

wall and the flowing water. The particular method of fabrication for our

model^^^ is illustrated in Fig. 9. By inserting the split brass plates labeled

B in Fig. 9a into the central copper piece C, water channels are formed 4 mm wide at their ends and 1 mm wide in the bent parts near the 2 mm diameter bore

as seen in Fig. 9b. In this way the water entering from two opposite sides is

accelerated near the arc channel, pressed towards the hot wall, and removed

through the two remaining adjacent ports. Pieces A, B and C are silver-soldered together and machined to the required thickness of 2 mm. A photograph

of an arc plate assembly with the central part cut open to expose the flow

channels is included in Fig. 9c.

The operation of the arc requires substantial electric, water, and gas

utilities. The electric current to the arc is supplied by a 1200 V, 110 A DC

power source with 0.05 percent current regulation and a ripple less than 0.2

percent of the average voltage. In order to divide the arc current equally, a 12 Q. ballast resistor is placed in series with each of the four arc anodes and

cathodes. These resistors, designed for loads up to 7.5 kW, are immersed and

cooled in a flowing tap water reservoir. A separate 12°C chilled water system

with a total flow rate of about 80 l min"^ at 2.5 atm pressure is used to cool

the arc plates and electrodes. Since arc ignition is best performed in pure

-18-
Figure 10. An end-on view of the hydrogen arc operated at 100 amperes. The temperature

for maximum continuum emission has been exceeded in the vicinity of the arc axis for wavelengths in the visible. This is manifested as a slight contrast

between the brighter outer edge and the darker center of the plasma.

-19-
argon gas, a simple gas handling system is required. The arc is struck at

20 A by shorting the cathode to the anode with a tungsten rod connected to the

cathode potential and slowly withdrawing the rod down the axis of the arc. After adjusting the position of the electrodes and raising the current to

60 A, the argon flow is replaced by a hydrogen flow of about 5 z min"i through

a valve and flow meter system. The high flow rate is necessary primarily to

purge the extension tube between the arc and the MgF2 window where air and

water vapor absorption can affect the calibration accuracy.

As was discussed above, calibrations utilizing the hydrogen continuum are most accurate when the arc is operated at power levels such that maximum

emission is achieved. The calibration procedure reduces then to a simple adjustment of the arc current for maximum signal at the wavelength of interest.

At this condition the response of the system is identified with the spectral

radiance calculated from the maximum continuum emission coefficient. If the

arc current is increased so that the axis temperature exceeds that for maximum

emission for a given wavelength, the intensity at that wavelength can be observed to decrease. This effect can be demonstrated by projecting white light

from the arc onto a screen and operating it at currents well above that needed

for maximum emission. A close inspection of Fig. 10 made at an angle close to

the plane of the paper reveals a circular shadow about 1/3 the arc diameter

and centered about the arc axis.

The long-term stability of the hydrogen arc source is excellent and depends primarily on how well one accounts for small changes in the transmission

of the windows. The source stability in the air ultraviolet range of 200 nm to 360 nm is measured to be about 1 percent over a 10 hour period. However,

since the vacuum ultraviolet signal of the arc with window decreases over the same period by 10-20 percent due to window degradation, calibration experiments are kept as brief as possible, and the shutter is closed between

measurements to protect the VUV optics.

-20-
2. Blackbody Line Arc

The short wavelength limit of the hydrogen arc at about 124 nm is determined by the onset of the Stark-broadened, optically thick Lyman line series

of atomic hydrogen that dominates the spectrum between 94 nm and 124 nm. Since the short wavelength cutoff of magnesium fluoride windows which are used

in many VUV instruments is at about 115 nm, it would be convenient to have a primary standard which covers the range 115 nm to 124 nm. This need has been

met with the development of the blackbody line arc,i3-i5

Blackbody radiances for a number of prominent UV spectral lines are determined in the following manner. A wal 1 -stabi 1 i zed steady-state arc source

is operated at about 12,000 K in argon with small admixtures of oxygen, nitrogen, and carbon dioxide. The admixtures produce very strong emission for

their principal resonance lines, which all happen to lie conveniently in the

far ultraviolet, spanning a wavelength range from 115 nm to 250 nm. These

extremely strong resonance lines reach the blackbody intensity limits even at

very small concentrations of the elements in the plasma, i.e., they become

"optically thick." More precisely, the central regions of these line profiles

reach, for a small wavelength range of a few hundredths of a nanometer, the

intensity a blackbody would have at the arc temperature. Thus, the arc represents, for a few narrow wavelength bands in the UV spectrum, a very high temperature blackbody. To utilize this arc as a standard source, the temperature

must be accurately determined using standard plasma spectroscopic techniques

including absolute radiometry in the visible region of the spectrum. Subsequently, using Planck's radiation law, absolute intensities are established

for these narrow spectral bands and are utilized as calibration points. Calibrations at other wavelengths are usually found by spectral interpolation.

This technique has been used in a number of experiments and has been found to

be reliable, with uncertainties ranging from about 10% at 250 nm to about 25%

at 115 nm. For general use, however, it has the significant deficiency that

large areas of the far UV have to be covered by interpolation since very few

blackbody limited lines are found outside the ranges 115 nm to 126 nm and

146 nm to 175 nm. Since we are primarily interested in the range 115 nm to

124 nm, this is no great handicap here.

The accuracy of the blackbody line method depends critically on the meas- urement of the arc temperature, which is applied in the Planck function to

determine the intensity of the blackbody ceilings of the optically thick res- onance lines. It has been estimated that no single spectroscopic method is

sufficient to determine the temperature of such a plasma with small O2 , N2

,

and CO2 additions to within an uncertainty of ±2%.i3 This represents an un- certainty in the blackbody ceiling of about 10% at 250 nm and about 25% at

115 nm as was mentioned above. To reduce this uncertainty, it was recommended

that several methods be applied, and the results averaged. However, a single

method, which is thought to be superior, was adopted here to determine the

blackbody temperature.

The blackbody ceiling of the C I emission line at 247.9 nm is measured

with a UV-cal ibrated tungsten strip lamp. The uncertainty in the spectral

radiance of the strip lamps calibrated by NBS at this wavelength is about

2%. The peak of the 247.9 nm C I line is actually calibrated in two steps:

-21-
first, the continuum emitted by the arc mixture at 250 nm is measured and

calibrated at low spectral resolution using the tungsten strip lamp and the

air-path predisperser and monochromator ; and second, the peak of the carbon

line is calibrated with respect to this continuum using the VUV instrument at

high resolution. The difference in the VUV system efficiencies between 247.9

nm and 250 nm is minimal but is measured with the hydrogen arc and taken into

account. Care is also taken to ensure that the contribution of the line wing

at 250 nm in the low resolution air system is negligible. For a 4.7 mm diameter arc at 60 A the axis temperature of the argon plasma with admixtures of

N, C, 0, and H was measured to be 11,900 K. A temperature uncertainty of ±160

K is estimated due to an estimated rms uncertainty of about 6% in the blackbody intensity determination at 247.9 nm. This results in an uncertainty of

10% in the blackbody line radiances between 115 nm and 140 nm.

Besides the temperature of the plasma, the character of the blackbody

limited lines is also critical. For example, if the boundary layers between

the mixed plasma and the pure argon buffer are too thick, the radiation transfer may be complicated by the density and temperature gradients through the

layers. Such conditions are more the rule than exception and are easily detected at high resolution as structure in the line peaks. However, by appropriate flow rate adjustments, this effect can be minimized so that the blackbody plateaus are essentially flat- topped. Referring to Fig. 6, we see that

for a high power hydrogen arc the calibrated spectral radiances obtained using

blackbody limited lines are consistent with the corresponding calculated

quantities, as was noted above in Section III-A,la.

B. Secondary Standards

1 . Argon Mi ni -Arc

a. Physical Principles

After the development of the hydrogen arc as a primary standard of radiance in the VUV, it was realized that a simpler, portable, more easily operated, and lower powered transfer standard would be of great value. The hydrogen arc is complicated and difficult to operate, and requires a massive

1200 V, 110 A dc power source. A secondary standard which could be compared

with our primary standard and then transported to the user's laboratory was recognized as being essential.

The most widely used transfer standard at that time was the commercially

available deuterium lamp. Although the deuterium lamp has attractive properties such as its relatively strong continuum, low power requirements, and

small size, it also has limitations. First, its region of applicability as a radiometric standard is restricted to wavelengths above 165 nm due to the

presence of a many-line molecular deuterium band system below 165 nm. Second,

the lamp exhibits a variability related to the positioning of the discharge on the electrodes following each ignition. Finally, the deuterium lamp has aging

characteristics which are not well known. At the least, this means that the

lamp must be frequently recalibrated to ensure accuracy.

-22-
-23-
To meet the lack of adequate transfer standards in the far UV, the argon

mini-arc was developed at NBS.^^ This source can be conveniently applied

as a radiometric transfer standard without the limitations of the deuterium

lamp. The wavelength range of the argon mini-arc overlaps the lower range of

the tungsten strip lamp in the near UV and extends beyond the short wavelength

limit of the low pressure deuterium lamp at 165 nm. The mini -arc was designed

to fulfill, insofar as possible, the following goals: (1) an intense linefree continuous spectrum between 115 nm and 330 nm; (2) stability and reproducibility over many hours of operation; (3) light source and power supply

both portable; (4) uniform output over a large solid angle; (5) radiant power

output adjustable over a range of several decades; and (6) simple alignment

and operation. We now proceed to discussions of the arc construction and

operating characteristics.

b. Description of the Arc

Figure 11 is a photograph of the arc source. The model described here

was designed specifically for use as a secondary radiance standard and is

different in some respects from arc sources previously designed for other

purposes. It was developed after experimentation as the simplest model which

meets the requirements listed above. Argon was chosen as the operating gas

because of its suitability in providing a stable discharge and an intense,

line-free UV continuum with minimal power requirements. In the photograph the

arc is mated to a monochromator with a stainless steel bellows. Water cooling

connections and a triple-tube manifold for supplying argon are also shown.

The two vertical threaded posts on the top of the arc are the electrical con- nections to the anode and cathode. The arc is mounted on an adjustable table

which allows precise translation and rotation about two axes. All sources

utilized in the NBS VUV radiometry program are mounted on this type of table.

As shown in Fig. 12, which is drawn to scale, the arc source is constructed essentially of five copper plates separated by silicone rubber insulating

rings and clamped together to form the device. The central plate forms a channel which guides and constricts the discharge. The plates adjacent to the

central plate contain the anode, a 3.2 mm diam thoriated tungsten rod pressed

into its plate, and the cathode, also of tungsten but mounted so that it can be moved into and out of the arc channel. The cathode was made adjustable for ease in igniting the arc. All five plates are water-cooled with the water

flowing inside each piece through holes drilled in the sides. These holes are seen in the photograph but not in the diagram. The arc constricting section

is 6.3 mm thick with a 4.0 mm diameter hole. Diameters smaller than this were considered unacceptable since they caused a higher plasma temperature and

significant Ar II line emission in the wavelength region of interest. Larger

diameters were rejected since the radiant power for a given current decreased

considerably. The arc plasma is observed end-on through the holes in the

electrode pieces, and the radiation calibrated is that emitted along the arc axis and emerging through the magnesium fluoride window on the cathode side.

This window is set back to avoid possible contamination from the discharge.

The gas purity necessary to maintain arc stability and reproducibility of the

continuum emission is insured by operating with a continuous flow of argon.

Maintaining a high degree of argon purity within the arc chamber also minimizes the radiant power of atomic resonance lines in the spectrum which arise

from small (ppm) concentrations of oxygen, nitrogen, carbon, and hydrogen.

-24-

These elements are present due to air and water vapor in the arc chamber and

gas handling system. The positions of the gas inlet and outlet ports are chosen to keep all sections of the arc chamber thoroughly purged. The argon

is admitted to the inlet ports through plastic tubing connected to a flowmeter. Under the flow rate used here, the pressure in the chamber is un- changed from the ambient atmospheric pressure. The arc may be operated most

conveniently with the outlet ports open and the data given below used- to ac- count for local and temporal differences in atmospheric pressure, if necessary.

Optical alignment can be performed by sighting with a telescope or laser

beam down the arc axis. The distance for proper focus should be measured to

the center of the middle copper piece.

The arc discharge is initiated by applying voltage between the electrodes

and then inserting a tungsten rod, which is externally connected to the anode

potential, until it touches the cathode protruding into the arc channel. The

discharge is transferred from the tip of the rod to the anode as the rod is

withdrawn. Finally, the cathode is withdrawn from the channel as shown in

Fig. 12, Power for the discharge is furnished by a current regulated supply,

the size of which is determined by the current required. For example, the

power supply used in some of our experiments was a 1,2 kW, high efficiency

switching regulator weighing only 10 kg,. For starting the arc without difficulty, it was found necessary to use a ballast resistor of about 0,5 o, in

series and to apply a potential of at least 40 V, The arc can be started at

currents above 10 A, and after ignition the resistor may be shorted out if

desired. For currents above 10 A the voltage drop across the arc is nearly

constant at 28 V,

In developing the arc model presented here, various designs were tested.

Arcs were constructed with different dimensions and numbers of confining

plates; several were provided with a window for measuring the radiation emitted side-on, i.e., perpendicular to the discharge axis. Upon observing the

discharge side-on midway between the electrodes, a feature in the continuous

spectrum was seen which is not apparent looking end-on. This was a broad peak

in the continuum centered at 130 nm, with a halfwidth of about 10 nm. By

scanning across the arc diameter, this radiation was observed to be emitted

most strongly from a region well away from the arc axis, where the temperature

is much lower than its value at the axis. This radiation arises from a molecular argon conti nuum. ^ Its spatial distribution and intensity were found

to be strongly dependent upon gas flow rates. For this reason, it was decided

that the radiation emitted side-on was not appropriate for application as a reliable standard and that the radiation emitted end-on would be the quantity

to be investigated further and finally calibrated.

Wal 1 -stabi 1 i zed arcs employed for end-on measurements usually are con- structed of many constricting plates or disks, in order to obtain a nearly

homogeneous plasma extending along the axis to the boundary regions near the

electrodes at each end. These boundary regions emit a negligible part of the

radiation emerging along the arc axis. The hydrogen and blackbody arcs which

we use for primary radiance calibrations are necessarily of this type, since

the lengths of the emitting plasma columns must be well defined. In such

cases, however, the solid angle over which uniform radiation can be observed

-26-
10" C

1 1 > r

110 150 190 230 270 310 350

X [nm]

Figure 13. Spectral radiance as a function of wavelength for

a mini-arc with an arc plate diameter of 4.0 mm operated at two different currents: 50.0 A and

25.0 A.

-27-
OO-^THCOCOOOOOOOCDOOOiCiOO-^OC^incNCM^OOO^ OOO^O^OOt>t>tDiOT}<TfC<lTHO^OO«OTj<<MOl>iOCDTt(MOlCOCO

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St

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rHOaCOOJi-tCCCD'^CO

ooc3io^ooi>coTfcoc^oooi>iO"^c^oo6u:jcooa>tDTtc<io*^

B O lO O lO O lO O lO O O lO O lO O o" to O lO o" lO o" LO O lO o

CO<N(Ni-HrH00050iOOOOl>t>COC£>iOiO'^TtcOCO(N(Mi-iT-(00

(:ocococococococ<^(Nc^<Nc^c^c^(NC<^(NC^c^(N(^^c^(N(^^(^^(N(^^

-28-
is snail (f/200 is used for calibrations with the hydrogen and blackbody arcs)

and not convenient for the calibration of many optical systems. For a transfer standard, on the other hand, the arc length does not need to be precisely

known as long as it is a reproducible quantity. Thus, in order to provide a much greater solid angle, the argon mini-arc was made with only one constricting plate. Measurements discussed below show that this construction provides

a rather large angular beam of radiation (f/10) having a practically uniform

intensity distribution. This should be considered an advantage even if one does not require such a large solid angle, since having the larger available

angle makes the angular alignment of the mini-arc less critical.

c. Spectrum

Figure 13 illustrates the spectral radiance of the argon mini -arc light source for two arc currents: 25.0 A and 50.0 A. The continuous spectrum of

the argon arc arises primarily from atomic recombination radiation. Near the

short wavelength end the molecular continuum mentioned above contributes to some degree, and below 125 nm the increased output is from the wings of the

argon resonance lines at 106.7 nm and 104.8 nm. Below the MgF2 window cutoff, one should expect to have strong emission on the wings of the resonance lines

with complete absorption at their centers. Below 80 nm there should be com- plete resonance continuum absorption by ground state argon atoms.

The spectral radiance was determined by direct comparison to the NBS

wal 1 -stabi 1 i zed hydrogen arc^ between 130 nm and 330 nm and to a plasma blackbody line radiator below 130 nm.^'^'^^ Data were taken with a spectral reso- lution of 0.01 nm. The temperature of the blackbody line radiator was determined to be 11,800 K ± 100 K by directly measuring the blackbody ceilings at

193 nm, 174 nm, 165 nm, 149 nm, and 146 nm, using the hydrogen arc as a primary standard of spectral radiance. With this temperature the radiant power

of other blackbody limited lines emitted below 130 nm could then be calculated. For the calibration of longer wavelengths (>210 nm) , the second-order

spectrum from shorter wavelengths was eliminated by purging the small volume

between the arc and the spectrometer with oxygen. Below 210 nm, argon was used as the purge gas.

The spectral radiance values illustrated in Fig. 13 and listed in

Table II apply to the plasma radiation emitted along the axis of the mini-arc

which is imaged on a 0.30 mm diameter aperture (magnification =1) at a solid

angle f/200. The basic uncertainty in the spectral radiance is ±5% above

140 nm and ±10% below 140 nm due to uncertainties in the primary radiometric

standards. Also included in Table II for reference purposes is the measured

transmission of the MgFp window used on the mini-arc. If one wishes to know

the spectral radiance of the plasma itself, the figures in the table must be

divided by the transmission. Known systematic deviations or uncertainties in

the radiant power due to different operating or imaging conditions are described in subsequent sections.

There are no Ar I lines in the spectrum between 114 nm and 330 nm. Below

200 nm there are several narrow Ar II lines and atomic nitrogen, carbon, and

oxygen lines in the mini-arc spectrum due to air impurities in the argon gas

(99.999% pure). In addition the hydrogen Lyman alpha line at 121.6 nm is

-29-
1VN9IS 1VN9IS

O) t—

I

t- o

•r—

-30-
Table III. Survey of Emission Lines

Appearing in Spectrum of Mini-arc

Radiance relative

X(nm) Identification to continuum^

115.22 0 I 1

116.79 N I 1

117.69 N I 0.3

118.94 C I 0.3

119.38 C I 1

119.99 N I 30

121.57 H I 100

124.33 N I 5

126.13 C I 2

127.75 C I 5

128.98 C I 1

130.22 O I 15

130.55 O I 10

131.07 XT T N I

-11

131.95 XT T N I 0.5

loz.9o C I 4

loo. DO C II 8

1 O C CO loo. DO C I 0.2

141. ly XT T JN I 0.7

i4o.iy I U. 1

\~> I U. 1

14d. oo ^ I 1

14d. / 0

r* T

I n 1 U. 1

1 /I 8 1 fi 14o. 10 n o 14y.ZD IN I A4

t AQ Al i4y .4 / M T IN I D

loo. lU L/ I 4

10/. OU Aw T T u. z 160.04 Ar II A o

U. Z

160.35 Ar II 0.1

160.65 Ar II 0.05

165.72 Cl 2

174.27 N I 1

174.53 N I 0.5

175.19 C I 0.1

187.31 Ar II 0.05

188.90 Aril 0.1

193.09 C I 2

247.86 C I 0.05

^ In column three are listed line radiances relative to the

continuum, measured with 0.25-nm spectral resolution.

-31-
present due to trace quantities of water vapor throughout the system. These

impurity lines can be seen in Fig. 14 which is a photoelectric scan of the

spectrum between 115 nm and 320 nm. The halfwidths of the lines are on the

order of 0.01 nm, and the lines are well separated. Their presence has no significant influence on the ability to make absolute continuum measurements

except when extremely coarse wavelength resolution is used, as would be the

case, for example, if the monochromator were replaced by relatively wideband

filters. All lines observed in the spectrum are listed in Table III, From

this list one may readily determine if, for a given wavelength and bandpass,

one is free of lines. This precaution is necessary since the amount of trace

impurities in the arc and the corresponding line intensities may vary somewhat

from one system to another.

Figure 15 compares the spectrum of the mini-arc with several other light sources which have been applied as spectral radiance standards. Included in

this figure is a representation of the spectra of the two primary standards,

the hydrogen wal 1 -stabi 1 i zed arc and the blackbody line radiator, which were

used to calibrate the mini-arc spectrum. The figure clearly illustrates the

main advantages of the new transfer source. The mini-arc has substantially

larger output and longer UV wavelength range than either the deuterium lamp or the tungsten strip lamp. Also, scattered visible light is not a significant

factor, as it is when a tungsten lamp is used as a UV light source. The visible light from the argon arc is of the same order of magnitude as the near UV

light, and there are less than 2 orders of magnitude difference between the

radiance at 110 nm and at 330 nm,

2, Argon Maxi Arc

A recent advance in the arc standards program at NBS is the development

of the argon maxi-arc. This source is essentially a high powered version of

the mini-arc with three or four arc confining plates instead of one and power

ratings of 5-10 kw instead of 1,5 kw. The maxi- arc with three plates has an irradiance approximately thirty times that of a mini-arc and was designed to

enable calibrations to be performed at a level comparable to the solar irradi- ance in the near UV (250-350 nm) , The discussion of the mini-arc given above

generally applies also to maxi -arcs except for the powers and radiances.

Figure 15 gives a comparison of the spectral radiance of the maxi-arc with

that of several other UV primary and transfer standard sources. Several of

these arcs have been supplied for use in calibration of space experiments,

3, Deuterium Arc Lamp

a. Introduction

A source which has advantageous properties as a radiometric standard and

is quite convenient to use is the molecular deuterium arc lamp,i8 xhis lamp

has considerable UV radiant power, is light and compact, is low-powered

(30 W) , and with maximum radiant power at 190 nm has a very favorable ratio of

UV to visible radiant power. This last fact is important in avoiding systematic errors due to visible scattered light effects when making measurements in

the UV. Its disadvantages are that it is restricted to wavelengths above

165 nm due to the presence of a many-line molecular band system below 165 nm;

-32-
10-

10

10^

10

-4

4'

^-"l"" "^IISOOK BLACKBODY

BLACKBODY LIMITED LINES

FROM 11800K ARC PLASMA

U

Ly a

NBS HYDROGEN ARC

ARGON MAXI ARC

ARGON MINI ARC

D2 LAMP

TUNGSTEN STRIP/ LAMP

110 150 190 230 270 310 350

X [nm]

Figure 15. Comparison of the spectral radiance of several UV

primary and transfer standard sources.

-33-
-t->

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it may exhibit a variability in the positioning of the discharge on the electrodes following each ignition; and its aging characteristics are not well

known. On balance, however, the advantages of the deuterium lamp outweigh its

disadvantages making it an essential component of any program of UV radiometry. The following discussion will describe the calibration of the deuterium

lamp for spectral irradiance in the range 167-350 nm.

Discussion of the spectral radiance calibration of the deuterium lamp is

not presented here since it is not a standard calibration service offered by

NBS. However, this type of calibration can be requested under the category

"Special Tests of Radiometric Devices in the Near and Vacuum Ultraviolet."

b. Description and Operation of the Lamp

A schematic illustrating the operation of the deuterium lamp is shown in

Fig. 16. In order to start the lamp, the cathode coil is first heated for 5 sec by a dc power supply (10 V at 0.8 A) in order to provide some free electrons which facilitate initiating the discharge. When a voltage of about

400 V is applied to the lamp, an arc forms between cathode and anode in the

general form of an L. Most of the UV light is generated at the constricting

aperture (1 mm diameter in our case) located in front of the anode. The main

dc power supply is a 500 V, 300-mA constant current supply, with 0.1% current

regulation. A ballast resistor (Iks^, 100 W) is used in the anode circuit

because most power supplies cannot react fast enough to maintain a constant

arc. After the arc is started, the voltage across the arc drops to about

100 V. At this point the heater current is switched off, and the lamp output

stabilizes in 20 minutes or less. If the lamp is switched off, it should be

allowed to cool back to room temperature before restarting.

Because of variations in fabrication from one deuterium lamp to another,

optical alignment of the deuterium lamp with respect to one of its physical

characteristics does not necessarily result in a lamp whose radiant output is

least sensitive to alignment uncertainties. Therefore, optical alignment of

lamps calibrated at NBS is achieved with respect to a precision bipost base in

which each lamp has been individually positioned and potted to yield maximum

irradiance. Through this technique any number of deuterium lamps can be put

into the system in a reproducible way such that uncertainties in the spectral

irradiance due to small variations in alignment are minimized. Also, since

the bipost base is identical with the one used to mount NBS calibrated

tungsten-quartz-halogen lamp standards, these potted deuterium lamps are compatible with calibration systems designed to accommodate the quartz-halogen

lamp. Figures 17 and 18 show a line drawing of the deuterium lamp and a photograph of the deuterium lamp and the quartz-halogen lamp emphasizing their

common bipost base. The orientation procedure is as follows. First, the

bipost base, mounted in a kinematic support, 20 is positioned so that the optical axis of the spectroradiometer optics forms a perpendicular with the plane

defined by the front surfaces of the posts and passes midway between the

posts, 9.5 cm above the bottom of the posts. This adjustment was made precisely and quickly with the use of an auxiliary alignment jig made by sealing

a piece of glass plate onto the posts of an identical base mounted in the

kinematic support; an etched cross on the glass serves as a target for a laser

beam, and the mount is adjusted so that the laser beam reflects back on itself. With the position of the base so determined, the deuterium lamp is

-35-
3 cm

9.5 cm

Figure 17. Line drawing of the deuterium arc lamp standard,

-35-
C

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-37-
placed in the base so that the laser beam passes through the center of the

1 mm diameter discharge-constricting aperture inside the lamp. The pitch and

yaw of the lamp are then adjusted about the center of the 1 mm lamp aperture

for the condition giving maximum spectral irradiance at 250 nm, as measured

using a 0.3 mm diameter field stop and diffusing window as the monochromator

entrance slit optics. The lamp is then potted in the base. It can now be

positioned in a precise, reproducible way anywhere in the optical system by

properly adjusting the orientation of the kinematic support through use of

laser retrorefl ecti on from the alignment jig and then substituting the potted

deuterium lamp in place of the alignment jig.

The deuterium arc lamp with quartz window was calibrated for spectral

irradiance using the method of collimating apertures. »2i j22 jh-js method is

described in detail below in Sec, IV-B. Also, the spectral irradiances of

several other sources were determined using this method, and the results along

with two reference spectra are shown in Fig. 19. Further aspects of the deuterium lamp as an irradiance standard are covered in Sec. V-C.

C. Measurement Apparatus

1 . Monochromators

A key element of the vacuum ultraviolet portable source radiometry program at NBS is the set of three monochromators on which all spectroradi ometric

measurements are made. The monochromators are commercial models which until

recently were all available as stock items. Two of the instruments operate in vacuum with the third operating in air.

Each of the units has been generally dedicated to a specific type of

measurement. The 3 m focal length vacuum instrument is set up for measurements of spectral radiance, whereas the 1 m Seya-Namioka vacuum and 1 m ai r instruments are usually reserved for measurements of spectral irradiance. The

3 m monochromator is equipped with a 1200 L/mm grating with a ruled area of

65 mm X 150 mm. Its first order reciprocal linear dispersion is 0,278 nm/mm

with wavelength coverage from central image to 400 nm photographically and

from central image to 382,5 nm photoel ectri cal ly , The use of a magnesium

fluoride entrance window limits the low-wavelength cutoff to about 115 nm. The bilaterally adjustable slits have a continuous range of widths from 10 ym

to 2 mm, and a synchronous motor driven gear box provides eleven scanning

speeds from 0,025 nm/min to 50 nm/min.

The 1 m focal length Seya-Namioka vacuum monochromator has a 1200 L/mm

grating with a ruled area of 30 mm x 50 mm. Its first order reciprocal linear

dispersion is 0,83 nm/mm with a wavelength coverage from central image to

300 nm. With a magnesium fluoride entrance window the low-wavelength cutoff

here is also about 115 nm. The bilateral entrance and exit slits are continuously adjustable from 10 ym to 2 mm, and twelve synchronous scanning speeds

from 0,05 nm/min to 200 nm/min are provided. The 1 m ai r monochromator has a 1200 L/mm grating with a ruled area of 102 mm x 102 mm. Its first order reciprocal linear dispersion is 0,833 nm/mm with a wavelength range from 200 nm to

1300 nm. The bilateral slits are adjustable from 5 pm to 2 mm, and the twelve

scanning speeds range from 0,05 nm/min to 200 nm/min. To reduce scattered

-38-
10

0 50 100 150 200 250 300 350

A (nm)

Figure 19. Absolute spectral irradiance measured as a function of wavelength at a distance of 50 cm from the field aperture for five different continuum sources with the

indicated power requirements. The spectrum of the lamp with MgFj window below

170 nm, measured for a 1 nm bandpass, is a pseudo-continuum made up of blended lines.

Shown for comparison purposes are spectra of the 250 MeV National Bureau of Standards

synchrotron radiation facility, for a beam current of 10 mA and a field aperture distance of 19 m, and the tungsten-quartz-halogen lamp, for the standard 50-cm distance.

-39-
light, a 0.25 m focal length Ebert monochromator has been mated as a predisperser to the 1 m ai r monochromator. The Ebert instrument is equipped with

a pair of interchangeable gratings with ruled areas of 64 mm x 64 mm. One

grating with a ruling of 2365 L/mm is blazed at 300 nm for use at shorter

wavelengths, and the other has 1180 L/mm with a blaze at 600 nm for longer

wavelength use. The reciprocal linear dispersions are 1.65 nm/mm and 3.3

nm/mm, respectively. For irradiance measurements an integrating sphere with a 0.6 cm diameter entrance aperture is used as the diffusing element at the

entrance to the pre-di sperser. The inner surface of the sphere is coated with

barium sulfate. For radiance measurements the integrating sphere is replaced

with a 0.3 mm diameter entrance aperture.

2. System Components

a. Photomul tipl iers

Photomul ti pi i ers are used as detectors on our three monochromators . The

two vacuum instruments are equipped with VUV-sensiti ve solar blind tubes,

while the tube on the air monochromator has a UV-through-visible sensitivity.

The solar blind tubes are operated at voltages such that the anode current

does not exceed 1 nA. This is done to eliminate possible hysteresis effects

which have been observed at higher currents. Scattered light is removed from

the air system by the use of the previously described double monochromator

setup. With the vacuum monochromators the scattered light is strongly reduced

through the use of the solar blind photomultipliers.

b. Detection Electronics

The detection systems utilize picoammeters with ranges from 10"2 A to

3 X 10"^^ A full scale as amplifiers, with the output of each picoammeter

being fed simultaneously into a strip chart recorder and either a data logger

or a voltage-to-frequency converter. The output of the converter is registered on an electronic counter, whereas the output of the data logger is processed by a computer. The strip chart recorder serves to monitor the data as

it is being taken. The picoammeter ranges are related by factors of 3 and 10,

and they are calibrated relative to one another using a "bootstrap" technique

with a constant current source.

c. Specifications

The solar blind photomultipliers are end-on 18-dynode tubes having cesium

telluride photocathodes and lithium fluoride windows, while the tube on the

air monochromator (1P28) is side-on with nine dynodes, a cesium-antimony

photocathode, and a UV transmitting glass envelope. The solar blind tubes are used over the range 115 to 360 nm, and the 1P28 is used over the range 195 to

750 nm. The solar blind tubes are potted for operation in vacuum, where one

is used on the 3 m monochromator. On the Seya instrument the window on the

solar blind tube serves as a vacuum interface.

The photomultipliers are operated with power supplies which show no more

than 0.0025% variation in output voltage for combined line and load variations

within the operating range 0 to 10mA at constant ambient temperature. The

-40-
temperature variation in voltage is within ±50 ppm/°C in the operating range 0

to 50°C after a 30 minute warmup, and the long-term drift in voltage is less

than 0.01%/hr or 0.03%/24-hr at constant input line voltage, load, and ambient

temperature, also after a 30 minute warmup. Finally, the output ripple is

less than 15 mV peak to peak at frequencies from 5 Hz to 5 MHz.

The accuracies of the picoammeters are ±2% of full scale on the 10~2 to

10-9 A ranges and ±4% of full scale on the 3 x lO'^ to 3 x 10-13 A ranges.

Zero drifts are less than 0.5% of full scale per week plus 0.02% per °C on the

10-2 to 10-12 1^ ranges and 1.5% per week plus 0.06% per °C on the 3 x 10-13 A

range, with source potentials greater than 1 V and after a warmup of 10 minutes. The amplifier outputs are ±1 V for full-scale meter deflection, with

noise less than 1% of full scale peak-to-peak on the 10"3 to 10"ii A ranges.

The noise increases to 5% peak-to-peak on the 3 x 10-13 /\ range. Since our methods of calibration involve ratios of signals measured with a common picoammeter, we profit from the phenomenon that the systematic errors in the

ratios are smaller than the systematic errors in the measured signals. This

is shown by the following expression,

where and are the measured signals of the unknown and standard sources and ASj^ and ASf^ are the corresponding systematic errors. If, for example,

and Sf^ were equal and the measurements were made on the same picoammeter

range, AS^ and ASf^ would be equal and the systematic error in the ratio of

signals would be zero.

The strip chart recorder has an accuracy of 0.2% of full scale with re- settability and linearity each 0,1% of full scale. Its full scale response

time is 0,5 sec. The data logger has a range of 2 V with a resolution of

1 mV, Its accuracy is ±0,05% of the reading or ±1 count full scale, and its

linearity is 0,05% of the reading. The voltage-to-frequency converter has a linearity referred to a straight line between zero and the full scale calibration point of ±0.02% of full scale. Its stability at constant line voltage

and temperature is ±0.02% of full, scale per day, and its line voltage effect

is less than ±0.006% of full scale for a ±10% line voltage change. Its temperature coefficient is ±0.001% of reading and ±0.0005% of full scale per °C in

the range 10 to 50 °C.

-41-
o

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-42-
IV. Calibration Methods

A. Radiance Calibrations

1. Introduction

Spectral radiance, L;^ , is defined in the Introduction as the radiant

power emitted by a source per area per solid angle per wavelength interval:

L;^ = [watts cm'^ sr"i nm"i], A calibration of radiance is performed by

directly comparing the source to be calibrated with a standard radiance

source. An optical system is used to limit the geometrical quantities, i.e., area and solid angle and to direct the radiation to a detector. A mono- chromator selects the wavelength band desired. First, one source is placed in

position and signals are measured at the various wavelengths. This source is

then replaced by the second source and the measurements are repeated. The

radiance of the source to be calibrated is then obtained from the ratio of the

signals times the radiance of the standard source. In general the signal

obtained from a source is

S (amps) = ej-^) . ''f )

• A(cm2) . fi(sr) . AX(nm), (2)

watt cm sr nm

where A is the area, n the solid angle, ax the wavelength bandpass,

L;^ the spectral radiance, and e\ the efficiency of the detection system. If

subscript R refers to the standard source and subscript x refers to the one to

be calibrated, then

Sr"^^,R • \ • "r • ^^R • ^

^

If A, and AX are identical for each source, then

Sx. -X,x " ^Sp^ • 4,R*

2. Hydrogen Arc

a. Setup

Figure 20 shows the setup for the measurement of signals from the hydrogen arc, our principal primary radiance source. The monochromator used is a 3

meter vacuum instrument with a grating of 1200 L/mm, The hydrogen arc is a standard spectral radiance source for 124 < x < 360 nm, if observed as shown,

along its axis. The mirror images the arc with magnification M = 1 onto the

entrance aperture of the monochromator. The aperture A, placed just outside

the entrance slit of the monochromator, is 0.15 mm in diameter and limits the area observed to a circle of this diameter. The distance from the center of

the source to the mirror is 150 cm. Aperture B, 0.63 cm in diameter and located 125 cm from the source center, limits the solid angle to 2.0 x 10"^ stera-

-43-
dians or f/200. These are rather small geometrical limits, but are necessary

to limit the variation in radiance to 1% within the observed optical beam,

b. System Alignment

Before placing the arc in position, the position of the telescope is

adjusted so that its crosshairs are coincident with both the vacuum window

and aperture B. The mirror is then adjusted so that aperture A is coincident

with the telescope crosshairs. The arc is next placed in position and adjusted so that its axis is coincident with the optical beam. The arc is ignited

and operated as discussed in Sec. III-A, lb. After operating the arc in

hydrogen at about 40 A, the mirror is adjusted further. This is done by observing a signal with the monochromator set at 350 nm and carefully rotating

the mirror to maximize this signal. This insures that the arc alignment is

correct.

c. Arc Current

The arc current is increased while observing the signal at 350 nm. At

about 100 A the signal reaches a maximum and decreases with further increase

in current. To avoid saturation of the photomul ti pi i er (PM) tube, the maximum

signal should be limited to 1 mA by reducing the PM voltage if necessary.

This value of current should be recorded since the current values corres- ponding to the maximum signal depend upon wavelength. In this manner a set of

current values for maximum signal are obtained as a function of wavelength for

use during the calibration.

These settings for arc current are determined prior to measuring any

signals from the arc, because radiation from the arc gradually degrades the

optical system, especially the vacuum window, by polymerization of condensed

hydrocarbon molecules. Once measurements are begun, therefore, the exposure

time is limited to only that necessary to obtain a good measurement at each

wavelength. This is done by using a shutter to block the beam while measurement is not actually in progress, as when moving from one wavelength to

another. In fact even when the exposure time is limited, some change in signal may be observed when a measurement at a given v^avelength is repeated during a run. This change is a function of wavelength, with the shortest wave- lengths being most affected, and thus contributes to the uncertainty of the

calibration.

d. Dark Current and Scattered Light

The signal from the arc increases with increasing wavelength over our range of operation. To cover our complete wavelength region, different ranges

of the picoammeter must be used. For each range, the signal with the shutter

closed is taken, and this "dark current" is to be subtracted from the detected

signal at each wavelength. Also necessary to be subtracted is the portion of

the signal due to scattered light. Scattered light is the part of the signal

which comes from radiation at wavelengths other than the wavelength being

properly diffracted by the grating, i.e., other than that wavelength at which

the monochromator is set. The scattered light signal may be evaluated by

scanning the monochromator grating to cover the range of wavelengths near

-44-
110 nm. As the grating is scanned toward shorter wavelengths, the signal

gradually decreases and finally becomes constant at a value greater than the

dark current. Since no radiation is transmitted at these wavelengths below

about 113 nm (the MgF2 window does not transmit radiation beyond this wave- length), the signal is the sum of the scattered light from longer wavelengths

and the dark current. It is subtracted from the detected signal to obtain the

arc signal. At settings of the monochromator which transmit much longer wave- lengths, the grating is at a different position, and the scattered light may

be somewhat different from its value with the grating set near 110 nm. However, for these longer wavelengths the signal level is much higher than for

the short wavelengths, and therefore an error in estimating the scattered

light will have little effect.

For wavelengths X > 230 nm a contribution to the signal from second order

radiation is possible. To avoid second order radiation, a quartz filter is

included in the optical system for 200 < x < 320 nm, and a glass filter is

used for X > 320 nm. In actuality, the second order contribution to the signal is extremely small.

In the wavelength range near 130 nm the continuum radiation from the

hydrogen arc is modified somewhat by both molecular emission lines and absorption lines. The absorption occurs in the cooler transition zones near the

ends of the arc column along the axis. To get a good measurement of the con- tinuum level in this wavelength range, the monochromator is scanned and the

signal observed with a chart recorder to avoid wavelengths at which lines occur.

3. Argon Arc

The hydrogen arc primary standard is generally used to calibrate an argon

arc. After measuring its signals, the hydrogen arc is removed and replaced

with the arc to be calibrated. The telescope is used to align the argon arc,

and once it is operating, a fine adjustment in position is accomplished by

translating the arc both horizontally and vertically to maximize the signal.

As with the hydrogen arc, there is a volume in between the arc source and the vacuum window. This volume is purged with argon at a flow rate of 5 liters/-

min. The purging must continue for at least 10 min before measurements are begun in order to sufficiently displace the air from this region. This flow

rate is continued during the entire calibration. By monitoring the signal at

170 nm where air absorption is strong, the rise in signal during purging may

be observed, and the time at which essentially all air is displaced can be

determined. There are two cases to consider: (1) if the arc to be calibrated

contains a window, the purge region is isolated from the arc, and any purge

rate which is sufficient to maintain the region free of air may be used; and

(2) if the arc to be calibrated is one without a window, the flow in the

"purge" region is not independent of the arc, and the same flow arrangement

must be used whenever the arc is used as a secondary standard. This is

necessary because the flow rate in the purge region can have an effect on the

arc operation and hence on the radiance of the arc. Once the argon arc is

operating and the purge region is clear of all air, signals are measured at

the various wavelengths as with the hydrogen arc. In addition to the wave-

-45-
lengths covered by the hydrogen arc, signals are also measured at shorter

wavelengths extending to 115 nm. In this region, however, there are numerous

lines in the spectrum from minute air impurities. These lines are typically

resonance lines and are extremely strong even at very low concentrations. The

wavelengths at which the continuum is measured are selected to avoid interference from these relatively strong lines. Table 1 of App. F-1 lists wave- lengths typically included. Also, in order to avoid interference from the

lines, the wavelength bandpass must be no greater than 0.2 nm (700 ym slits on the 3 m monochromator) . The continuum from this short wavelength region is

calibrated using a second primary standard, the blackbody line arc.

4.. Blackbody Line Arc

Another primary standard source, the blackbody line arc, is used to ex- tend the calibration to wavelengths below 124 nm. This is also a wallstabilized arc, operated with argon but including small admixtures of H, 0, C,

N and Kr. These admixture elements furnish spectral lines which can be made

to be optically thick , i.e., they reach a maximum radiance value at the wavelength of the line.

In practice the arc to be calibrated is subjected to measurement and then

replaced by the blackbody line arc. The latter arc is then aligned using the

telescope and initially ignited with pure argon. After setting the current at

40 A and completing a fine adjustment of the arc position, measurements are made at wavelengths adjacent to the positions of the black body lines to be

measured. The admixture elements are next added gradually while the spectrometer is scanned slowly and repeatedly over the respective lines. Prior to

measuring the lines, the spectrometer slits are reduced to 10 ym each, giving

a spectral bandpass of 0.0028 nm. The elements are added by admitting H-

,

CO2 , N2 and Kr at flow rates of a few ml/min. The flow arrangement limits the

admixtures to a volume near the center of the arc. Initially, a line will

increase in both height and width as the admixture flow rate is increased

until the top of the line becomes flattened. Further increase of the admixture increases the line width, but the height remains constant. The flow rate

of the various gases should be adjusted until all lines are flat-topped, but

no more admixtures should be added than necessary. This is because the addition of these gases changes the temperature of the arc slightly, thus affecting the accuracy of this calibration method, which depends upon having a nearly uniform temperature in the region where the lines are emitted. For this

reason, the admixtures are limited to the central part of the arc. If they were present at the end boundaries of the arc column where the temperature is

lower, self absorption would distort the flat tops of the line profiles. In practice it usually proves difficult to achieve a good flat top for each line,

and a compromise must be accepted since adjusting the flow rates to improve

one line may worsen another.

The process of extending the calibration to shorter wavelengths is a "bootstrap" procedure. The lines at the longer wavelengths are first calibrated relative to the hydrogen arc, and each such line furnishes a value of

temperature via the Planck blackbody relation. Using several lines, an average value of arc temperature is obtained. With this value the radiances of

the lines below the range covered by the hydrogen arc can be obtained, thereby

-46-
extending the range of calibration down to 115 nm. By interpolation the calibration is extended to all wavelengths in between the lines. A sufficient

number of lines are available to restrict the interpolations to short intervals.

Since the slits had to be greatly reduced in width to observe the blackbody lines, the line radiances cannot be directly compared to the continuum

level of the argon arc or the hydrogen arc. This comparison has to be done in

steps

:

1. With the arc operating in pure argon, continuum levels at wavelengths

adjacent to the lines are first measured with the same monochromator slit

settings used for the previous two arcs.

2. With 10 ym slit settings, the radiances of the lines are measured

relative to the adjacent continuum levels. This may require changes in PM

voltage to get sufficient signals from the continuum.

3. The gas admixtures are stopped and the change in continuum level is

measured. This will be only a slight change, but it is significant.

From these ratios, the ratio of the blackbody line peaks to the continuum

levels of both the hydrogen and argon arcs may be obtained. The spectral

lines used in determining the temperature of the arc have wavelengths in nm of

193.1, 174.5, 174.3, 165.7, 149.5, 149.3, and 146.3; and the spectral lines

whose radiances are determined have wavelengths in nm of 126.2, 124.3, 120.0,

119.4, 118.9, 117.8, 116.8, 116.5, and 115.2.

5. Tungsten Lamp Standard

In the calibration of radiance one additional standard source, the tungsten strip lamp,^^ is applied. The tungsten lamp is used for two reasons:

(1) to increase the accuracy of the calibration, and (2) to extend the

calibration to longer wavelengths. The wavelength ranges of the hydrogen arc and tungsten lamp overlap in the near UV region, and although the uncertainty

in the radiance of the hydrogen arc is ±5%, tungsten lamps are calibrated for

radiance with an uncertainty of ±2.3% in the near UV. Therefore, the calibration of a source extending to the near UV may be increased in accuracy by

comparing it directly to the tungsten strip lamp. If this is done, a calibration using the hydrogen and blackbody line arcs is considered to furnish only

the relative spectral distribution of the radiance of the unknown source. A

calibration with the tungsten lamp at the long wavelength end of the range of

the hydrogen arc then serves to set the absolute scale of the radiance.

The calibration with a tungsten lamp is carried out on a different

instrument, namely the double monochromator in air. The double monochromator

is necessary because the tungsten lamp is relatively weak in the near UV compared with its output in the visible range. With only a single monochromator

the scattered light level would be too high to perform precise signal measurements in the near UV region. Figure 21 shows a diagram of the double mono- chromator setup. The tungsten lamp is first aligned according to the

procedure described in its calibration report. This is accomplished by using

-47-
-48-
a laser with an alignment device to set the angular orientation. After

ignition, the lamp is translated slightly to precisely adjust the image position on the entrance aperture as specified in the lamp calibration report.

Then the concave mirror between the predisperser and monochromator is adjusted

to give a maximum signal at an arbitrary wavelength setting before measurements are begun. No further adjusting of this mirror should be done during

the run. A 0.3 mm diameter circular aperture is used as the entrance aperture

of the predisperser. Due to the fact that the tungsten lamps are calibrated

using a larger area than that necessary for the arc, mapping of the lamp image

is required to evaluate the slight difference in radiance due to the different

areas used in the measurement. The exit slit of the predisperser and the

entrance slit of the monochromator are set at 2 mm, and the exit slit of the

monochromator is set at 300 ym. These settings give a bandpass of 0.5 nm. The aperture at the focusing mirror gives a beam of f/30. As with the previous setup the sources are aligned in succession, and signals are measured

from each source at the various wavelengths. With the monochromator set at

the wavelength of interest, the predisperser is set to maximize the signal.

Usually the setting of the predisperser is sufficiently insensitive to allow

the wavelength counter to be used to obtain the proper setting. The wave- length counter on the monochromator is calibrated by scanning over known

spectral lines from a mercury source. Signals are usually measured in 10.0 nm increments from 250 to 320 nm. This gives an overlapping range with the hydrogen calibration, and the measurement at each wavelength in this range furnishes an absolute radiance of the unknown source at that wavelength. Although a measurement at one wavelength would in principle suffice to set an absolute scale, the results for several wavelengths are averaged to get a more

accurate result.

As before, the scattered light and dark currents must be determined and

subtracted from the total signals. In this setup scattered light is very

weak; it is determined by measuring a signal at a wavelength beyond the res- ponse of the instrument (x < 180 nm).

6. Calibration of Argon Arcs Relative to an NBS Argon Arc

For most calibrations the hydrogen arc and blackbody line arc are not

employed, but rather calibrations are performed using a mini-arc (the NBS

argon arc) as a transfer standard.

The transfer standard is employed since calibrations with the hydrogen

and blackbody arcs are much more difficult and time consuming than calibrations performed relative to an argon arc. Moreover, the comparison of two

similar sources (two argon arcs) can be accomplished in reasonable time with

less error than is involved in a calibration involving the hydrogen and blackbody line arcs. Therefore only a slight decrease in accuracy results from

employing the transfer standard. The calibration of one argon arc relative to

another proceeds similarly to the calibration of an argon arc relative to the

hydrogen arc. In this case the apertures A and B in Fig. 20 are changed to

0.3 mm in diameter and 1.3 cm in diameter, respectively. An exit slit of

700 ym is used, giving a bandpass of 0.20 nm. The NBS argon arc is operated

without a window using a purge rate of 5£,/min. The arc to be calibrated usu-

-49-
ally contains a window, and thus the calibration of the arc must include a separate measurement of its transmission.

The window transmission measurement is carried out by removing the window

from the arc and installing it in a holder which enables the window to be

positioned in or out of the beam just in front of the arc. The window is

replaced in the arc by a disc having a 1 mm aperture in its center; this re- duces the difference in the gas flow within the arc produced by positioning

the window in and out of the beam. The window holder is located in the region

purged with argon, so it need not be vacuum tight. With the window near the

arc, the area of the window for which the transmission is measured is approximately the same as the area which the radiation traverses when the window is

mounted on the arc.

The transmission of the window at each wavelength is given by the ratio

of the signal with the window in the beam to the signal with the window out of

the beam. However, as in the arc calibrations the dark current and the scattered light contributions to the signal must be subtracted from the total

signal. The scattered light levels are determined as before by measuring the

signals at 110 nm with the window in and out of the beam. The window transmission at a given wavelength x is

(Signal, - Dark Current - Scattered Light),,. . J

_

^ ^ X ^ '^Window in ,c\

X (Signal^ - Dark Current - Scattered l-ight)^j^.^^Q^ ^

'

B. Irradiance Measurements

1. Establishment of Irradiance Scale: General Method

Spectral irradiance is the radiant power incident upon a target area per

area per wavelength band: E = [watts cm"2 nm"^]. A source of radiation may serve as a standard source of irradiance by operating it at a given distance

from the target area. If one had a radiance source with uniform radiance over

its emitting area, the irradiance at a given distance from this source could

be easily computed. In general however, radiance sources are non-homogenous,

and the radiance is known only for a small portion of the source, i.e., an area element which can be approximated to be homogeneous. Thus, the irradi- ance at some distance from the source can be determined only if radiation from

outside the calibrated area is prevented from reaching the measurement system.

This can be done by the use of optical imaging or collimating apertures. For

applications in the VUV, the method using collimating apertures22 has proved

to be more practical and is the basis for the measurements described here.

The collimating aperture method is as follows, A radiation source that

is homogeneous over a certain emitting area and whose spectral radiance has

been previously determined is situated a given distance from a monochromator,

A pair of apertures, one at the entrance slit of the monochromator (the field

aperture) and the other as close to the source as possible (the source aperture), is chosen so that only the radiation from the homogeneous portion of

the source is measured. The spectral irradiance at the field aperture is

given by the product of the known spectral radiance of the source and a geo-

-50-
= 0) O Q. O < I o E X 5 <o

•r—s

<L) (/)

C

O

O

x: -r- E -a ,— t_

O) •--

-a 'o +j 1/1

•r- <u o >

<X3 -r- t_ 4-) t- fa

0)

(_

O) c:

4-> o

O)e -a O O)

•r- c

-O -r-E

t_ {_ O O)

C- +J O T3

<Da. (/) 00 •-

>^

<J

O) • u

0) u4- o a>

<DOO

(_Q. • A "O o OJ

•r— +J

+J «3

c_

TO> • Ol o o 0) TO n -oc o o TO

4-> +-> o CO to +J <Uo E <u c

OJ

u TO x: c u T3

•r- TO o (-

TO5_

C

C • o

CM T3

CM 1 TO T3 t

l- o; MO

Ol <U o

U- (/)

-51-
metric factor dependent on the aperture dimensions and their locations relative to the lamp. This geometric factor contains the information on the

effective area of the emitting source, the solid angle of the radiation beam

incident upon the field aperture, and the irradiated area.

In principle, these quantities can be determined by measurement. However, a direct determination of the geometric factor is unnecessary, as is

shown by the following discussion. The response of a spectroradi ometer as a function of wavelength to a suitably collimated radiance standard is essen- tially a measure of the system detection efficiency on a relative scale. If

the same spectroradi ometer is irradiated with an unknown source and the res- ponse is measured again, the spectral irradiance of the unknown source can be

determined, also on a relative scale. Then, provided that the wavelength

range of calibration extends to the visible or near-UV region in which standard sources of irradiance are available, the absolute spectral irradiance of

the unknown source can be determined at one wavelength within the calibrated

wavelength range. This absolute value can then be used to normalize the relative scale of irradiance to an absolute scale.

The calibration procedure is

spectral irradiance at wavelength x determined, the response S^(x) of source [see Fig. 22(a)] is

illustrated in Fig, 22, If E^Cx) is the

of the source whose irradiance is to be

the spectroradi ometer to this radiation

I S^(X) = E^(x)a(x), (6)

where a(x) is the spectroradi ometer system efficiency. The response S|^(x) to

the apertured radiance standard [see Fig, 22(b)] is

Sf^(X) = LR(x)a(x)Y, (7)

where y is the geometric factor discussed above and L^{x) is the spectral

radiance of the radiance standard. The relative spectral irradiance can then

be determined:

SM)

The absolute spectral irradiance is determined by comparing the unknown source with an already existing irradiance standard at some convenient wavelength. If it is more suitable, this can be done on a separate spectroradi o- meter setup, for example, one that is dedicated to spectral -i rradiance

measurements in the air. The responses of this system (primed) both to the source to be calibrated, S'^, and to the already existing irradiance standard,

S' J

, are measured at one wavelength, Xq, so that the spectral irradiance of

the unknown source, Ex(Xo)» given by

S\(Xo)

-52-
G)C

CO

<uo

t-3o

in

Q- e S 13 O 13 a ua

(- > o

Q.

C 3 -r-

+-> CU (U

(/) oc

XT r-

+-> -o

-O

<+- t_ o «- o

IT3 t_ E 4-> 0) U ^ <U o a.

on i/>

CO

cvj

CU3

0)

-53-
where Ej(Xq) is the spectral irradiance of the standard source.

If Eq. (8), with X = Xq, is combined with Eq. (9), y can be eliminated to

give

S'x(Xo) S,^(Xo) E^(Xo) S^(x) ^^^^ ^ s'iIXq) l^TTJ "-R^^^- ^^^^

From Eq. (10) the absolute irradiance of the unknown source can be determined

over the complete wavelength range of the radiance standard.

The considerations given above assume first that the system efficiency of

the spectroradiometer employed does not depend on the angle at which radiation

enters. A diffuser located directly behind the field aperture is usually

necessary to ensure that this assumption is correct.

2. Argon Arc Calibrations

Two types of sources are calibrated as irradiance standards, the argon

mini-arc and the deuterium lamp. Calibration of an argon arc will be discussed first.

The light source used as a standard of spectral radiance in the VUV and near UV is an argon mini-arc previously calibrated by the hydrogen arc. This source is used according to the method discussed above to calibrate a second

mini-arc as an irradiance standard. The measurements in the VUV spectral

range are performed using the SEYA monochromator described above in

Sec. III-C,1.

Figure 23 is a diagram of the setup. The telescope is first aligned with

the optical path through the entrance slit and the center of the grating. The

monochromator must be brought to atmospheric pressure temporarily to accomplish this. A 2-mm diameter aperture mounted on a ground MgF2 diffusing window is then positioned on the optical axis 5 cm in front of the entrance

slit. The entrance and exit slits are set at 0.5 mm, giving a spectral bandpass of 0.4 nm. The arc source is connected to the system with a bellows as shown to permit alignment. A 0.5 mm diameter aperture is used in place of the

arc window. After alignment, the arc is ignited, and the volume in front of

the arc is purged for 10 minutes. Signals are then measured at wavelengths

listed in Table 1 of App. F-2. For X<i38 nm the signal/noise is too small

(mainly due to the diffuser) to obtain a useful measurement, so 138 nm is

usually the lower wavelength limit in irradiance calibrations. The arc is

then removed, and replaced by the arc to be calibrated. This second arc is

aligned with the telescope as before. The unknown arc should be operated as it is intended to be used, e.g., with a window instead of the aperture used

with the transfer standard arc above. Signals are then measured from the

second arc at the respective wavelengths. The irradiance from the unknown source at each wavelength is proportional to the ratio of signals from the two sources times the radiance of the standard source (see. Eq. 8 above).

-54-
Q.

-55-
Thus, the above measurements determine the irradiance of the unknown

source, but only on a relative scale as long as the value of the constant ^ in

Eqs. 7 and 8 is unknown. To obtain an absolute scale, the irradiance of the

unknown source is compared in the near UV region to that of a calibrated tungsten-quartz-halogen irradiance standard. This measurement is carried out on the double monochromator , with an integrating sphere used as its entrance

aperture. For this measurement this setup has two advantages. First, the

double monochromator greatly reduces scattered light which otherwise would be

a serious problem for the quartz-halogen source. Second, the integrating

sphere serves as a better method of rendering the signal insensitive to the

direction of radiation from the source. This is important in comparing the

small sized arc source with the much larger quartz-halogen source area.

Figure 24 is a diagram of the doubl e-monochromator setup for irradiance

measurements. On this setup the arc is operated with the front of its window

exposed to air, since all measurements are for x_>200 nm. Using the laser, the

arc is aligned with the integrating sphere aperture which is 0.6 cm in diameter. The baffle shown is «10 cm from the source; it is an aperture 3 cm in

diameter. The arc is positioned so that its center is 50 cm from the aperture

of the integrating sphere. Measurements are carried out on this setup as described in Sec. IV-A,4. The quartz-halogen lamp is aligned and operated

according to the instructions included in its calibration report supplied by

the Radiometric Physics Division of NBS. With this type of lamp light is

emitted in all directions, so it is especially important to prevent light from

entering the integrating sphere or predisperser by reflection from some object. This is accomplished by positioning baffles to avoid any possible re- flecting paths.

3. Deuterium Lamp Calibration

Deuterium lamps are calibrated as spectral irradiance standards in the

near UV range 200 _< X <_350 nm by another calibration group in the Radiometric

Physics Division of NBS. The lamps calibrated are of the side-on type. Prior

to calibration they are potted in a base identical to that of the tungstenquartz-halogen lamps. Our calibrations of such lamps usually consist of

extending the spectral range in the vacuum UV down to 165 nm. The measurements are performed on the SEYA monochromator setup, using the same method

as was described in the previous section for calibrating an argon arc as an irradiance source. The deuterium lamp is placed inside a stainless steel

container which is purged with argon during the measurements. The container

is constructed so as to isolate to a large degree the upper (emitting) portion

of the lamp from the potted base. This helps to keep the volume through which

the radiation passes free of impurities so as to minimize absorption. The

deuterium lamp is aligned with the spectrometer in the same manner as is the

quartz-halogen lamp, i.e., an alignment fixture is substituted for the lamp

and aligned with a laser beam as described in the quartz-halogen lamp calibration report. In this procedure the alignment device is first translated until

the laser beam intersects the designated position on the device; the alignment

device is then rotated to reflect the laser beam back along its path. The

alignment fixture is then removed and the deuterium lamp is inserted in the

base. This technique aligns the lamp in the orientation in which it was calibrated for X > 200 nm. Since the measurements to be made are only to deter-

-56-
nine the relative spectral distribution, the exact distance from the lamp to

the entrance aperture is not critical. This distance is set at approximately

50 cm. The lamp is ignited and operated for 10 minutes with an argon purge of

at least 10 il/min. This purge rate is essential because of the possibility of

radiation absorption due to air contamination in the radiant heated purge

chamber between the lamp and the vacuum system window. After this warm-up

signals are measured at the following wavelengths in nm: 260, 200, 195, 190,

187, 184, 182, 180, 178, 176, 174, 173, 172, 171, 170, 168, 167, and 166.

The lamp output is highly sensitive to changes in wavelengths near

165 nm. Therefore, measurements are made for small increments in wavelength

in this region, so that the shape of the spectrum near 165 nm depends somewhat

on the wavelength bandpass. For calibrations a bandpass of 0.4 nm is used.

This is the full width at half maximum of a triangular slit profile obtained

by setting the monochromator entrance and exit slits both at 0.5 mm.

After signals are measured from the deuterium lamp, the lamp is removed

and replaced by the argon arc transfer standard source previously calibrated

by the hydrogen arc. This argon arc is set up and operated as described in

the previous section: a 0.5 mm aperture is used in place of a window, the arc

is aligned with the telescope, and the volume between the arc and the spectrometer window is purged with argon. After igniting the arc, signals are measured at the same wavelengths as was done with the deuterium lamp. Then the

irradiance of the deuterium lamp at each wavelength is given by Eq. 8 above,

i.e., the ratio of signals from the two sources times the radiance of the

standard source. This gives the irradiance of the deuterium lamp to within a constant factor y. The value of y is found by applying Eq. 8 with x = 260 nm. This then enables the relative irradiance values at all wavelengths to be put

on an absolute scale. Table 1 of App. F-3 lists the values of spectral irradiance determined for a typical 30 W deuterium lamp.

-57-
10 20 30 40 50 60

ARC CURRENT (A)

Figure 25. The dependence of the mini-arc spectral radiance

on arc current for several representative

wavel engths

.

-58-
V. Uncertainties

A. Argon Mini -Arc Radiance Standard

1 . Primary Standards

The basic uncertainties in the spectral radiance of the mini-arc are due

to uncertainties in the primary radiometric standards. As was discussed in

Sees. III-A,1 and 2, the hydrogen arc contributes a 2a uncertainty of ±5% to

the mini-arc radiance between 140 nm and 330 nm,i while the blackbody line arc

is responsible for a 10% contribution between 114 nm and 140 nm.i^

2. Arc Current

As the arc current and consequently the input power to the discharge are changed, both the electron density and temperature change. For an atmospheric

pressure argon arc with a 4.0 mm diameter channel, the electron temperature

varies from about 14,000 K to 11 ,000 K and the electron density from about

1.5 X 10^^ cm"^ to 3 X 10^^ cm"^ as the arc current is decreased from 60 A to

20 A. 2^ Since the continuum radiation varies as the electron density squared,

a large variation in intensity is expected. The measured dependence of the

mini-arc radiance on arc current is shown in Fig. 25 for four representative

wavelengths. The radiance, normalized to unity at 60 A, varies smoothly over

a factor of about 100 for a 40 A current range. The wavelength (x) dependence

is due primarily to ^(X, T) ,2*+ a quantity which depends on the photoioni zation cross sections of the various Ar I energy levels and which enters as a multiplicative factor in theoretical calculations of the emission coefficient of

a plasma in thermal equilibrium at temperature T.

The uncertainty in the spectral radiance due to an uncertainty in the arc current measurement can be obtained from Fig. 25. In general, there is

generated approximately a 4% uncertainty in the spectral radiance due to an uncertainty of 1% in the arc current. Using a 0.1% current regulated power

supply, a calibrated shunt and digital voltmeter, the current may be set

typically to within 0.02 A, giving an uncertainty in radiance of 0.4% or less

for currents above 20 A.

The ability to vary the radiance over such a wide range by a simple adjustment of arc current suggests another mode for applying the arc as a radiation standard, namely, as a source of constant spectral radiance over the

complete wavelength range. Table IV shows the arc current values required at

several representative wavelengths such that the spectral radiance is constant

at 2.7 X 10"3 W cm"2 nm"^ sr"^ for each wavelength. Similarly, the arc current could be adjusted so that one's spectrometer response would be equal to its

response to the unknown source or experiment being calibrated. In this way

scaling difficulties and problems with the linearity of the detection system

may be avoided.

3. Gas Flow

The arc is stable over a wide range of argon flow rates. Relatively

large flow rates are preferred since the degree of purity in the arc chamber

-59-
c

•*-» c 7 0)

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4, c

1 cr

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O E

rodu 1E

30-n

U CO

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> o >

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O^OI>l>rHOiOCOCDiOLOOO

O O rH rH (>i CO 00 <N Oi lO O l>

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coi-io^i>ir:)corHaii>ir:)^co(NTH

-60-
LU

>

I-

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-J

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LU

CJ

<

DC

-J

<

CC

\-

o

LU

a.

0.8 0.9 1.0 1.1

PRESSURE (ATM)

1.2

Figure 26, The dependence of the mini -arc spectral radiance

on the arc chamber pressure. The arc chamber

pressure, in the absence of an exhaust manifold,

is equal to the local atmospheric pressure. Data

are shown for the following wavelengths: 300 nm

{+); 250 nm (o); 200 nm (x); and 140 nm (•).

-61-
is greater when the purge rate is high. However, too large a flow rate can cause changes in the spectral radiance of the continuum. It can cool the

plasma (an effect which tends to decrease the radiant power) or cause a pressurization of the arc chamber (an effect which tends to increase the radiant power), or it may cause a slight shift in position of the constricted

discharge or even an instability.

The total argon flow rate used for the arc calibration is 5 Ji min"i

divided equally among the three input ports in Fig. 12. This rate was chosen because an increase beyond this amount resulted in little further de- crease in the gas impurity level. Also, a change in the flow rate of a factor

of 2 in either direction from this amount resulted in no significant change in

the radiance of the arc. Thus the possible effects of flow rate changes were either negligible or compensated for each other to within our measurement

preci si on.

4. Pressure

If the arc is operated open to the air, the radiative properties of the

arc discharge will depend somewhat on the prevailing local atmospheric pressure. To first order, a change in pressure is expected to be relatively significant since the continuum depends directly on the square of the electron

density. However, there are other effects which tend to compensate; for

example, the electron temperature increases as the pressure is lowered thus

increasing the ionization fraction. Deviations from local thermodynamic equilibrium may also become significant. To obtain some experimental data, an exhaust port manifold was used to vary the pressure over a range from 0.8 atm

to 1.2 atm. Figure 26 shows the measured dependence of the spectral radiance

on the pressure for four different wavelengths. The data are normalized to

unity at one standard atmosphere. There is no significant wavelength dependence, but there is a linear relationship between spectral radiance and pres- sure over the 0.80 - 1.2 atm range. A linear fit to the data in Fig. 26 gives

L(p) = (1.57p - 0.57) L(o), where L(p) is the spectral radiance at pressure p in atmospheres, and L(o) is the radiance at one standard atmosphere. Since

pressure measurements in this range can be easily made to within uncertainties

of 1%, the uncertainty in the spectral radiance can be made negligible with

reasonable care in monitoring the pressure,

5. A1 ignment

Figure 27 illustrates the results obtained by translating the arc across

a 0.300 mm diameter spectrometer entrance aperture for four different wave- lengths at 50 A. All data are normalized at the point of maximum signal when

the discharge is properly centered on the optical axis. Only a weak dependence on wavelength is seen in that the arc appears slightly narrower at

shorter wavelengths. At 300 nm the results are also given for a current of

20 A. No significant dependence on arc current is observed.

These data give the sensitivity of a calibration to possible translational alignment errors. It can be seen, for example, that an error in the

arc alignment of ±0.20 mm, i.e., 5% of the radius of the central plate, would

lead to a decrease of about 2% from the calibrated spectral radiance shown in

-62-
COo

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-63-
0)o

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a» -c c c o ^ +J CT> (O -r4-> CD-r- 4-> C I— » <o

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u

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X -M c c O) o -f- oj re I- Q. CL CO re 0) CO -f— •

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00 re a» re OJ ^ •!- •> I— 4-> t- +J

O) re re c

t_ t_ O) > OJ 3 O) t. c_ cn > oj 5 t-

•I- oj xr re 3

-64-
Fig. 13 and Table II. However, this uncertainty usually can be avoided en- tirely by providing fine adjustments for maximum signal, for example, by

translation of the light source itself or of the arc image through use of an adjustable reflecting mirror which can be incorporated into the optical system. The latter technique was used in obtaining the absolute intensities in

Table II, and therefore no translation alignment uncertainty needs to be included in the error budget for the calibration.

If one must observe a somewhat larger portion of the arc than the

homogeneous 0.3 mm diameter circle, a correction to Table II must be made.

A transl ational scan across a 0.015 mm x 0.15 mm rectangular aperture, also

illustrated in Fig. 27, was indistinguishable from the other data. This implies that the data are not limited by spatial resolution and that they can be

taken to represent the actual radial distribution. Using these data and knowing that the discharge is cyl i ndri cal ly symmetric, one can readily calculate

the spectral radiance over a larger area.

Figure 28 illustrates for two wavelengths and currents the results obtained by tilting the arc through an angle of 0.1 rad (5.7°) with respect to

the optical axis. Referred to a Cartesian coordinate system centered at the arc with the z axis defined as the arc axis, the terms pitch and yaw correspond to rotation about the x and y axes, respectively. The data were taken

with a 0.300 mm entrance aperture and an f/200 optical beam. All these data

show no significant dependence on angle, indicating that most of the radiation

comes from the region of the central constricting plate. Tilting in two mutually perpendicular planes (pitch and yaw) resulted in slight differences attributable to the nonsymmetri c, unconfined portions of the arc discharge near

the electrodes.

In addition to the above measurement of the angular distribution of the arc radiant power, the radiant power measured over larger solid angles was compared to the radiation from a flat-filament, tungsten strip lamp as a function of the solid angle of observation. The comparison was made for arc cur- rents of 20 A and 50 A at a wavelength of 250 nm. The ratio of signals from

the arc and tungsten filament was constant within 1% for apertures smaller

than f/9. For f/7 this ratio decreased by about 2% and for f/6 by 8%. Data

taken at 20 A and 50 A showed no difference. Assuming that the tungsten strip

lamp is a good Lambertian source, we conclude that the arc emission is indeed

uniform over quite a large solid angle. This property is important since

calibration of the arc is done at f/200 due to restrictions on the primary

standard, whereas applications for the arc usually require larger aperture

beams.

6. Reproducibility and Stability

To test the reproducibility of the arc, it was ignited ten times; after

each ignition the current was set at 50.00 A ± 0.05, and the radiance at

200 nm was measured. The standard deviation of these measurements was 0.5%.

This variation can be accounted for almost entirely by the uncertainty in our measurement of the arc current (see Fig. 25). Thus to within the precision to

which the current was set, no reproducibility problems were encountered.

-65-
In addition, both short and long term stability must be considered to

evaluate the reliability of the mini-arc as a secondary radiation standard.

Short term instabilities might be caused, for example, by electrode effects in

which the arc termination points on cathode and anode shift slightly, by an excessive amount of air leaking into the discharge, by too large a flow rate, or simply by poor current regulation by the dc power supply. With the operating conditions described in connection with Table II, the radiance at 200 nm was monitored photoelectrical ly over periods of 10 sec, 1 min, and 1 hour.

With a 0.1 sec time constant for the recording system, the SNR was 100, and

any shifts in the signal level were measured to be less than 1%. The behavior

at all currents between 60 A and 10 A is essentially the same until the arc extinguishes at a current of about 5 A.

Long term stability is influenced primarily by the aging of the UV window. Changes in the local atmospheric pressure also have an effect, of

course, but these can be monitored or controlled to the accuracy desired. The

spectral radiance of the mini-arc was measured every 3 hours over a 24 hour

period for several representative wavelengths: 325, 300, 275, 250, 225, 200,

178, 153, 139 and 125 nm, A shutter between the argon mini-arc and the vacuum spectrometer shielded the vacuum optics and detector system from illumination

by the arc except during the actual measurements. The sensitivity of the

measuring system, thus protected against possible radiation damage, was measured at the beginning and end of the experiment using the hydrogen arc primary

standard. No change in sensitivity was detected to within the ±1% precision

of the calibration procedure. The result of the aging test was that no systematic changes in the arc radiance were observed to within ±1% over the 24

hour test period. However, it is important to realize that the 2 mm thick

MgF2 arc window, through which observations were made, was exposed on the

outside surface to a constant flow of argon at atmospheric pressure and not to

the vacuum environment. An additional window, usually protected by the shutter, was used to make the vacuum seal. In this way, any possible decrease in

transmitted radiant power due either to radiation damage to the arc window or accumulated tungsten evaporate on the window surface could be distinguished

from a possible decrease due to contamination of the vacuum side of the sealing window. This latter effect is caused by UV photodissociated hydrocarbons

which are present in trace amounts in most oil pumped vacuum systems. It was purposely uncoupled from other aging effects because it is a characteristic

not of the lamp but rather of the system used to measure the radiance of the

1 amp.

Although no aging effects were noticeable, it should be noted that it is

possible also to calibrate and operate the argon mini-arc with no window, thus

eliminating any risk of window contamination. The open end then acts simply

as an additional exit port for the flowing argon gas. Under these conditions

and for a total flow rate of 5l min~i at 1 atm pressure, the arc remains stable, the continuum spectral radiance is unchanged, and the strengths of the

impurity lines are not increased. The main reason for retaining the window in

the basic mini-arc design is to make the light source compatible with VUV

measuring systems.

-66-
7. Gas Impurity

It was standard procedure during the various experiments described here

to use ultrahigh purity argon (99.9995% pure). If commercial grade argon

(99.9990% pure) is used, the continuum intensities are unchanged, the only

effect being an increase in the strength of the impurity resonance lines.

This increase is not as much as one would expect from an analysis of the gas

cylinder impurity content because there are usually impurity contributions due

either to small air leaks or to outgassing of materials within the arc chamber

or within the gas handling system. In our case, this impurity content was on the order of 5 ppm or about the same as that of the argon gas cylinder.

In order to investigate the effect on the continuum due to the presence

of larger impurity concentrations which conceivably might be encountered in a worst case situation, air and hydrogen were mixed with the argon to produce

impurity concentrations as high as the 0.1% level. If air is mixed with the

argon flowing into all three gas ports, as might be the case if a large leak

developed in the gas handling system, there can be obvious problems in making

VUV radiance measurements because of molecular oxygen absorption in the gas

volume between the cathode and the UV transmitting window. With an air impurity concentration of about 1/3000, there was no measurable effect on the

continuum intensities above 200 nm. With a higher concentration of 1/1000,

the continuum signal increased by not more than 3% above 200 nm, but the radiant power at 140 nm was observed to decrease by 60% due to absorption. When

air was mixed only with the argon flowing into the arc plate ports (A and B in

Fig. 12), the absorption effect was much smaller, 16% at 140 nm.

The effect of adding hydrogen is of interest mainly because there is

always a residual hydrogen impurity which is due to trace amounts of water

vapor throughout the system. Unless a bakeable system is used, this impurity

content will be a function of the local relative humidity. Since even small

quantities of hydrogen can alter the arc properties, it is important to estimate the effect of variations in the hydrogen impurity concentrations. Indeed, when hydrogen is added to the argon to form a 0,2% mixture, the con- tinuum spectral radiance is increased about 10% with a slight wavelength dependence. With a 1% admixture, an increase of 30% was observed. From relative measurements of the intensity of the optically thin far line wing of the

hydrogen Lyman alpha line, it was estimated that the normal residual hydrogen

impurity concentration in our experiments was 0,02%, By interpolation using

the above figures, it is estimated therefore that the spectral radiance fig- ures in Table II are about 1% or 2% higher than they would be if there were no hydrogen impurities at all. Because the local humidity in the NBS laboratory

is controlled to a rather normal value of about 45% and because no extraordinary drying techniques were used, it is not expected that large variations

from this figure will be encountered in the field. Therefore, the effect of

residual hydrogen impurities will be included as an additional uncertainty of

±1% affecting the spectral radiance determinations,

8, Summary

An argon mini-arc has been designed, tested, and operated as a transfer

source of spectral radiance for the wavelength range from 114 nm to 330 nm.

-67-
Table V. Error Budget for the Spectral Radiance

Calibration of the Argon Mini -Arc

Source Contribution {%)

Primary Standard (s)

above 140 nm below 140 nm

Arc current ( r)

Gas flow (r)

Pressure (r)

Alignment (r)

Reproducibility (r)

Short term stability (r)

Long term stability ( r)

Gas impurity (r)

Combination of errors in quadrature

above 140 nm below 140 nm

5

10

0.4

<1

1

1

5.3

10.1

Random and systematic errors are denoted by (r) and (s).

-68-
Calibration has been performed using two primary standard sources: the hydrogen arc from 130 nm to 330 nn and the blackbody line radiator from 114 nm to

130 nm. The mini-arc has the following principal features: (1) a steadystate reproducible source with dc power requirements of less than 1.5 kW;

(2) an intense continuum which is line- free between 194 nm and 330 nm and

which has only a few narrow and widely spaced impurity lines between 114 nm and 194 nm; (3) a uniform output over a solid angle as large as f/9; (4) negligible aging effects over at least a 24 hour period; and (5) uncertainties

(2a) in the absolute spectral radiance of 5.3% above 140 nm and 10.1% between

114 nm and 140 nm, A budget of the errors contributing to these uncertainties

is given in Table V. The radiant power emitted by the mini -arc is influenced

primarily by the arc diameter, the arc current, and the transmission of the IJV window material

.

B. Argon Mini -Arc Irradiance Standard

The determinations of spectral irradiance are based on the assumptions

that the system efficiencies of the measuring spectroradiometer are independent of the angle at which the radiation enters and that diffraction effects

are not significant. Since the monochromator grating used must be assumed to

have a nonuniform reflection efficiency, a diffuser located directly behind

the field aperture is used to ensure that the first assumption is met, as is

shown in Fig. 22. A magnesium fluoride window, ground on one side, was used

as the diffusing element. Although it cannot be expected to be as good a diffuser as an integrating sphere, it was shown to be suitable, at least over

a relatively small range of angles. For the argon mini-arc the uncertainty in

the spectral irradiance that is due to the nonideal properties of the diffusing window can be as high as 6%.

The possible effects of diffraction must also be considered. For an extended homogeneous radiation source whose dimensions are large compared with

the source aperture, it can be shown that Frauenhofer diffraction effects

introduce no wavelength dependence and that the geometric factor y in Eq, (7)

is wavelength independent. This can be understood qualitatively by realizing

that some percentage of the radiation from each radiating point does not pass

through the field aperture because of diffraction. However, a complementary

point can always be found in the domain of the extended homogeneous source that exactly makes up for such a loss. As will be discussed below, by substituting a different-sized aperture we show that the inhomogeneity of the

mini-arc is insufficient to cause any measurable effects due to diffraction.

The vacuum spectroradiometer, as indicated schematically in Fig. 22,

consists of a solar blind photomul tipl ier and a spectrometer with a 2-mm diameter field aperture. This aperture is mounted on a MgF2 diffusing window

located 50 mm in front of the entrance slit. The diffuser was located some distance in front of the entrance slit so as not to overfill the grating and

thereby increase the scattered light, which was less than 2% of the weakest

signal.

The light source used as a standard of spectral radiance in the near and vacuum ultraviolet was an argon mini-arc calibrated with respect to a primary

standard, the NBS wal 1 -stabi 1 i zed hydrogen arc. Both sources have been sub-

-69-
Table VI. Error Budget for the Spectral Irradiance

Calibration of the Argon Mini -Arc

Source Contribution (%)

Standard (s) 5.3

Diffuser (s) <6

Diffraction (s) , 0.5

Spatial resolution (s) 4

Wavelength dependence

from above effects (s) 0-4

Arc curent (r) 0.4

Gas flow (r)

Pressure (r)

Alignment (r)

Reproducibility (r)

Short term stability (r) <1

Long term stability (r) 1

Gas impurity (r) _1

Combination of errors in quadrature 9-10

Random and systematic errors are denoted by (r) and (s).

-70-
in in o o; x:

-l-J -t-> "O > c: (/) (/) 3 c o "4- u 14- O) en «- o 0) x: o cn. r3 (/) fa QJ

I/) a> -a QJ c c_ o TJ

*'"* •»-) T3

»-> cr o 'O {_ * CO

CT1 4-) ^ O) I/) o

-4-> •r— O) 4-5

u

d

-a 13 O) •r— *^ u "O •1— c fO

TJ o C—

t_ -o o o. t c cr

cz c fO cr 1 •r" •(-> •r- 1 1 L.CJ

fO <u E ^ % t_ o

+-> n3 to o ^ > ai CTi CL-O C 1 I/) fO

(_ 1/1 QJ c c_ 3

O) •r— 1 ^ 0) >^ c (— <u 1 \U n o •r"

4-> > (a ZD OJ

(/) t_ N

"3 •»-> O) * OJ C_ v)

(O 1 i to

<u cr1 o r—

C=1 OJ 4-) c M 03 4->

(/I C_

CT> O) C 4—' (J cr

cr c -M

q; (J

c_ TJ

<u > 14- C71 3)

l4-

C > 1 _

•r" •r- t_ -o t/1 o

OJ QJ

Q

t_ n3 -o

--> C (O o

O) •r- 3 QJ QJ o M T3 t_ (_ C O

OJ O) t_ cr CO 4-> 00

0) •r" sz xr

O) 4-> • </) 0) OJ 4-i CM sz • c

0)

Ol o c_ o o (U 3 j:: JC -a s_ CD O 4-> a.

•r— O) OJ (U U. 4J e 5_

-71-
jected to radiometric scale i ntercomparisons , both internal i »18 and international. ^5 The 2a uncertainty in the absolute spectral radiance of the miniarc has been determined in Sec. V-A to be 5.3% between 140 and 330 nm. The

mini-arc was located 50 cm from the field stop, and a 0.3 mm diameter aperture

placed 5 cm from the mini -arc center was used to restrict the size of the

radiating area to about 0.5 mm in diameter.

The total uncertainty in the irradiance calibration of an argon mini -arc,

including uncertainties in the primary source calibration and the transfer

procedure, is estimated to be ^10%. A budget of the errors contributing to

this uncertainty is given in Table VI. Although the arc emits radiation at

shorter wavelengths than that shown in Fig. 19, the short wavelength limit is

taken to be 140 nm because of the low signal obtained from the stopped-down

mini-arc radiance standard at shorter wavelengths. The bandpass for the radiance-to-i rradi ance transfer here was set at 1 nm. Not shown in Fig. 19 are several lines in the argon arc spectrum that are due to residual gas

impuri ties.^^ The irradiance of the mini-arc was put on an absolute scale by

determining its absolute spectra"* irradiance at 280 nm, using a tungstenquartz-halogen lamp as an irradiance standard. A separate spectroradi ometer

utilizing a BaS0^-coated integrating sphere, a predi spersi ng monochromator,

and an analyzing spectrometer was used for this measurement. As can be seen from Eq. (10), one measurement of this type is sufficient to determine the

absolute irradiance at all wavelengths.

As a check on the method, it is possible to carry out tungsten-lamp calibrations at other wavelengths between 250 and 350 nm and thus overdetermine

the system of measurements. Figure 29 illustrates the percentage difference

between the spectral irradiance of the deuterium lamp based on the tungstenlamp measurements (on the integrating-sphere-double-dispersion spectroradiometer) and the spectral irradiance based on the argon mini -arc measurements (on the di f f usi ng-wi ndow-vacuum- monochromator spectroradiometer) . The

measurements are in agreement within 3%. However, perhaps because the point

at 250 nm is a little high, one can perceive a slight trend toward an increasing difference at the shorter wavelengths, i.e., the radiance-based values

seem to go lower than the tungsten-lamp based values. Such a trend would be

expected if diffraction played an important role. In order to check on whether the argon-arc plasma was being sufficiently resolved by the collimating system and whether diffraction effects were indeed being accounted for by

virtue of the extended nature of the source, additional measurements were taken with a 1-mm aperture used as the field aperture. These measurements

should be more sensitive to possible diffraction effects since a smaller

percentage of the diffracted beam from each radiating point passes through the

field aperture. The spectral distribution of the signal was exactly the same

as it was with the 2 mm diameter aperture, indicating that the data are not

significantly affected by diffraction. The measurements also indicate that

the spatial resolution was sufficient to define a relatively homogeneous region about the arc axis and that the radiance values applied to the data were appropriate. Thus, if there is a trend in the data of Fig. 29, diffraction

effects do not seem to be the cause of it. Our irradiance scale is presently

being compared with that of the NBS synchrotron ultraviolet radiation facility

(SURF II) to check the consistency of the two scales.

-72-
200 250 300 350

WAVELENGTH (nm)

Figure 30. Typical spectral irradiance of a 1000 W

quartz-halogen lamp and a 30-W deuterium lamp.

-73-
AI347

ROTATION

-2** -1* 0 +r +2*

ROTATION

Figure 31. The dependence of the spectral irradiance upon

pitch and yaw of a deuterium lamp located 50 cm from a l-cm^ aperture. A 2° angular change is

equivalent to a 1.76-cm transl ati onal change.

-74-
C. Deuterium Lamp Irradiance Standard

1. Spectral Irradiance Calibration

Figure 30 illustrates the spectrum of a typical deuterium lamp as com- pared with a calibrated 1000-W tungsten quartz-halogen lamp. The spectral

irradiance of the two types of lamps is equal at about 260 nm. At 350 nm the

quartz-halogen lamp is stronger by a factor of 100. At 200 nm, the converse

is true, the deuterium lamp being a factor of 100 stronger than the quartzhalogen lamp. The spectrum of the deuterium lamp above 350 nm is not illustrated, but is a continually decreasing continuum, about a factor of 10 lower

at 550 nm than at 350 nm. Superimposed on the continuum there is an atomic

hydrogen line at 486 nm and a combination of emission and absorption line

structure in the region between 550 and 700 nm. However, the dominant feature

in the spectrum is the intense UV continuum.

Since the shapes of the spectra of the two lamps illustrated in Fig. 30 are so different, a critical test of a spectroradiometer' s performance at some specific wavelength can be applied by irradiating the instrument with each

lamp. If the ratio of signals from the two lamps is not equal to the ratio of

their calibrated spectral irradiances, it would follow that a systematic

source of error is present in the measuring system such as scattered light,

second order radiation, poor out-of-band rejection characteristics, or detector nonl i nearity . We have experienced such an inconsistency which was eventually traced to fluorescence of a particular integrating sphere coating

formerly used in our system.

2. Al ignment

The sensitivity of the spectral irradiance to lamp position is illustrated in Fig. 31 for two different lamps. The measurements were made for a wavelength of 250 nm. The contours were obtained by changing the pitch and

yaw of the lamps in 0.5° increments over a ±2° grid. A BaS0[^-coated integrating sphere with a l-cm^ diameter aperture 50 cm from the lamp served as the

field stop located at the entrance to the monochromator. Over such a small

range of angular changes the contours may also be considered a measure of the

sensitivity of the spectral irradiance to vertical and horizontal positioning

over a grid range of ±1,75 cm. The two sets of contours are shown to illustrate the typical variation from lamp to lamp. Lamp A4199 appears to be more uniform since it exhibits a falloff of 2% or less through most of the grid.

This is due mostly to the fact that the lamp was positioned very well for

maximum irradiance when it was being potted. It can be seen that the center

of maximum irradiance for the Al 347 lamp is not at the 0,0 grid position but

rather in the fourth quadrant. The potting procedure in this case was not as precise as for the A4199 lamp. Nevertheless, the results for both lamps indicate that if the lamp can be mounted so that the 0,0 position can be reproduced within ±0.9 cm, the alignment error will be no more than ±1%.

3. Lamp Current

The dependence of the spectral irradiance on lamp current is illustrated

in Fig. 32. Below 100 mA, the lamp is not stable. Above 1 A the lamp over-

-75-
1

1 1 1 1

1 1 1 1

1 T ~i i r i 1

1 1 1 1 1 1 •

w

A

•

•

t

... 1

i —

i

• 250 nm

i

350 nm •

' 1 - • •

1

1 1 1 1 1 1 1 1 1 1 0 .3 .6 .9 1 CURRENT (A)

Figure 32. Spectral irradiance as a function of lamp current

for two representative wavelengths. The values

are normalized to the spectral irradiance at a current of 0.3 A.

-76-
1000

I

I

100 I \ \ \ \ I 0 10 20 30 40 50 60

DISTANCE (cm)

I Figure 33. Spectral irradiance measurements as a function of distance between the deuterium

lamp envelope and a l-cm^ field stop. Results expected by assuming the inverse square law and

taking into account the finite size of the field stop and the position of the radiation center

(determined by least squares fitting) are illustrated by the solid line. The data are taken at

three representative wavelengths.

-77-

i
heats and is in danger of failure. Between these limits, the spectral irradiance can be varied about a factor of 10 and is nearly linear with current,

with a slope weakly dependent on wavelength. It can be seen that high precision current measurements are not required in order to maintain lamp accuracy. For example, a precision of 1 mA at a lamp current of 300 mA causes an uncertainty of only 0.3% in the spectral irradiance,

4. Solid Angle

Figure 33 illustrates the dependence of the spectral irradiance of a deuterium lamp on solid angle. The field stop was a circular l-cm^ aperture

on the entrance port of a BaSO^-coated integrating sphere. If the source and

this aperture were infinitely small and distance measurements were made between the aperture and the precise radiation center rather than the lamp pins,

the data points on this plot would be expected to fall on a straight horizontal line. As can be seen, when the distance between the deuterium lamp and

the field stop is reduced from 50 cm to 5 cm, the irradiance gradually departs

from the ideal inverse square dependence. For distances greater than 20 cm,

the deviation is less than 2%. The curve in Fig. 33 represents the results one expects strictly from geometric considerations when the finite aperture

size and radiation center are taken into account. Because the radiation cen- ter is inside the sealed lamp and cannot be precisely measured, a least

squares fitting of the curve to the data was carried out. Although the 50 to

10 cm data fit the curve to within 1%, the 5 cm datum does not. This is ex- pected in view of the variation with position illustrated in Fig. 31. Thus,

when the detector aperture size and radiation center are properly accounted

for, the calibrated deuterium lamp may be operated at distances as close as 10 cm, i.e., at solid angles up to f/10 (0.008 sr) , with an uncertainty in

spectral irradiance of no more than 1%.

5. Vari abi 1 ity

Because there was a fairly large variation in the stability, reproducibility, and aging characteristics from one lamp to another, much of the discussion in this section is in the form of observations rather than generali zati ons.

Warmup time varied from lamp to lamp. Some required none at all; others

required up to 20 minutes. For the latter, typically the signal would rise to

a maximum during the first minute and then slowly decrease, reaching an equilibrium value about 20% lower than the maximum after about 20 minutes. Be- cause of this variation, it was decided that the lamps should be switched on routinely for 30 minutes prior to application. With this precaution, the

radiative output of lamps, already aged for 100 hours, was measured to be

stable at all wavelengths to within 1% over a 24 hour period of continuous

operati on.

Aging over the first 100 hours was more significant, amounting in general

to a decrease of about 15% from the initial values. However, a slight wave- length dependence was observed in some lamps, and the ability of several lamps

to recover from their diminished values during the first 100 hours implied

that the aging process was not simple. Consequently, this study does not

-78-
attempt to generalize on the lamp behavior during the first 100 hours of its

1 i fetime.

There was also a variation associated with striking the lamps. Some

lamps would produce more signal if, after the normal warmup time, they were switched off and on again. Because the magnitude of this effect was not re- producible and certainly not characteristic of all lamps, it was decided always to start the lamp when it was at room temperature. The reproducibility

of twelve lamps was tested by igniting the lamps every 2 hours for a total of

100 hours. After 30 minutes of warmup the spectral irradiance of the lamp was measured between 200 nm and 350 nm. The lamp was then turned off and allowed

to cool down before the next ignition. Six of the lamps were reproducible

with a standard deviation from their respective mean values of 1%. The worst

of the twelve lamps had a standard deviation of 5%. For this particular lamp,

the values of spectral irradiance seemed to cluster into two distinct groups,

indicating that the lamp may have been bistable. Significantly, the relative

spectral distribution of all the lamps tested was constant to within a standard deviation of 1%, i.e., the absolute values may have changed by about 5%,

but the shape of the spectral irradiance vs wavelength curve did not vary by

more than 1% on the average. From these tests, it is concluded that the standard deviation of a single measurement of a typical deuterium lamp may be as large as 5%, although one may preselect for greater dependability.

6. Calibration Checks and Accuracy

As a check on the accuracy of the method, it was standard procedure to

compare the calibration of a deuterium lamp between 200 nm and 330 nm immediately after it was obtained using the argon mini-arc with a calibration of the same lamp between 250 nm and 350 nm obtained with a high accuracy calibrated

quartz-halogen lamp. The uncertainty in the absolute values based on the

argon mini-arc is 6%; based on the quartz-halogen lamp, it ranges from 3% at

250 nm to 2% at 350 nm. The variability of the deuterium lamp does not enter

as a factor in this comparison since all measurements were done consecutively

during the same run. On the average, the values based upon the arc calibration were 2% lower than those based upon the quartz-halogen lamp. The standard deviation of the differences was 3%. We interpret this agreement, to

within the calibration uncertainties of the standards, to indicate that the

radiance-to-i rradiance calibration method used was free of significant systematic error and that the scale traceable to the hydrogen wal 1 -stabi 1 i zed arc (through the mini-arc) is consistent with the scale traceable to blackbody

based methods. Confident that the scales are consistent to within 3%, we can effect a reduction in the overall deuterium lamp uncertainty by normalizing

the mini-arc based values at 250 nm to the higher accuracy tungsten lamp values. Since most of the mini -arc uncertainty is traceable to a 5% uncertainty

in the measurement of the hydrogen arc plasma length, the normalization essen- tially means that the hydrogen arc length is being determined more precisely,

to within an uncertainty of 3%, based upon a radiometric measurement instead

of a physical measurement. Thus, if we sum in quadrature the first six uncer- tainties given in Table VII, we find the total 2a uncertainty in the deuterium

lamp calibrations above 200 nm to be 6%. Below 200 nm we have an additional

contribution due to the possibility of absorption by air contamination in the

purge chamber between the deuterium lamp and the vacuum window. This contamination is apparently caused by outgassing from the chamber wall, which is

-79-
Table VII. Error Budget for the Spectral Irradiance Calibration

of the Deuterium Arc Lamp

Source Contribution (%)

Standard (s) 3

uirfuser (s) <3

Alignment (r) 1

Lamp current (r) 0.3

Sol id angle ( r) 1

Variability (lamps preselected) (r) 4

Absorption by air contamination

(195 nm to 167 nm) (r) 1-8

Combination of errors in quadrature

above 200 nm 195 to 167 nm6

6-10

Random and systematic errors are denoted by (r) and (s).

-80-
heated by radiation from the deuterium lamp. The inclusion of this final

contribution which is given in Table VII gives an uncertainty of 6% to 10% in

the wavelength region from 195 nm to 167 nm,

7. Summary

A set of commercially available deuterium lamps was calibrated for spectral irradiance in the 167-350 nm spectral range. At 250 nm and above the

spectral irradiance values were obtained using a tungsten-quartz-halogen lamp

whose calibration is based on an NBS blackbody. Below 250 nm the relative

spectral distribution of the deuterium lamps was determined through the use of

an argon mini-arc spectral radiance transfer standard whose calibration is

based upon the NBS wal 1 -stabi 1 i zed hydrogen arc. The absolute values assigned

to the deuterium lamps were then obtained by normalizing to the spectral irradiance values at 250 nm and above. Confidence in this procedure was established by comparing the absolute spectral irradiance of the deuterium lamps as determined independently by the argon mini -arc radiance standard and the

tungsten-quartz-halogen irradiance standard in the 250-330 nm range. The

agreement was within 3%. The large UV flux and high stability of the mini -arc

are the properties which make possible the calibration below 250 nm.

The deuterium lamps are aligned and potted in bipost bases identical to

those used in mounting tungsten-quartz-halogen lamps. The two standards are thus interchangeable in a given optical system. Since the shapes of the spectral distributions of these lamps are so different, the presence of systematic

errors in a measurement system may be detected by intercomparing the two sources. The spectral irradiance of the 30 W deuterium lamp is equal to that

of the 1000 W quartz-halogen-lamp at about 260 nm, 100 times stronger at

200 nm, and 100 times weaker at 350 nm.

The uncertainties in the absolute values of the deuterium lamp spectral

irradiance are: 200 nm and above, 6%; 171 nm to 195 nm, 7%; and 170 nm and

below, 10%. A major contribution to each uncertainty is the variability asso- ciated with the striking of the deuterium lamps. As a result all calibrated

deuterium lamps supplied to customers by NBS are preselected for variabilities

of 4% or less. For higher accuracy and confidence, one may take advantage of

the result that the relative spectral distribution of the deuterium lamps is

reproducible to within about 1%, Thus, the uncertainty may be reduced by

renormal izing the absolute scale with a quartz-halogen lamp after each ignition to eliminate any variability in the strike. This procedure makes possible a reduction in the uncertainties of about 3%.

Additional work has shown that under certain conditions the deuterium

lamp with an MgFg window may be used as a radiometric standard down to

115 nm.^'^ This effort was directed at determining the circumstances under

which portions of the many-line molecular spectrum below 167 nm can be used as

a continuum. See App. A for a discussion of this investigation.

-81-
VI. Quality Control Through Intercompari sons

A, Introduction

In radiometric work the demonstration of the consistency of independent

primary standards is essential to the foundation of valid systems of calibration. This process involves both internal and external comparisons of primary

standards. At NBS the independent internal primary standards to be compared

are a wall stabilized hydrogen arc discharge and a gold-point blackbody. The

external comparison of primary standards involves the NBS hydrogen arc and

radiation from the DESY synchrotron in Hamburg, Germany. A comparison of the

NBS hydrogen arc with the SURF II synchrotron at NBS is in process, but no results are as yet available.

B. The Hydrogen Arc Versus the Gold-Point Blackbody

1. Tungsten Strip Lamp

A calibration of the spectral radiance of the hydrogen arc obtained using

an NBS-cal i brated tungsten strip lamp was compared to the same quantity determined from the calculated maximum hydrogen continuum emission coef f icient .1 The spectral radiance calibration of the tungsten strip lamp was based on an NBS gold-point blackbody and was carried out for twenty wavelengths in the

range 230 nm to 800 nm. The uncertainties in the twenty measurements range

from 2% at the shortest wavelength to 0.7% at the longest wavelength. To

determine the spectral radiance of the hydrogen arc, the transmission of the

arc end-window must also be measured. This contributes an additional uncer- tainty of 4%. Figure 6 shows a plot of the spectral radiance of the hydrogen

arc as a function of wavelength as determined by several methods. The curve

in the figure is the theoretical spectral radiance, which is the product of

the theoretical maximum hydrogen plasma emission coefficient and the arc length. The curve is graphed as a shaded area which represents the rms uncer- tainty due to the previously described uncertainties in both the calculated

emission coefficient and the arc length. The solid circles represent the

measured continuum values obtained using the calibrated tungsten strip lamp

for spectral radiance normalization. The discontinuity in the wavelength

scale is due to the interruption of the hydrogen spectrum by the Balmer line

series of atomic hydrogen, which dominates the high temperature arc spectrum

above 360 nm except for a small region around 560 nm between H^ and H^ , The

solid circles are the average of ten runs performed on different days, and the error bars represent uncertainties associated not with the hydrogen arc but

with the tungsten strip lamp. As can be seen from Fig. 6, the theoretical

values and the experimental results based on the tungsten strip lamp are in

excellent agreement, thus showing consistency between calibrations based on the hydrogen arc theory and those based on a gold-point blackbody for a wavelength range 360 nm to 250 nm.

2. Tungsten Strip Lamp with Blackbody Line Arc

In the preceding comparison of the hydrogen arc and the tungsten strip

lamp, the minimum wavelength was determined by the short wavelength cutoff of

the tungsten lamp at 230 nm. By use of the blackbody line arc along with the

-82-
tungsten strip lamp, the minimum wavelength can be lowered to about 124 nm.i

In the present comparison a wall stabilized arc discharge is seeded with C, N,

and 0 atoms to produce optically thick blackbody limited resonance lines in

the UV and VUV. The calibration of one of the lines in the UV with the

tungsten strip lamp permits the calibration of others at wavelengths shorter

than the low wavelength cutoff of the tungsten strip lamp. These calibrated

blackbody limited lines can then be used to calibrate the hydrogen arc at

their wavelengths. In particular, the blackbody ceiling of the C I optically

thick emission line at 247.9 nm is calibrated with the tungsten strip lamp.

This calibration is applied to the Ki rchhof f-Planck function to determine the

temperature of the blackbody line arc. For a 4.7 mm diameter arc at 60 A, the

axis temperature of the argon plasma with admixtures of N, C, 0, and H was found to be 11,900 K. A temperature uncertainty of ±160 K is estimated due to

an estimated rms uncertainty of about 6% in the blackbody intensity determination at 247.9 nm. The temperature was then used with the Ki rchhof f-Pl anck

function to obtain the radiances of the following blackbody limited lines:

C I, 193.1 nm; N I, 174.3 nm; C I, 165.6 nm; N I, 149.2 nm; C I, 146.3 nm;

0 I, 130.2 nm; C I, 126.1 nm; and N I, 124.3 nm. Finally, these line calibrations were used for the spectral radiance normalization of the hydrogen arc,

the results of which are plotted as open circles in Fig. 6. Each point is the

average of ten runs, and the error bars represent uncertainties associated

with the tungsten strip lamp and the blackbody line arc. We see that the open

circle points based on a gold-point blackbody are in excellent agreement with

the shaded curve of theoretical results, and thus calibrations based on the

hydrogen arc and gold-point blackbody primary standards are seen to be consistent in the wavelength range 200 nm to 124 nm.

3. Argon Mini-Arc and Tungsten-Quartz-Halogen Lamp with Deuterium Lamp

In the initial calibration of the deuterium lamp, the argon mini-arc and

the tungsten-quartz-halogen lamp were used as calibrating secondary standards

of spectral radiance and irradiance, respectively. The mini- arc is calibrated against the hydrogen arc, whereas the calibration of the quartz-halogen

lamp is traceable to a gold-point blackbody. Thus, the calibration of the

deuterium lamp by the above secondary standards gives information on the

radiance-to-i rradiance transfer method and the consistency of the hydrogen arc

and blackbody as primary standards.

Using the radiance calibrated mini-arc, the procedure for radiance-toi rradiance transfer given above was applied to calibrate the deuterium lamp

for spectral irradiance in the range 200 nm to 350 nm. Immediately thereafter, the deuterium lamp was calibrated in irradiance in the range 250 nm to

350 nm using the irradiance-cal ibrated quartz-halogen lamp. The 2a uncer- tainty in the radiance-to-i rradiance calibration using the mini-arc is estimated to be less than 10%, 22 while the uncertainty in an irradiance calibration

using the quartz-halogen lamp ranges from 3% at 250 nm to 2% at 350 nm. Comparing the results, the values based on the mini-arc were found to be about 2%

lower than those based on the quartz-halogen lamp, and the standard deviation

of the differences was about 3%. We interpret this agreement, to within the

uncertainties in the two methods, to indicate that the radiance-to-i rradiance

transfer procedure is valid and that the scale traceable to hydrogen arc is

consistent with the scale traceable to a gold-point blackbody.

-83-
C. The Hydrogen Arc versus the DESY Synchrotron

1. Deuterium Lamp

In this section we compare the UV radiometric scales of three laboratories through the spectral radiance calibration of deuterium lamps. 25 jhe

laboratories are the Max Planck Institut fur Astronomie (MPI), the Physikalisch Technische Bundesanstal t (PTB), and NBS. As fundamental radiometric

standards the NBS group used the optically thin continuum radiation from its

wall-stabilized hydrogen arc, whereas both the MPI and PTB groups used the

DESY synchrotron radiation facility in Hamburg. Although the MPI and PTB

groups used the same primary standard, their instrumentation and measurement

methods were different. Because of the differing experimental requirements of

the groups, the area of the deuterium lamp calibrated by each group is different. This affects the absolute values obtained by each group but not the

relative spectral distribution, which is essentially independent of spot size.

Thus this intercomparison is a direct check of the consistency of the relative

scales between 165 nm and 350 nm.

The deuterium lamps used here were of the end-on type. Model D-15.25

Such lamps have been flown in rocket spectroradiometers for calibration purposes^^ and have operating characteristics very similar to those of the sideon type which has been described above. The lamps were subjected to extensive

aging and reproducibility tests and preselected to minimize any short term

variations in radiative output and undesirable wavelength aging characteristics. Following are the relevant operating conditions:

1. lamp current: 200 mA;

2. lamp orientation: the lamp was translated and rotated for maximum

signal at 220 nm;

3. cooling conditions: there was no lamp housing, and the lamp was exposed to normal room temperature and ventilation;

4. lighting procedure: with the lamp at room temperature the filament

power was switched on for 5 sec and switched off after the application of the

anode voltage and lamp ignition;

5. area calibrated: NBS, 0.07 mm^ circular; PTB, 0.64 mm2 circular; and

MPI, 0.05 mm2 circular. A detailed investigation of the lamp properties is

published elsewhere.^^

The primary standard of spectral radiance used in the MPI and PTB calibrations was synchrotron radi ation.^o »3i jhe electron synchrotron provides

intense, continuous radiation over a wide spectral range. The instantaneous

power radiated by an electron accelerated in a synchrotron can be accurately

calculated^2 depends on measurable machine parameters, the final energy of

the electrons, the mode of acceleration, the radius of the electron orbit, and

the solid angle used for measurements. The total flux emitted from the synchrotron in a given direction is proportional to the number of circulating

electrons.

-84-
The PTB calibration facility at DESY allows the simultaneous measurement

of synchrotron radiation in the visible and VUV spectral regions with a twomonochromator system. One monochromator looks at the visible radiation from

the synchrotron and compares it with a tungsten strip lamp. The other mono- chromator operates throughout the near and vacuum UV and compares the synchrotron radiation with that of the deuterium lamp to be calibrated. A detailed

description of the principle of calibration and the instrumentation is given

elsewhere. 33 The rms uncertainty (a) in the absolute spectral radiance of the

calibrated deuterium lamp is estimated to be _<2%. This uncertainty is made up

of the following individual contributions: uncertainty of the parameters of

the synchrotron 0.4%, uncertainty in the determination of the current of ac- celerated electrons 1.2%, and uncertainty in the experimental device 1.3%.

The calibrated spectral radiance of the deuterium lamp was checked by a com- parison with the spectral radiance of the tungsten strip lamp in the wave- length region 290 nm to 340 nm using a different spectroscopic system. The

agreement was within ±1.5%.

The MPI calibration facility at DESY has been described previ ously ,3'+ although the spectroradiometer used in these measurements was a modified ver- sion of the one described. Additional baffling and stray light precautions were necessary to overcome a systematic measurement error that was evident in an earlier calibration above 260 nm.

The MPI spectral radiance calibration, besides depending on the spectral

radiance of a tungsten strip lamp in the visible and the known spectral distribution of the synchrotron radiation, depends upon measurements of the re- flectivity of one mirror and the polarization characteristics of the spectroradiometer. The total rms uncertainty in the measurements is estimated to be

±3%. The absolute scale was compared separately at MPI by calibrating the D2

lamp with a tungsten strip lamp above 250 nm. The agreement was within ±4%.

The primary standard of spectral radiance used in the NBS calibration was the wal 1 -stabi 1 ized hydrogen arc discharge described above. The hydrogen arc provides an almost completely ionized hydrogen plasma for which the continuum

emission coefficient can be very accurately calculated. By operating the arc at a current for which the spectral radiance reaches a maximum at a given

wavelength, the absolute value of the spectral radiance depends only on the arc length, the atmospheric pressure, and the condition of local thermodynamic

equilibrium.^ The rms uncertainty in the absolute spectral radiance of the

hydrogen arc is estimated to be ±5%.

The NBS calibration facility has been described above. The radiation of

the sources is focused, with magnification M = 1, on the entrance aperture

(0.300-mm diam) of a vacuum monochromator. A mask in front of the focusing

mirror restricts the radiation to an f/200 beam. The possibility of second

order radiation is eliminated through the use of a cutoff filter inserted in

front of the solar blind photomul tipl ier for wavelengths longer than 220 nm. The UV end-window of the hydrogen arc serves as the vacuum seal and remains in

the system for all measurements. Thus, both the primary and transfer sources illuminate the UV window, the optical surfaces, and the detector photocathode

in the same way, eliminating possible sources of systematic error. The short

chamber which separates the deuterium lamp from the UV vacuum window was

-85-
' ' ' —. CD

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Q

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r

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r ' o ' ' i—j _i I I I

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isest O•LO

LOQ oI I I I I—I r

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+-> Q. -a +1 E O)N E •-> 3

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m

</) 4-> c0 (UQ. 0

t/)

Q-2. >

A) "O

0)0 4-* c • i "O

10 4-' • 0 to Z c

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"O 0. C

i/i T3 0.

•4— (U 0

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a. +->

00 c c

•r— 0) <u

C- <v <oCL +-> > e0u 0

>^

{- t-> 0

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03

<_ c 0 • 0 0 0

t_

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+-> c 0

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(U x: q: J3 4->

CO3

-86-
purged with high purity argon gas, thus protecting the lamp window from possible deterioration due to condensation of UV photo-dissociated hydrocarbons.

Following the calibration of the lamp on the vacuum UV calibration facility, the lamp was calibrated on a different spectroradiometric system using a tungsten strip lamp as the spectral radiance standard. This was done in air

in the 250-350 nm wavelength range. Since the two calibrations gave the same results, one has increased confidence that the deuterium lamp alignment was proper and that no significant systematic errors were present in the measurements.

The intercompari son was performed using two lamps: No. 6 was calibrated

by NBS and MP I, and No. 14 was calibrated by PTB and MP I. Figure 34 shows the

absolute spectral radiance of the two calibrated lamps as determined by NBS

and PTB, The MPI measurements, illustrated by circles, have been normalized

to give a best fit to the NBS and PTB results. No emphasis was placed on a comparison of the absolute scales since the calibrated area of the D2 lamp was different for each group.

The agreement everywhere is within ±3% except below about 170 nm, where

several blended emission lines (not shown in the figure) belonging to the

molecular deuterium Lyman band system appear in the spectrum. Similar results

have been obtained by Einfeld et al.^s in a separate intercomparison with the

Technische Universitat in Munich. Our agreement indicates that the spectral

radiance scales based upon the DESY synchrotron and the NBS hydrogen arc are consistent, at least in a relative sense. Since each of the calibrations was repeated in the near UV using calibrated tungsten strip lamps, it may also be

inferred that the absolute scales are consistent since the tungsten lamps

calibrated by PTB and NBS have been shown to be consistent, 36 »37 Furthermore,

because the spectral distributions of the synchrotron radiation and the hydrogen arc radiation are so different, one increasing toward shorter wavelengths

and the other increasing toward longer wavelengths, the agreement indicates

that the three calibration facilities are reasonably free of systematic

sources of error such as scattered light, second order radiation, nonlinearity

effects, and fluorescence phenomena.

-87-
VII. Acknowl edgments

The authors are grateful for the suggestions made by Wolfgang L. Wiese,

Chief of the Atomic and Plasma Radiation Division, James R. Roberts, Group

Leader of the Plasma Radiation Group, and the CALCOM committee members who

reviewed this Document. The members of this committee were: Dale D. Hoppes,

Donald McSparron, William R. Ott, and Bert G. Simson. Special thanks are due

Dr. Ott, who served as chairman of the CALCOM committee. The helpful suggestions of Lois S. Gallahan and Helen R, Felrice in preparing this manuscript

are also gratefully acknowledged. Finally, the authors wish to acknowledge

the continuing support of the VUV Radiometry program at NBS by the NASA Solar

and Heliospheric Physics Branch. Most of the work described here was carried

out under partial funding by this agency.

-88-
VIII. References

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(1973). —

4. W. R. Ott and W. L. Wiese, Opt. Eng. 12^, 86 (1973).

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37. R. D. Lee et al., in Temperature, Its Measurement and Control in Science

and Industry , H. H. Plumb, Ed. (Instrument Society of America,

Pittsburgh, 1972), Vol. 4, p. 377.

38. G. H. Mount, G. Yamasaki, Walter Fowler, and W. G. Fastie, Appl. Opt.

26, 591 (1977).

39. J. Z. Klose, Radiometric Calibration of the Space Telescope Vacuum

Optical Simulator (calibration report for Martin Marietta Denver

Aerospace), October, 1984.

-90-
40. J. Z. Klose, Portable UV Radiometric Standards (contract report for the

National Oceanic and Atmospheric Administration), September, 1981.

41. J. Z. Klose, J. M. Bridges, and W. R. Ott, J. Opt. Soc. Am. 72., 1804

(1982).

42. R. T. Brackmann, W. L. Fite, and K. E. Hagen, Rev. Sci . Instrum. 29^, 125

(1958).

43. W. E. Kauppila, W. R. Ott, and U. L. Fite, Rev. Sci. Instrum. 38, 811

(1967).

44. K. Watanabe, E. C. Y. Inn, and M. Zelikoff, J. Chem. Phys. 21^, 1026

(1953).

45. J. Z. Klose, J. M. Bridges, and W. R. Ott, Appl . Opt. 24, 2263 (1985).

-91-

IX. Appendices

A. The Deuterium Lamp as a Radiometric Standard Between 115 nm and 167 nm

At NBS an investigation has been carried out with the aim of extending

the VUV range of spectral irradiance of the deuterium lamp down to the MgF2

cutoff at 115 nm.^^ The effort was directed at determining under what conditions portions of the blended line spectrum below 167 nm can be used as a continuum. In this connection computer recorded continuous scans of the spectrum of a Do lamp with a side-on MgF2 window were made over the wavelength

range 115-17/ nm at several different bandpasses. The data from these scans were recorded as intensities at intervals of 0.25 nm and stored in a computer

as a function of bandpass for each wavelength. All measurements were made

with identical entrance and exit slits. Figure 35 shows computer generated

plots of intensities versus wavelengths for the extreme bandpasses of 0.83 nm

(1 mm slits) and 4.98 nm (6 mm slits). The traces were normalized in the pure

continuum region above 170 nm. Below 170 nm the curves are contrasted by the

presence in one case and complete absence in the other of spectral line structure. Additional measurements were made at bandpasses of 1.66, 2.49, 3.32,

and 4.15 nm to investigate the fading of the line structure with increasing

bandpass.

In order to quantify the deviations of the spectral distributions at the

various bandpasses from the spectral distribution of the broadband pseudocontinuum, a procedure of analyzing the data by comparing the normalized

ratios of the intensities at the narrower bandpasses to the intensities at the

broadest bandpass was adopted. The computer was therefore programmed to produce such ratios, listing them for each wavelength in columns according to

their bandpass ratios. If the intensity ratio for each column is 1 over some wavelength interval, the spectral region can be treated as pure continuum. If

the maximum difference in the intensity ratios over the columns is 10% for a certain wavelength region, that spectral region can be regarded as a continuum

only to a precision of 10%. By comparing the intensity ratios as a function

of bandpass ratio for each wavelength, one can determine wavelength intervals over which the intensity ratios vary less than a chosen amount. For example,

over the range 139.75 nm through 153.75 nm the maximum difference in the intensity ratios is 15% for the set of bandpass ratios 1/6, 2/6, 3/6, 4/6, 5/6,

and 6/6. Figure 36 shows the wavelength ranges over which the maximum differences in the intensity ratios are 10, 15, and 20%. From the figure one sees a significant extension of the ranges in going from a difference of 10% to a difference of 15% and ranges added, especially one near 120 nm, in going from

15% to 20%. As an experimental check several intensity ratios obtained by the

continuous scans were compared with ratios obtained by static measurements at

given wavelengths and found to be in good agreement.

In conclusion, the maximum differences given in Fig. 36 can be taken as indications of how closely the blended line spectrum approaches a pure con- tinuum throughout the different wavelength ranges. Thus, the figure shows

different combinations of precision and wavelength for which this deuterium

lamp can be used as a continuum source for calibration in the VUV. Similar

graphs can be constructed for any such D2 lamp to determine the wavelength

ranges for any desired maximum difference.

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B. Hollow Cathode Lamps

A source which is gaining in favor for the calibration of space experiments is the metal rare gas hollow cathode discharge lamp. Two versions of

this lamp having a platinum or platinum-chromium cathode and a filling of neon gas^^ are being used to calibrate two instruments on the Space Telescope.

These lamps with MgF2 windows are spectral line sources which cover the wave- length range from the VUV at 115 nm to the infrared at 878 nm. Figure 37

shows a portion of the spectrum of a Pt-Cr-Ne lamp. Sets of the lamps were calibrated at NBS and were then used for preflight calibrations of the Faint

Object Spectrograph (FOS) in wavelength and irradiance^^ and the High Resolution Spectrograph (HRS) in wavelength only. Two Pt-Cr-Ne lamps will be on board the FOS to monitor its operation and serve as inflight wavelength standards, while two Pt-Ne lamps will be on board the HRS as inflight wavelength

standards. Sets of these NBS-cal i brated lamps are also being used in preflight throughput tests of the Space Telescope with all of its instruments

installed. The additional instruments are the Wide Field/Planetary Camera

(WF/PC), the Faint Object Camera (FOC), and the High Speed Photometer (HSP).

These lamps have excellent physical characteristics for inflight use in

that they are rugged, dependable, small, light, and low powered (4 W). They

are also readily available commercially at reasonable prices. The VUV radiometry group at NBS is investigating these types of lamps further for the purpose of determining their suitability as secondary radiometric standards.

B-1
B-2
C. Rare Gas Dimer Lamps

Another type of lamp which has received considerable attention as a calibration source in the VUV for space experiments is the rf-excited rare yas

dimer lamp,'*° These lamps emit significant radiation in the VUV but only over limited wavelength ranges of about 60 nm or less. Nevertheless, the lamps,

which separately contain argon, krypton, and xenon, can provide between them a calibrated continuum from 115 nm to 200 nm.

The dimer lamps consist of pyrex cylinders with MgF^ end-windows. In the

original design each lamp is excited by a thin wire antenna which extends into

a central channel in the barrel of the lamp. This design produced a non- uniform shell of radiation concentric with the channel, which resulted in

annular emission through the window. With emission in this form the lamps

could be calibrated in irradiance, but not conveniently in radiance. However,

a new design, suggested by the NBS VUV Radiometry Group, was utilized to produce a lamp in which the radiation was emitted from the end of a capillary

tube with a concentric coil antenna. The emission through the end-window then

appeared as a circular spot, suitable for radiance calibration. This model

lamp, calibrated in spectral irradiance, was used in the preflight calibration

of the Faint Object Spectrograph, an instrument on the Space Telescope. 39

These lamps are conveniently mounted in cylindrical exciters, 20 cm or less in length and 7.6 cm in diameter. The unit is compact, light, and low

powered (15 W) , making it a good candidate for use on experiments in space.

The lamps and exciter are readily available commercially at reasonable prices.

The usable wavelength ranges of the lamps are: argon, 115-155 nm; krypton,

125-180 nm; and xenon, 148-200 nm. Figure 19 shows the spectral irradiance of

a xenon dimer lamp in comparison with other UV sources, and Fig. 38 shows the

spectral radiance of a krypton dimer lamp.

C-1
Figure 38. The spectral radiance of an rf-excited krypton diner lamp.
D. Monochromatic Source of Lyman-g Radiation

For some time the VUV Radiometry Group at NBS has been concerned with

development of a calibrated source emitting the radiation of an individual

atomic transition.**^ Such a source would be of value since it could be used

to calibrate detectors at its wavelength without the need of a monochromator.

Furthermore, several such sources at different wavelengths could be used to

calibrate spectrometers over the range of wavelengths of the sources. The

Lyman-a radiation from the hydrogen atom at 121.6 nm was chosen for the initial development of a monochromatic source because of its high-intensity, its

ready availability from hydrogen lamps, and the possibility of isolating it

using a molecular oxygen f i 1 ter.'+i"'*^ The oxygen filter is needed since interference filters with reasonable peak transmissions at 121.6 nm (15-20%)

have bandwidths (FWHM) of 15 nm or greater. Watanabe et al_.'+'+ give the absorption coefficient of oxygen as a function of wavelength. They show a sharp

minimum in the absorption coefficient at the wavelength of Lyman-a, providing

a window for the transmission of Lyman-a radiation through an atmosphere of

oxygen. Above the low-wavelength cutoff of MgF2 at »115 nm, the behavior of

the absorption coefficient indicates that most other emissions could be eliminated up to «170 nm. However, these authors show a decreasing absorption

coefficient with further increasing wavelength indicating that a Lyman-a

transmitting oxygen filter probably will not cut out lamp emission above

«170 nm. Our work at NBS verifies the above inferences in that the oxygen

filter transmits Lyman-a radiation and absorbs most other emissions up to

170 nm."*^ A Lyman-a transmitting interference filter had to be inserted to

eliminate emissions above the upper wavelength cutoff of the oxygen filter.

As our emitter of Lyman-a radiation, we chose a commercially available

15-W rf-excited hydrogen-helium lamp with the helium at low pressure to en- hance the Lyman-a emission and reduce visible radiation. This lamp has a MgF2

window 1 cm in diameter, is equipped with a sidearm containing uranium hydride

and metallic uranium, and is filled with helium at a pressure of 4 Torr. The

uranium hydride (UH3) is decomposed by heating to add hydrogen to the fill

gas, and the metallic uranium acts as a getter to remove atmospheric contaminants. The commercial exciter used to operate the lamp has the capability of

maintaining the UH3 at preselected temperatures in the range of 100-200°C,

enabling the hydrogen to be supplied reproducibly.

When a small flow of oxygen was added to the argon flow (0.3% O2) purging

the volume between the lamp window and the MgF2 entrance window of the scanning monochromator, the Lyman-a peak at 121.6 nm was virtually unchanged, but

the many-line spectrum of the Lyman bands of hydrogen out to «170 nm was almost completely eliminated. Only structure around 126.9 nm remained. Furthermore, increasing the flow of O2 decreased the intensity of Lyman-a but at

a slower rate than the decrease of the integrated intensity in the vicinity of

126.9 nm. This is shown in Fig. 39 and Table VIII, where the "impurity radiation" refers to the structure around 126.9 nm. Thus, in the region below

«170 nm the purity of the Lyman-a radiation shows a positive gradient with

oxygen flow. Finally, to obtain the pure Lyman-a source, the hydrogen conD-1
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tinuum above «170 nm was eliminated through the addition of a standard interference filter with the specifications given above.

The irradiances of the source were obtained by applying the radiance-toirradiance transfer method described above in Sec. IV-B,1 along with the previously described procedure for the determination of line i rradi ance.'+s in

this determination the relative system efficiencies and the irradiance of the

continuum were obtained by irradiating the spectroradiometer with an argon

mini-arc calibrated for spectral radiance. Table VIII gives the irradiances

of the source for different oxygen flows at a UH3 temperature of 100°C and a source-to-def i ni ng-aperture distance of 50 cm.

In summary, we have found that by regulating the flow of oxygen through

the filter cell, we can almost completely eliminate the Lyman bands

(115-167.5 nm) from our hydrogen lamp spectrum. By next adding an interfer- ence filter with a transmission peak at 121.6 nm, we removed the hydrogen

continuum (above 167.5 nm) , giving a highly pure spectrum of Lyman-a

(121.6 nm) radiation. We have also noted that the intensity of Lyman-a decreased with increasing oxygen flow but at a lower rate than the decrease in

the intensity of the impurity radiation around 126.9 nm. Finally, we determined the line irradiances of our source along with the fractions of impurity

radiation, the reproducibilities of the Lyman-a signals, and the accuracy of

the irradiances. The out-of-band radiation around 126.9 nm ranges from «1%

down to 0.2%, the reproducibility with oxygen flow is within 2%, and the overall 2a uncertainty is 11%.**^ Thus, we have a useful source in that the line

irradiance is not strongly dependent on oxygen flow, the spectral purity is

high, and a region exists in which both the irradiance and spectral purity are relatively insensitive to oxygen flow. These properties make this source a good candidate for further development as a radiometric standard.

V

[

\

D-4
E. Safety Considerations

1. Extreme caution should be exercised when dealing with high voltages

and currents.

2. Before an arc operation is begun, a check to ensure the electrical

isolation of the anode and cathode should be made.

3. Before an arc operation is begun, a check to ensure electrical isolation of the arc plates should be made.

4. Insulating gloves are to be worn when adjusting the hydrogen arc electrodes.

5. Smoking is not allowed in the laboratory while the hydrogen arc is

being operated, and the room ventilation must be sufficient to prevent an explosive mixture from building up.

6. Deuterium lamps in air are to be operated in the open with normal room air circulation only. Any UV-produced ozone will be dispersed by normal room convection currents.

7. Tungsten-quartz-halogen lamps are to be operated in the open clear of

any flammable materials.

8. Precautions should be taken at all times to prevent the exposure of

the eye to UV and high-intensity visible radiation.

E-1

0-M>

U.S. Department of Commerce

NATIONAl. SUMKAU OT STANDAftm

Appendix F-1 washimoton. mzm

REPORT OF TEST

October 22, 1985

VUV Spectral Radiance of Argon Mini-Arc

An argon mini-arc light source was calibrated in the wavelength range 118-335 nm. The lamp was calibrated

according to procedures described in the accompanying report, "Argon MiniArc Calibration". The results of the calibration are given in Table 1.

Uncertainties are given in Table 3.

For the Director,

Wolfgang L. Wiese, Chief

Atomic and Plasma Radiation Division

Center for Radiation Research

Test # 531/226179

Atomic and Plasma Radiation Division

Center for Radiation Research

F-1-1
Argon Mini -Arc Calibration

Argon mini-arc light sources are calibrated in the wavelength range 118 nra to 335 nm with the NBS argon mini-arc. The latter is a secondary standard and

had been calibrated previously with three primary radiometric standards, the NBS

wall -stabilized hydrogen arc from 140 nm to 335 nm, a plasgia blackbody-line

radiator from 118 nm to 165.6 nm, and a tungsten strip lamp from 250 nm to 335

nm. The overlap of the wavelength bands of applicability of these three

standards ensured consistency of the calibration base. The spectral radiance of

the customer's mini-arc, in units of W cm-^ nm-i sr-^ , is listed in Table 1 for

an arc current of 38.00 A.

The values of spectral radiance listed in Table 1 apply to the radiation

emitted by the mini -arc light source through a sealing window. Also listed in

Table 1 is the transmission of the MgF2 ultraviolet window attached to the light

source and that of an additional MyF;, window. The spectral radiance of the arc plasma itself is obtained by dividing the values given in Table 1, Lj^, by the

transmission of the appropriate window. The spectral radiance of the light source using another window, for example window #2, is then obtained by

multiplying the quotient by its transmission, for example

U (#1)

L (#2) = — X T{#2). (Where, for example, the arc was

X T(#l) calibrated with window #1).

In this way damage to the arc window does not result in the loss of a calibrated

light source. Also, it is possible through such substitution of a calibrated

window, to periodically check for deterioration of the window due to, for

example, condensation on the window of uv-photodissociated hydrocarbons in the

vacuum system.

1. Calibration Conditions . A continuous flow of technical grade argon,

(99.996% pure), was used to purge the arc source. The total flow rate was set to

give a reading of 80 units on the flowmeter accompanying the arc. This argon

flow was divided equally in a gas manifold and directed to the three input gas

ports. The flowmeter, manifold, and gas connections used were those supplied by

the customer. The output ports vented directly into the laboratory.

The arc was ignited by shorting out the anode {+) and cathode (-) with a tungsten striking rod inserted into the arc channel. Some tungsten "smoke" may

be generated in this process, but the relatively high gas flow protects the

window and purges tne arc cnamoer within seconds.

The current was measured using a calibrated shunt and digital voltmeter.

The optical conditions for which the calibration is appropriate are as follows:

a) The radiation from the center arc plate of the mini-arc was focused on the spectrometer entrance aperture.

b) The calibrated area of the mini-arc was determined by the optical

magnification (M =l ) and by the entrance aperture (0.30 mm diameter).

(F-1) F-1-2
c) The f number of the optical system was defined by a field stop located

in front of the focusing mirror (f/100).

0) The spectrometer resolution was 0.1 nm.

e) A short gas cell is located between the mini -arc and the vacuum system.

It is normally purged with argon at atmospheric pressure. Thus, the mini -arc uv window is not exposed to vacuum system contaminants during a calibration.

The light source was calibrated with its axis of symmetry coincident with

the axis of the optical measuring system. Course adjustment was made using a telescope. Fine adjustment of the arc position was obtained photoel ectri cally by

gently scanning the mini-arc image across the spectrometer entrance aperture

until maximum signal was obtained.

2. Mini-Arc Characteristics . The radiance of the mini -arc is a function of

pressure. Tne values in Table 1 apply to an ambient pressure of 1 atm. The

radiance at other pressures is given by

L{p) = (1.57 p - 0.57) L(o)

where p is the pressure in atmospheres and L(o) is the radiance at 1 atm. This

relation holds for all wavelengths.

Although the mini-arc was calibrated for an optical aperture f/100, tests on the NBS mini-arc indicate that the spectral radiance is constant to within 1% for

apertures as large as f/9. The spectral radiance decreased by about 2% for f/7

and by about 8% for f/6.

The horizontal and vertical alignment are critical for absolute

calibrations. Figure 1, taken from reference 1, illustrates the dependence of

the signal on arc position for four representative wavelengths. If the alignment

is off by about 0.75 mm, the radiance is decreased to about 70% of the

calibration value. The area calibrated is the central 0.3 mm diameter region

where the discharge is nearly homogeneous. It should be clear that a correction

must be made if an area larger than the 0.3 mm diameter calibration region is

observed. The area of observation should be limited, nowever, to approximately a 2 mm diameter since the relative spectrum changes significantly for larger

di ameters.

Tests have shown that the MgF2 window is the component most likely to cause

a systematic change in the arc radiance. Measurements on the NBS arc indicated a negligible change in the radiance of the arc plasma itself after 24 hrs

continuous operation. Window deterioration may arise from radiation damage, from

vacuum system hydrocarbons, or from dust deposited on the arc side of the window.

Windows have generally shown a wavelength dependent change in transmission of a few percent after several hours use. Therefore, periodic checking of the window

transmission is recommended, as described on page 1.

The ultraviolet continuum spectrum is interrupted by several atomic and

ionic emission lines, mainly in the wavelength region from 114 nm to 135 nm. These lines arise from air and water vapor impurities in the argon gas cylinder,

the gas handling system, and the outgassing arc chamber. Table 2. lists the

wavelengths of the lines emitted from an arc similar to the one calibrated, and.

(F-1) F-1-3
indicates the approximate relative intensity of the lines relative to the aryon

continuum for a bandpass of 0.25 nm. Figure 2 is a strip chart recording of a photoelectric scan of the spectrum under these conditions. The width of the

lines is instrument-limited in this case. High resolution measurements indicate

that all the lines have a halfwidth of about 0.01 run. The line intensities will

generally show large variations, since they arise from small impurities of the

elements involved; other lines also may possibly appear at larger impurity

levels.

3. Uncertainties . The uncertainties are a function of wavelength since

three different primary standards were used in the calibration procedure and

since the signal-to-noise ratio is a function of wavelength. In addition, the

transmission of the vacuum sealing window (not the arc window) in our measurement

system decreases slightly at the shorter wavelengths during the entire

calibration process. This has been taken into account, but results in a loss of

precision and contributes to the increased uncertainty below 140 nm. The

uncertainties are summarized in Table 3.

References

1. J. M. Bridges and W. R. Ott, Appl. Opt. J16, 367 (1977).

2. W. R. Ott, K. Behringer, and G. Gieres, Appl. Opt. 14, 2121 (1975).

3. G. Boldt, Space Sci. Rev. 11, 728 (1970).

4. H. J. Kostkowski, D. E. Erminy, and A. T. Hattenburg, in Advances in

Geophysics, Vol. 14, p. Ill (Academic Press, New York, 1970J

.

(F-l) F-1-4
Table 1. Calibrated spectral radiance in units of W cm-^ nin"^ sr~i. The area of

the light source which is calibrated is the central 0.3 nm diameter region.

Values of the spectral radiance are given for an arc current of 38.00 A.

X(nm)

Spectral radiance,

using window #79

W an"^ nm"^ sr~i

Transmi ssion

Window #79 Window #80

335.0 4.66 E-2 0.920

320.0 4.57 t-2 0.915 n Q on u. y^iu

310.0 4,3d L-2 o.yiu n D on U.ycU

300.0 4.15 r o t-2 n Q 1 n 290.0 3.89 E-2 r\ D0

1

o.byi n ono 280.0 3.50 E-2 U.o/

5

n QQn

270.0 3.31 E-2 n OCT

250.0 E-2 0.835 0.835

240.0 2.56 E-2 0.850 0.854

230.0 2.32 E-2 0.872 0.874

220.0 2.06 E-2 0.877 0.884

210.0 1.81 E-2 0.878 0.888

200.0 1.49 E-2 0.878 0.888

191.0 1.25 E-2 0.876 0.887

186.0 1.18 E-2 0.874 0.885

176.0 8.91 E-3 0.866 0.875

170.0 7.52 E-3 0.855 0.861

164.0 6.51 E-3 0.850 0.856

159.0 5.42 E-3 0.847 0.848

154.5 4.84 E-3 0.839 0.840

151.5 4.27 E-3 0.832 0.834

145.0 3.20 E-3 0.810 0.814

140.0 2.67 E-3 0.791 0.790

137.0 2.39 E-3 0.765 0.771

134.5 1.95 E-3 0.737 0.753

129.0 1.32 E-3 0.632 0.675

126.8 1.05 E-3 0.581 0.633

125.4 0.95 E-3 0.548 0.593

118.4 0.96 E-3 0.361 0.355
Table 2. Survey of emission lines present in the 38 A argon mini-arc

spectrum. The spectrometer bandpass is 0.25 nm.

Radiance of line peak

XCnm) Elenent relative to continuum

115.22 -

•

_ _- ._ 0 I

'

1 - - 116.79 N I i 117.69 R I 0.3

118.94 C I 0.3

119.38 C I 1 119.99 N I 30

121.57 H I 100

124.33 N I 5

126.13 C I 2

127.75 C I 5

128.98 C I 1 130.22 0 I 15

130.55 0 I 10

131.07 N I 1 131.95 K I 0.5

132.93 C I 4

133.53 C II 8

135.58 C I 0.2

141.19 K I 0.7

143.19 C I 0.1

145.91 C I 0.1

146.33 C I 1 146.75 C I 0.1

148.18 C I 0.2

149.26 N I 4

149.47 H I 6

1.56.10 C I 4

157.50 Ar II 0.2

160.04 Ar II 0.2

.160.35 Ar II 0.1

160.65 Ar II 0.05

165.72 C I 2

174.27 N I 1 174.53 N I 0.5

175.19 C I 0.1

187.31 Ar II -• 0.05

188.90 ft w. T T Ar 11 0. 1 193.09 C I 2

247.86 C I 0.05

328.0-334. 0 0.1

336.5-342. 5 0.15

334.5-347. 0 Ar II and Ar I 0.1

348.5-350. 5 Cbroadened lines) 0.05

353 and above

(F-1) F-1-6
Table 3. Uncertainties in the mini -arc

spectral radiance calibration.

X(nni) uncertainty

(2a)

250-335 4%

140-250 9X

118-140 20%

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U.S. DEPARTMEhTT OF COMMERCE

Appendix F-2 NATIONAL. BUREAU OF STANDARDS

WASHINGTON, O.C. 20234 April 16, 1984

An Argon mini -Arc light source was calibrated for spectral

irradiance in the wavelength range 138mm to 335mm based upon the KBS

argon mini-arc^. The latter is a secondary standard of spectral

radiance and had been calibrated previously at 40.00A with three

primary radiometric radiance standards, the NBS wall -stabilized

hydrogen arc^ from 140mm to 335mm, a plasma bl ackbody-1 ine radiator^

from 114.5mm to 165.6mm, and a tungsten strip lamp** from 250mm to

335mm. The overlap of the wavelength bands of applicability of these

three standards ensured consistency of the calibration base.

The method used to transfer from a spectral radiance source (The

NBS mini-arc) to a spectral irradiance source is outlined in

Reference 5. The short-wavelength limit is set at 138mm due to low

signal from the stopped-down radiance standard at shorter

wavelengths. The long wavelength limit of 335mm is due to the

increased density of argon spectral lines at longer wavelengths.

The spectral irradiance of the mini -arc, in units of W cm~^

nm ^, is listed in Table 1 for an arc current of 40.00A.

The values of spectral irradiance listed in Table 1 apply to the

radiation emitted by the mini-arc light source through a sealing

window. Also listed in Table 1 is the transmission of the MgFj

ultraviolet window attached to the light source and that of an additional MgFo, window. The spectral irradiance of the arc plasma

itself is obtained by dividing the values given in Table 1 by the

transmission of the appropriate window. The spectral irradiance of

the light source using another window, for example window 2, is then

obtained by nultiplying the quotient by its transmission, for

exampl e

(where, for example, the arc was calibrated with window#l)

F-2-1
In this way damage to the arc window does not result in the loss of a calibrated light source. Also, it is possible through such

substitution of a calibrated window, to periodically check for

deterioration of the window due to, for example, condensation on the

window of uv-photodi ssoci ated hydrocarbons in the vacuum systeri.

1. Calibration Conditions. A continuous flo-.v of high purity

(research grade) argon, with total gas cylinder impurities quoted to

be less than 10 ppm (99.999% pure), was used to purge the arc source.

The total flow was 5000 cc min"^ and was divided equally in a gas

manifold and directed to the three input gas ports. The output ports

vented directly into the laboratory. The pressure in the arc chamber

was equal to the ambient pressure, measured to be 1.00 atm, or 760 mm

Hg, + 1%.

The tungsten cathode was positioned in its holder using the

adjustment fixture supplied.

The arc was ignited by shorting out the anode (+) and cathode

(-) with a tungsten striking rod inserted into the arc channel.

After ignition, the cathode was withdrawn from the arc axis by

unscrewing the cathode holder exactly five rotations from its

inward most position.

The current was measured using the calibrated shunt, leads, and

digital voltmeter supplied with the arc.

The optical conditions for which the calibration is appropriate

are as follows:

a) The arc was aligned with its axis of symmetry along the

optical path. The center arc plate of the arc was 50 cm from the

entrance aperture, v/hich was .5 mm x 1 mm. (See fig. 1)

b) The spectrometer resolution was 0.4 nm.

c) A short gas cell is located between the mini -arc and the

vacuum system. It is normally purged with high purity argon at

atmospheric pressure. Thus, the mini -arc uv window is not exposed to

vacuum System contaminants during a calibration.

2. Mini -Arc Characteristics . Some properties of the mini -arc,

determined from measurements on models si mi liar to the one calibrated, are described below.

The irradiance from the mini -arc is approximately uniform over a much larger solid angle than that used for the calibration. By

observing the signal as the arc is rotated about its center, the

variation of irradiance with angle from the arc axis was measured.

This was done with signals at both 220 ^ 320 nm. The result is shown

in Fig. 2; no change with wavelength was observed. The irradiance is seen to be essentially uniform over an angle of 8 degrees,

corresponding to a solid angle of .015 rad (f/7).

(F-2) F-2-2
The irradiance of the mini -arc is a function of press- re. The

values in Table 1 apply to an ambient pressure of 1 atm. The

irradiance at other pressures is given ty

E(p) = (1.57p - 0.57) E(0)

where p is the pressure in atmosphere and ^(0) is the irradiance at 1 atm. This relation holds for all wavelengths.

Tests have shown that the MgFj window is the component most

likely to cause a systematic change in the arc irradiance.

Measurements on the NBS arc indicated a negligible change in the

irradiance of the arc plasma itself after 24 hrs continuous

operation. Window deterioration may arise from radiation damage or from vacuum system hydrocarbons. Some windows have shown a wavelength dependent change in transmission of a few percent after

several hours use. Therefore, periodic checking of the window

transmission is recommended, as described on page 1.

The ultraviolet continuum spectrum is interrupted by several atomic

and ionic emission lines. These lines arise from air and water vapor

impurities in the argon gas cylinder, the gas handling system, and

the outgassing arc chamber. Table 2 lists the v/avelengths where

these lines occur for an arc current of 40 A and indicates the

approximate relative intensity of the lines relative to the argon

continuum for a bandpass of 0.25 nm- These line intensities will

show large variations however, since they arise from small impurities

of the elements involved.

Figure 3 is a strip chart recording of a photoelectric scan of the

spectrum under these conditions. The width of the lines is

instrument-limited in this case. High resolution measurements

indicate that all the lines have a halfwidth of about 0.01 nm.

References

1. J.M. Bridges and W.R. Ott, Appl. Opt. 16, 367 (1977).

2. W.R. Ott, K. Behringer, and G. Gieres. Appl. Opt. 14, 2121

(1975).

3. G. Boldt, Space Sci. Rev. 11, 728 (1970).

4. H.J. Kostkowski, D.E. Erminy, and A.T. Hattenburg, in Advances

in Geophysics , Vol. 14, p. Ill (Academic Press, New York,

(1970).

5. W.R. Ott. J.M. Bridges and J.Z. Klose, Optics Letters 5^, 225

(1980).

(F-2) F-2-3
For the Director,

Wolfgang L. Wiese, Chief

Atomic and Plasma Radiation

Division

Center for Radiation Research

John M. Bridges, Physicist

Atomic and Plasma Radiation

Division

Center for Radiation Research

Attachments.

(F-2) F-2-4
Table 1. Calibrated spectral irradiance of Arc in units of W cm"^

nm"^. Values of the spectral irradiance are given for an arc current of 40.00 A, and at a distance of 50.0 cm from

the center of the arc. Estimated uncertainty is ^ 6% for X >^ 200 nm and ^ 10%

for X < 200 nm. The uncert€iinties are 2 standard deviations (2a).

Spectral Irradiance

Wavelength with window #71 Transmission

(nm) (W cm-^ nm'^ Window #71 Window #72

335 8.45x10-7 .931 .941

320 8.45x10-7 .929 .938

310 8.01x10-7 .929 .937

300 7.55x10-7 .925 .934

290 7.03x10-7 .917 .920

280 6.46x10-7 .905 .894

270 5.88x10-7 .893 .871

260 5.40x10-7 .890 .866

250 4.95x10-7 .892 .881

240 4.52x10-7 .900 .901

230 4.05x10-7 .905 .915

220 3.55x10-7 .904 .921

210 3.04x10-7 .904 .924

200 2.53x10-7 .903 .924

195 2.29x10-7 .902 .923

190 2.06x10-7 .899 .923

185 1.82x10-7 .895 .920

180 1.61x10-7 .891 .916

176 1.45x10-7 .887 .913

170 1.21x10-7 .879 .907

163 .995x10-7 .868 .895

159 .891x10-7 .861 .886

152 .734x10-7 .847 .864

144.5 .614x10-7 .830 .837

138 .559x10-7 .810 .808

(F-2)

F-2-5
Table 2. Survey of emission lines present in the 40 A argon mini -arc spectrum. The spectrometer bandpass is 0.25 nm.

Radiance of line peak

XCnm) Element relative to continuum

115.22 0 I 1 116.79 N T 1 117.69 K I 0 3

118. 94 r T 0 3

119.38 rw T 11 11 9. 99 T 3D

121 .57 H1 4 T 100

124.33 1

«

T 126.13 c T 2

127.75 c I 5

128.98 c I T 130.22 0 I 15

130.55 0 I 10

131 .07 N I 1 131.95 H I 0.5

132.93 c I 4

133.53 c II 8

135.58 c I 0.2

141.19 N I 0.7

143.19 C I 0 1 145.91 c I 0 1 146.33 c I 1 146.75 c I 0W.I1 148.18 c I n ?

149 26 N I 149.47 N I cD

155.10 c I 157.50 Ar II

160 04 Ar II n 7

160 35 Ar II n 1 160 65 Ar II

165.72 C I 2

174.27 K I 1 174.53 M I 0.5

175.19 C I 0.1

187.31 Ar II 0.05

188.90 Ar II 0.1

193.09 C I 2

247.86 C I 0.05

328.0-334. 0 0.1

336.5-342. 5 0.15

334.5-347. 0 Ar II and Ar I 0.1

348.5-350. 5 (broadened lines) 0.05

353 and above

(F-2)

^-'^-^
Figure 1 . Position of arc relative to entrance aperture

t I 8 I I t t I

{ L_

-5^ --f -S -/ O / 2^ 3 -fAngle of Rotation (degrees)

Figure 2

Relative signal vs angle of rotation of arc about its center.

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U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

Appendix F-3 Washington, d.c. 20234

REPORT OF TEST

VUV Spectral Irradiance of Deuterium Lamp

The Spectral Irradiance of Deuterium Lamp EF132 was calibrated for the

wavelength range 166-260 nm. An argon mini-arc radiance source was used to

calibrate the relative spectral irradiance as a function of wavelength over

this spectral range. An absolute scale was determined by calibrating the

irradiance at 260 nm with a tungsten lamp irradiance standard furnished by

the Radiometric Physics Division, CRR, NBS.

1. Method . A brief description of the method used to transfer from

spectral radiance standard (the NBS mini-arc) to a spectral irradiance

standard follows.

The calibrated area of the NBS mini-arc is the central 0.3 nm diameter

region where the discharge is approximately homogeneous. In order to use this radiance source to calibrate the irradiance of another source, the

radiation of the NBS arc from outside the central 0.3 mm region must be

blocked. This was done by the use of a set of two collimating apertures

located between the arc and the spectrometer. (One aperture also served as the entrance aperture to the spectrometer). The response of rhe spectroradiometer to the radiation from the region of the arc axis is then a relative measure of the sensitivity as a function of wavelength. For

measuring the irradiance of the source to be calibrated, the aperture

closest to the source was removed, so that the complete area of the unknown

source v/as seen by the spectrometer. To account for slightly different

solid angles of radiation entering the spectrometer from the two different

sources, a diffuser was placed behind the spectrometer entrance aperture

for both measurements.

2. Calibration Conditions . The D2 lamp was operated at a current of

300 ma. The lamp v/as at a distance of 50 cm from the entrance aperture of

the spectrometer, which was 2 mm x 1 .0 mm. The spectrometer entrance and

exit slits were of equal widths, giving a triangular slit function

(wavelength bandpass) with width at half maximum of 0.84 nm. The lamp was

F-3-1
operated inside an enclosure y^hi ch was purged with argon to prevent

absorption. An argon flow rate of 20i min"'^ was chosen, since an increase

in this rate produced no change in the signal at 170 nn. The spectral irradiance of D2 lamp EF132 is given in Table 1 for a current of 300 ma, at a distance of 50 cm.

3. Uncertainty . The estimated uncertainty of the spectral irradiance

values is ± 7% at 190 nm, and ± 10% for X < 170 nm. Uncertainties are 2a.

For the Director,

W. L. Wiese, Chief

Atomic and Plasma Radiation Division

Center for Radiation Research, NKL

Attachment

John M. Bridges

Atomic and Plasma Radiation Division

Center for Radiation Research

References

1. R. D. Saunders, W. R. Ott, and J. M. Bridges, Appl . Opt. 17, 593

(1978).

2. W. R. Ott, J. M. Bridges, and J. Z. Klose, Opt. Lett, 225 (1980).

(F-3)

F-3-2
Lamp EF132

Table 1. Spectral Irradiance of Lamp at 50 cm for current of 300 ma.

Wavelength Spectral Irradiance

X(nm) (W/cm3)

260 0.261

200 0.496

195 0.486

190 0.4 56

187 0.431

184 0.402

182 0.377

180 0.352

178 0.323

176 0.284

174 0.260

173 0.248

172 0.244

171 0.243

170 0.250

168 0.273

167 0.303

166 0.384

(F-3)

F-3-3
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS/SP-250/3

2. Performing Organ. Report No. 3. Publication Date June 1987

4. TITLE AND SUBTITLE

NBS MEASUREMH^IT SERVICES:

Radiometric Standards in the Vacuum Ultraviolet

5. AUTHOR(S)

Jules Z. Klose, J. Mervin Bridges, William R, Ott

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE ~

Gaithersburg, MP 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Same at item 6.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 87-619833

Document describes a computer program; SF-I8S, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bi bliography or literature survey, mention it here)

The radiometric calibration program carried out by the vacuum ultraviolet radiometry group in the Atomic and Plasma Radiation Division

of the National Bureau of Standards is presented in detail. The

calibration services are first listed, followed by descriptions of the

primary standards, which are the hydrogen arc and the blackbody line arc,

and the secondary standards, which are the argon mini- and maxi-arcs and

the deuterium arc lamp. Next, the calibration methods involving both

spectral radiance and irradiance are discussed along with their

uncertainties. Finally, the intercompari son of standards as a method of

quality control is described.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) arc (argon); arc (blackbody); arc (hydrogen); irradiance; lamp (deuterium);

radiance; radiometry; standards (radiometric); ultraviolet; vacuum ultraviolet

13. AVAILABILITY

Unlimited

I I For Official Distribution. Do Not Release to NTIS

[X] Order From Superintendent of Documents, U.S. Government Printing Office. Washington, D.C. 20402.

\23 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

137

15. Price

USCOMM-DC 6043-P80


PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

SP 250-1 N8S Measurement Services: SP 250-12

Spectral Radiance Calibrations

J.H. Walker, R.D. Saunders, and A.T. Hattenburg

SP 250-2 NBS Measurement Services: SP 250-13

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

SP 250-3 NBS Measurement Services:

Radiometric Standards in the Vacuun Ultraviolet SP 250-14

J.Z. Klose, J.M, Bridges, and W.R. Ott

SP 250-4 NBS Measurement Services:

Irradiation of Ferrous-Ferric Dosimeters

(Fricke- Dosimetry) SP 250-15

C.G. Scares, E.L. Bright and M. Ehrlich

SP 250-5 NBS Measurement Services:

Alpha-Particle Calibrations SP 250-16

J.M.R. Hutchinson

SP 250-6 NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J. Hsia, K.D. Mielenz, SP 250-17

and V.R. Weidner

SP 250-7 NBS Measurement Services:

Radiance Temperature Calibrations

W.R. Waters, J.H. Walker, and A.T. Hattenburg SP 250-18

SP 250-8 NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J. Hsia SP 250-19

SP 250-9 NBS Measurement Services:

Calibration of Beta-Particle-Etnitting

Ophthalmic Applicators

J.S. Pruitt SP 250-20

SP 250-10 NBS Measurement Services:

Radioactivity Calibrations with the "4ir" Ganma Ionization Chamber, and Other

Radioactivity Calibration Capabilities SP 250-21

J.M. Calhoun

SP 2.50-11 NBS Measurement Services:

Dosimetry for High-Dose Applications

J.C. Humphreys, D. Hocken, and W.L. McLaughlin

NBS Measurement Services: Neutron Personnel Dosimetry

R.B. Schwartz

NBS Measurement Services:

Activation Foil Irradiation

with Cal iforniuTi Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Activation Foil Irradiation by Reactor Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Photometric Calibrations

R.L. Booker and D.A. McSparron

NBS Measurement Services:

Calibration of X-Ray and Gamma-Ray

Measuring Instruments

P.J. Lamperti, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

NBS Measurement Services: Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

J.T. Weaver, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

Spectral Irradiance Calibrations

J.H. Walker, R.D. Saunders, J.K. Jackson,

and D.A. McSparron

NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrumentation and Sources

J.S. Pruitt, C.G. Scares, and M. Ehrlich
U.S. Department of Commerce

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