REFERENCE

IITED STATES RTMENT OF IMMERCE

LICATION

A*10' Co. Woi /

NBS TECHNICAL NOTE 729

I

Design, Construction, and Testing

Of a New High Accuracy

Spectrophotometer

U.S.

ARTMENT

OF

)MMERCE

100

U575S

10.72.1

1172.
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety.

The Bureau consists of the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, the Center for Computer Sciences and

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THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

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THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition,

and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination

and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
15753

17^

Design, Construction, and Testing

Of a New High Accuracy

Spectrophotometer

K. D. Mielenz and K. L. Eckerle

Institute for Basic Standards

\J. i -> < National Bureau of Standards

Washington, D.C. 20234

\

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary

NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acting Director,

Issued June 1972
National Bureau of Standards Technical Note 729

Nat. Bur. Stand. (U.S.), Tech. Note 729, 60 pages (June 1972) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:729). Price 60 cents.
TABLE OF CONTENTS

Page

Introduction 1

Spectrophotometer 2

A. Light Sources 2

B. Source Optics 3

C. Intensity Attenuators 3

D. Predisperser 4

E. Monochromator 4

F. Sample Compartment Optics and Light Beam 5

G. Polarizer 6

H. Double Aperture Linearity Device 7

I. Sample Holders 7

J. Averaging Sphere 8

K. Detector 8

L. Anode Current Measuring System 8

M. Data Acquisition and Data Processing 10

N. Equipment 10

Alignments 10

A. Monochromator Alignment 10

B. Alignment of Optics 11

Performance and Operation of Spectrophotometer 11

A. Measurement of Stray Radiant Energy 11

B. Double-Aperture Linearity Testing 12

C. Transmittance Measurements 16

Results of a Specific Test 18

Appendices 19

A. Discussion of Beam Non-Uniformity 19

B. Derivation of Efficiency of Averaging Sphere 20

References 24

XII

DESIGN, CONSTRUCTION, AND TESTING OF A

NEW HIGH ACCURACY SPECTROPHOTOMETER

K. D. Mielenz and K. L. Eckerle

A new spectrophotometer is described which has an accuracy of approximately 0.0001 transmittance units. The

spectrophotometer utilizes a collimated beam in the sample

area. This is accomplished by means of off -axis parabolic

mirrors in the monochromator and sample compartment. Also,

circular holes are used as entrance and exit apertures in

the monochromator. All components of this spectrophotometer

were chosen to achieve maximum accuracy. The result of

this work is a "state of the art" instrument. The instrument was tested to evaluate its performance. Systematic

errors such as detector non-linearity, stray radiant energy,

and beam non-uniformity are measured. A correction for

non-linearity of the photomultiplier and electronics is

applied.

Key Words: Beam non-uniformity; circular entrance and exit

apertures; high accuracy spectrophotometer; linearity;

parabolic mirrors; stray radiant energy.

I. Introduction

The purpose of this work was to build a spectrophotometer with

which transmittance measurements can be made accurate to within approximately 0.0001 transmittance units. This goal demands an instrument with

a minimal number of systematic errors which are as well defined as possible and which can be measured. At the same time, the spectrophotometer was designed to have flexibility in its immediate usage as well as

a capability for improving and extending its range of usefulness.

A predisperser and monochromator with wide wavelength range,

accurate wavelength settings, and narrow spectral bandpass were used.

The choice of spectral bandpass for the monochromator was dictated by

studies made by means of a commercial recording spectrophotometer and a high dispersion Czerny-Turner grating spectrograph [l] 1 on the resolution

necessary to measure accurately some absorption bands of a holmium oxide

glass. These measurements were assumed to exhibit a worst possible case

that would ordinarily be encountered. Figure 1 shows how the transmittance of the holmium oxide glass between 440 nm and 465 nm varies with

Figures in brackets indicate the literature references on page 24.

1
slit width of a commercial spectrophotometer. [2] The optical density

at the band peaks at 445.6 nm and 460 nm is plotted versus spectral

bandpass in Fig. 2. It is seen that for this case a spectral bandpass

of approximately 0.2 nm is necessary for an accurate measurement. In

view of the large dependence of transmittance on spectral bandpass, a time exposure plate of the absorption band at 445.6 nm was made by means

of the high dispersion grating spectrograph to determine that the band

has no fine structure. [3]

Off-axis parabolic mirrors are used in the monochromator and exit

optics. Circular apertures are used as entrance and exit apertures in

the monochromator so that a well-collimated beam is provided in the sam- ple compartment. The use of off-axis mirrors also eliminates interreflections that are present in lens systems. Since the beam is collimated, there is no change in focus upon insertion of a plane sample as there is in a converging beam.

A large sample space was incorporated into the spectrophotometer

to allow for flexibility both in type of measurement and in the methods

of measurement. For example, other light beam geometries can be simulated in this spectrophotometer. A detector and electronics that are both precise and linear were used to minimize corrections necessary for

any measurement. Extensive testing described in sections II, III, and

IV below was done to characterize the performance of the instrument.

It is believed that this design; namely, the exclusive use of

off-axis mirror optics, and the placement of the samples in a collimated

beam, constitutes the optimal beam geometry for high accuracy spectrophotometry. A detailed analysis supporting this belief may be found in

References [4] and [5].

II. Spectrophotometer

A schematic drawing of the spectrophotometer is shown in Fig. 3.

The spectrophotometer except for the electronics and controls is con- structed on a 122 cm by 244 cm surface plate. The optical path from the

predisperser through the photomultiplier tube is enclosed in a light

tight box. The interior of the box was coated with a flat black paint.

The optic axis was chosen to be 33 cm above the surface of the plate.

Photographs of the source side of the instrument, of the exit optics

and the detector apparatus, and of the control panel and electronics are shown in Figures 4, 5, and 6.

A. Light Sources

Three platforms for sources of various sizes and shapes were provided. Source mounting tables having three spring loaded screws for

adjustment were constructed for a tungsten lamp (rated for 6 V and 18 A)

with a ribbon filament 14 mm long by 2 mm wide (for photometric measurements) and for atomic line sources (for calibrating the wavelength

scale)

.
The current in the tungsten lamp is regulated by a power supply

using external sensing (.01% regulation) on a special 0.1 fl shunt rated

for 50 A. A 50 fl precision potentiometer and a 15 fl precision potentiometer in series provide coarse and fine current controls. The reason

for monitoring the voltage across the 0.1 SI shunt is greater stability

since one does not depend on the stability of a resistor internal to the

power supply. Since the resistance of the shunt is constant, the system

provides a constant current through the lamp. The lamp is operated in

the color temperature range of 1800 °C to 2600 °C as measured by an optical pyrometer. The lamp ribbon is aligned vertically by means of a plumb line.

A 2 mW helium-neon laser was installed in the system for alignment

of the optical components and of specimens to be measured. This laser

is mounted in two concentric rings each of which contains three threaded

screws 120° apart for leveling and adjustment as shown in Fig. 4.

B. Source Optics

The optical elements on the source side of the instrument are mounted on a 122 cm optical bench. One or more 2.5 cm diameter aluminized

optical flats are necessary to divert the light beam along the optical

bench. These mirrors are mounted on a support of two gimbal rings

independently suspended about vertical and horizontal rotation axes. A

7.6 cm diameter aluminized mirror is placed at the end of the optical

bench to divert the beam into the predisperser . This mirror is mounted

on a 4-point spring-loaded device that has micrometer driven pins to

rotate the mirror about two orthogonal axes. All of the mirrors can

be adjusted with an angular resolution of the order of 1 arc second. The

mirrors are all mounted on sturdy supports; and except for the two mirrors

immediately following the laser, the mirrors are also attached to translation stages which move perpendicular (horizontally and vertically) to

the light beam. The light beam is collimated over a portion of the

optical bench before being focused on the predisperser. This allows

convenient insertion of optical elements such as attenuators in the

entrance beam. The source collimating lens is a 60 mm diameter coated

glass achromat with 400 mm or 600 mm focal length depending on the size

of the source. The lens which focuses an image of the source on the

predisperser slit is a 200 mm focal length coated glass achromat having

a diameter of 41.5 mm. Both lenses are stopped down with 2.5 cm square

beveled apertures so that the light beam slightly over-fills the grating

of the monochromator

.

C. Intensity Attenuators

The tungsten source intensity reaching the detector can be varied

by changing the current in the lamp or by rotating an inconel coated

neutral density wedge. The neutral density wedge is 15.2 cm in diameter,

varies in optical density to 2.0, and is rotated remotely by means of a stepping motor attached to the circular wedge. It is used primarily to

attenuate the beam for linearity measurements rather than for transmittance measurements since it is placed after the collimating lens and

3
introduces additional inhomogeneity in the beam. It is also possible

to replace the neutral density wedge by a rotating Glan-Thompson placed

in the optical path as shown in Fig. 3 and discussed in Section II. G.

D. Predisperser

The predisperser is an Ebert quartz prism instrument with a 500 um

entrance slit. It has 15.2 cm focal length collimating and focusing

mirrors and a horizontal aperture ratio of f/6. The wavelength range of

this instrument is from 180 nm to 1.7 um. The wavelength driveshaft of

the predisperser is coupled to the monochromator wavelength drive to

synchronize their movements. [6]

E. Monochromator

The monochromator is a 1 m Czerny-Turner system with effective

aperture of f/8.7. The manufacturer states that its useful wavelength

range is from less than 200 nm to greater than 30 um if the appropriate

grating is chosen. The grating presently in use is a plane grating 102 mm

by 102 mm. The grating has 1200 grooves per mm, is blazed at 500 nm,

and has a useful wavelength range from approximately 185 nm to 1.3 um. The monochromator has a linear dispersion of 0.833 nm/mm.

The monochromator collimating and focusing mirrors are off-axis

parabolas. The surfaces of the mirrors were specified to be paraboloidal

to within 0.1 fringe in green light and to be free of zones. These

mirrors travel along a rail in the base and are both driven by micrometer

screws

.

The entrance and exit apertures of the monochromator can each be

chosen from a selection of 4 circular holes having diameters of 250, 500,

750, or 1000 um. These apertures can be rotated alternately into position. The exit aperture thus approximates a point source with bandpass of about 0.2, 0.4, 0.6 or 0.8 nm. This was checked by scanning the

atomic line of the Hgl spectrum at 546.07 nm. At the same time, the

deviation from concentricity of the four apertures was determined.

Repeatability of switching a particular aperture into place is within the

uncertainty of the measurements. The exit apertures were blackened

chemically so that their specular reflectance is approximately 7%.

The wavelength dial of this instrument can be set to a precision

of 0.01 nm, which is the least count of the output vernier. This dial is

calibrated using atomic lines of Hel, Nel, and Hgl with peaks

throughout the wavelength range of 380 nm to 850 nm. The results are repeatable to 0.01 nm. It was judged from the resulting calibration

curve (see Figure 7 for the data for the 250 urn apertures) that at any

arbitrary wavelength in the above range, the dial can be corrected

accurately to 0.02 nm. It was verified that wavelength corrections for

other size pinholes differed by a constant amount from the corrections

for the 250 um pinholes. The temperature coefficient for the wavelength

correction is approximately -0.01 nm for a 1 °C increase in temperature.
Since the light path through the monochromator is opposite to that

for which the monochromator was designed, [6] several changes in the

baffles were necessary to eliminate possible sources of stray radiant

energy in this instrument. The original entrance slit baffle was placed

at the new entrance slit. An identical baffle (with the beveled edge

on the opposite side) was installed at the new exit slit. Since the

oversize collimating mirror (original focusing mirror oversize to accom- modate a camera attachment) is now present , it was stopped down with a

mask. A baffle attached to the cover approximately midway between the

entrance slit and the new collimating mirror was enlarged since the

collimating mirror was stopped down. Since the interior mounting platform of the monochromator was highly reflective, it was covered with a black velvet cloth.

F. Sample Compartment Optics and Light Beam

After the light beam leaves the monochromator, a 2.5 cm diameter

optical flat diverts the beam into an off-axis parabolic mirror as shown

schematically in Figure 8. This produces a 20 mm square collimated light

beam. At the other end of a 122 cm precision optical bench, the light

beam is collected by another off-axis parabolic mirror and is diverted

into an averaging sphere by another optical flat. The optical flats

each have a beveled edge to allow the light beam to be unobstructed.

They are attached to rods and are mounted off center on three point

spring loaded supports. The angles of incidence (50°) on these flats are

close to the angle of restored polarization so that these flats will not

significantly depolarize the light. The parabolic mirrors have a 3.4 cm

diameter and 195 mm focal length. They are off-axis by an angle of

8.5° and are free of zones. The optic axes of the two parabolic mirrors

are on opposite sides of the optical bench i.e., the parabolic mirrors

are mounted in a

M z" geometry.

The parabolic mirrors are mounted in gimbal supports similar to

those discussed above which are in turn mounted to two micrometer driven

translation stages which give motion in two directions parallel and

perpendicular to the light beam. The translation stages are in turn

attached to a 5.1 cm diameter cylinder which slides in another hollow

7.6 cm diameter cylinder which is attached to the optical bench to give

a vertical adjustment.

The parabolic mirrors were tested individually and together in the

"z" geometry by means of a shearing interferometer [7] as shown in

Fig. 9a, b. The laser (632.8 nm) and microscope objective provided a point

source of light at the focal point of the mirrors. The angle between the

principal axis of the incident light and the reflected light was 8.5°.

The high quality 192 mm focal length lens was used when the mirrors were

tested individually. Pictures of the resulting fringe patterns were

taken and were evaluated perpendicular to the fringes at increments of

the shear. The deviation from straightness of the fringes was measured

at the center of the mirrors, and also at half their radius on either

side, both with the shear horizontal and vertical. No significant

difference was obtained from measuring the mirrors individually and

5
together in the "z" geometry. The average deviation from a spherical

wave front was found to be approximately 2/5 of a fringe for the horizontal shear and 1/5 of a fringe for the vertical shear.

The parabolic mirrors are aligned from geometrical drawings as a first step. The fine alignment of the first parabolic mirror is done

by moving a 5 mm diameter aperture from one end of the optical bench to

the other adjusting the mirror until the intensity along the optical

bench remained constant.

Measurements were also made of the uniformity of this collimated

beam. A 5 mm diameter aperture was scanned across the beam in a vertical

and horizontal direction at different wavelengths. The results can be

seen in Figs. 10 and 11. It is seen that in the vertical direction the

signal is constant to ± 0.1%. In the horizontal direction the light beam

has a gradient which was found to be increasing with wavelength. This

gradient is attributed to a variation of blaze angle across the grating

since the diamond head which cuts the grooves wears out as preparation

of the master grating is made.

The non-uniformity of the beam has no effect on the measured

transmittance of a homogeneous filter. If the filter is not homogeneous,

the corresponding measurement error is, according to Eq. A6 in Appendix

A,

AT = T

A

- T = jo" T aSx2

. (1)

This is negligibly small even if filter and beam are quite inhomogeneous

.

For example, if the beam non-uniformity is as large as 10% over a 10 mm

area (as in the worst case shown in Fig. 11), and if the filter transmittance varies by 1% over this area, then a = 10_3 /mm, 3 = 10~2 /mm, x = 10 mm, and

AT/T = 8 x 10" 5

. (2)

The error is eliminated altogether if the mean is taken of the two

transmittance measurements before and after the filter is rotated by 180°

so that its gradient is reversed. The above numerical example indicates

that these two measurements would differ only if the filter is grossly

inhomogeneous (e.g.; by more than_l% over a 10 mm area). In such extreme

cases, the average transmittance T is hardly a meaningful quantity, and

the filter should be mapped by measuring T(x) with apertures of small

width. Because of the dependence of AT on x

2 in Eq. 1, beam nonuniformity will not noticeably affect such measurements.

G. Polarizer

A Glan-Thompson prism in a rotatable mount is placed in the beam to

produce a defined state of polarization. This is necessary since it is
W'ell known that tungsten sources and grating monochromators produce

polarized light whose state of polarization varies with wavelength. An

example of the polarization of the beam is shown in Fig. 12 (polar plot

with an arbitrary scale of the transmittance of a sheet polarizer) at

564 nm and computed to be 23%. Zero degrees denotes the orientation of

the polarizer at which the electric vector is perpendicular to the

grating grooves. The presence of Wood's anomalies which are due to the

grating further complicates the state of polarization of the beam. The

polarization of the same models predisperser and monochromator illuminated

with a xenon source as measured at the manufacturer's plant is shown in

Fig. 13 for the wavelength range of 410 nm to 600 nm. The degree of

polarization is expressed as T^qo/Tqc where Tgg° and Tq° are the transmittances of a sheet polarizer with the electric vector parallel and

perpendicular to the grating grooves.

The Glan-Thompson prism can be placed in the collimated beam on the source side of the instrument or between the two off-axis parabolas in

the sample compartment. If the prism is placed on the source side, it

must be verified that the monochromator does not produce partially

polarized light for the wavelength at which the measurements are made.

It was found for several wavelengths at which measurements were performed

that the prism could be put on the source side as long as the electric

vector was either parallel or perpendicular to the grating grooves. If

the prism is placed in the sample compartment, it must be tilted with

respect to the optic axis to avoid inter-reflections between the prism

and the sample. However, the prism can not be tilted too much since it

polarizes for only small angles of incidence.

H. Double Aperture Linearity Device

A two aperture linearity tester with 5 mm diameter beveled holes

whose centers are separated by 10 mm was constructed and can be placed

in the sample compartment (see Fig. 14). These apertures are opened and

closed by two cylinders driven by compressed nitrogen. These cylinders

are opened and closed by two-way gas valves in which the direction of

flow is controlled by two solenoids located remote to the detector. The

linearity test is automated by means of a data acquisition and control

system as discussed below in Section IV.

I. Sample Holders

Two sample holders were constructed utilizing four-position Geneva

mechanisms—thus providing for 3 samples plus 1 clear space. These

Geneva mechanisms are driven by reversible synchronous motors which

rotate at 4 rpm and have a torque of 8600 gem. When the motor rotates

360°, the Geneva mechanism rotates 90°. The holders have adjustments so that the filters can be oriented relative to the beam in a normal mode

or tilted slightly. The filters can be scanned across the beam horizontally or vertically. Behind the sample holders, a compressed nitrogen

operated remotely controlled shutter was placed to determine the "zero"

level of the electronics.
J. Averaging Sphere

After the light is collected by the final parabolic mirror, it

passes into a six inch diameter averaging sphere whose interior is

coated with a 1/8 inch layer of pressed barium sulfate. This sphere

was carefully constructed so that no first reflections from the sphere

wall can reach the photocathode of the detector whose housing is attached

to the sphere. Another compressed nitrogen operated, remotely controlled

shutter was constructed between the sphere and the photomultiplier tube.

The design of the sphere was dictated by the size of the photocathode

(masked down to 44 mm diameter) and by questions of efficiency. The

light beam is focused at the entrance port of the sphere as seen in

Fig. 15. The efficiency of the sphere can be shown (see Appendix B) to

be:

(A'A^/TTe2A ) r

2

e = (3)

1 - r[l - (A + A')/As

]

In our case, AD % 14.8 cm2

, A

1 % 19.4 cm2

, As % 645 cm2, A %

2.58 cm2

, r % .98 and e ^ 3.8 cm; so that the calculated efficiency is

found to be % 20%.

K. Detector

The photomultiplier is an 11 stage end-on device with an S-20 surface. The dynode chain is a linear potted resistor chain with a zener diode between the cathode and the first dynode which was purchased from

the manufacturer of the detector. The high voltage power supply is

voltage regulated to .001%.

L. Anode Current Measuring System

The anode current is measured by means of a current to frequency

converter. This instrument together with a high accuracy counter (used

with 10 second integration time) was found to be linear to .01% or better

by measuring currents produced in precision resistors by a calibrated

voltage source (.01% accuracy). By this same method, the current to

frequency converter and counter were found to be precise to at least

1 part in 45,000 on the current ranges normally used. Low noise cable

is used.

The electrical output of the converter is not a true frequency;

the translation of the current input is events per unit time. The waveform at the output is a series of constant width rectangular pulses whose

number per unit time is a linear function of the input current to the

converter. The pulse rate (number of pulses per second at any instant)

is not necessarily constant for a fixed value of input current, for in

the presence of input noise the output pulse rate will be random over a short interval. Yet, it will yield the correct pulse count during the

relatively long 10 second measuring period of integration since the

8
random variations will be averaged.

The constructed converter is basically identical to the circuit

described by Taylor [8]. Some component changes were made in order to

enable the circuit to function in the desired manner—mostly at the

"single-shot" stage following the clock oscillator and the "flip-flop"

stage following the gate. Additional circuit components were added at

the input of the operational amplifier consisting of a fuse and pair of

low-leakage shunt diodes for the purpose of protecting the operational

amplifier from damaging overload by excessive excitation from the photomultiplier tube. A low pass filter was also added to the input line

for removing higher frequency noise components that may be present in the

input signal which could interfere with the subsequent gating function.

A shunt diode was added to the output line of the operational amplifier

stage for the purpose of negative clipping at the base of the transistor

stage driven by the operational amplifier thus keeping its emitter to

base voltage below the maximum rating.

The use of an event counter with a precise measuring gate time in

conjunction with the converter yields an input-output transfer expression

having the following equation for noise rejection:

rejection in dB = 20 log(irF T )-20 log(sin ttF.

tT_) (4)

where Fjj is the noise frequency and Tq is the integrating gate time within the counter. The first term is recognized as the attenuation characteristic of a single section low pass filter. The second term shows

additional notches of infinite rejection for periodic noise whose frequency, Fjj, represents multiple fractions of the period Tq. Experience

shows that it is the low frequency noise that precludes the precise

measurement of low level signals in the area of use by the converter.

For example: considering a noise frequency Fjj of 1 Hz and a gating time

Tq of 10 seconds, the achievable attenuation with the converter computed from the first term of the above expression is 30 dB on a voltage

or current basis. It is difficult to approach this performance with

either lump-passive or with active low-pass filters. The problem with

passive low-pass filters is the wasted time in waiting for the output

to reach final value; it takes 9.2 RC time constants to reach 0.01% of

the final value. Active low pass filters can exhibit fast response time

for an equivalent attenuation but their settling time precision is not

compatible in general with the desired 0.01% figure.

The gated-integrator feature of the described converter has a fixed reading accuracy figure that is dictated by the precision of the

gating time which can be a part per million or better in most counters

while the accuracy figure with a low pass filter is related to the time

in reaching final value. Carefully designed multi-section low-pass

filters can shorten the response time while preserving the desired

attenuation.
The dark current from the photo-multiplier tube is shown in Fig. 16 as a function of dynode voltage. This dark current was measured by

closing the shutter between the sphere and the detector. No measureable

stray light was detected upon opening this shutter.

M. Data Acquisition and Data Processing

Data are obtained by means of a data acquisition and control system

consisting of a teletypewriter, logic circuits, and four electrical

switches. The control system has start and stop codes, codes for recording data, and codes for sequentially actuating the electrical switches.

These can be programmed by means of a punched-paper tape. When the

counter finishes its integration, it signals the acquisition system to

record a datum point on punched-paper tape. For measurements which are

not yet automated, the count is initiated by pushing a start botton. For

automated measurements, the counter is set to repetitively count for

ten seconds after waiting ten seconds. The switches are used to open

and close apertures and shutters automatically for linearity measurements

by means of a network of diodes and relays. The teletypewriter is also

used as a computer terminal by means of an acoustic coupler to process

the punched-paper tape.

N. Equipment

The more important pieces of apparatus, their model numbers and

manufacturers are listed in Table I. [9]

III. Alignments

A. Monochromator Alignment

It was established by means of a Twyman-Green interferometer that

the surface of the grating was flat to approximately 2/5 fringe in both

the directions parallel and perpendicular to the grooves (see Fig. 17).

An autocollimator , focused on infinity for the observer's eye, was used

to align the parabolic mirrors as shown in Fig. 18 using a Hg lamp

focused on the entrance aperture. The light reflected from each mirror

individually via the grating in the zero order was observed and the

mirror under test was adjusted until the light reaching the autocollimator was parallel. However, since the grating had been found to deviate

from flatness, this was not the final setting since the parallelism of

the light was influenced by the grating. It was judged, however, that

the relative separation of the settings for the two mirrors should be

correct. Thus, the two mirrors were moved by fixed amounts in the

same direction to correct for abberations of the grating.

A fine adjustment can be made with the Foucault test. The direction in which the darkness crossed the grating was observed for a given

direction of rotation of the wavelength screw as observed at the exit

slit. Then, the mirrors could be adjusted (always moving them both in

the same direction and by the same distance) . The distance moved was

10
always halved between observations, thus converging on the true position. These settings were checked with the Hg lamp for the zero order,

for the first order green line (546.07 nm) , and for the second order

green line. Finally the Hg lamp was focused on the exit slit, and the

Foucault test was repeated at the entrance aperture.

B. Alignment of Optics

The optical system is aligned in the following manner. The laser

beam is adjusted until its path is 33 cm above the surface plate before

and after each reflection from a mirror and parallel to the optical

bench on the source side of the instrument. Lenses can then be introduced into the beam and adjusted so that they do not deflect the laser

beam. The monochromator ' s three leveling screws are then turned until

the laser light hits the center of the monochromator collimating and

focusing mirrors and the grating which are defined by masks with circular

holes in the centers. After the light leaves the monochromator, it is

33 cm above the surface plate, hits the centers of the parabolic mirrors,

and is parallel to the optical bench on the exit side of the monochromator. The positions of the optical flats and the parabolic mirrors are determined by the distances measured from a geometrical drawing of the

light path accounting for the 8.5° off -axis parabolic mirrors and the

20 mm by 20 mm size of the beam when the grating is fully illuminated.

The position of the averaging sphere is determined from a geometrical

drawing

.

IV. Performance and Operation of Spectrophotometer

A. Measurement of Stray Radiant Energy

Stray radiant energy which passes through the predisperser and

monochromator was measured by three independent methods . The largest

apertures were used for these tests (1 mm diameter) giving the worst

possible amount of stray radiant energy.

The first test follows the ASTM method E387 [10] using a .005%

methylene blue aqueous solution. A 1 cm cell of this solution was measured on a commerical recording spectrophotometer. The solution

transmits the yellow and blue light as well as some red and also has a strong absorption band at 660 nm. A 1 cm cell of this solution was measured at 660 nm on the spectrophotometer under discussion here and the

percentage of scattered light plus the transmission of the solution was

found to be 1.4 x 10-l+ which is of the same order of magnitude as measured on the commercial spectrophotometer by placing two screens in

its reference beam. It was therefore judged that a 2 cm solution would

have negligible transmission at 660 nm, and the percentage of stray

radiant energy as measured from a 2 cm cell was found to be about 5

parts in 100,000.

The second method employed the measurement of two selenium red

glasses each of which had a transmittance of less than 1 x 10_lf at

11
400 nm, 450 nm, and 500 nm as measured on the commercial recording spectrophotometer. The transmittance measured on the high accuracy spectrophotometer at 400 nm, 450 nm, and 500 nm for one selenium red glass and

two selenium glasses was found to be:

Wavelength Transmittance of Transmittance

(nm) one glass of two glasses

400 1.4 x 10" 6 1.8 x 10" 6

450 1.5 x 10" 6 1.4 x 10-6

500 5.3 x 10"6 3.0 x 10" 6

This indicates stray radiant energy of a few parts in one million from

the red portion of the spectrum.

Finally, the intensity of the helium-neon laser light whose spatial

coherence had been reduced by a beam expander and an oscillating ground

glass was measured at 632.8 nm. Fractions of this value were measured

at other wavelengths giving an indication of the stray radiant energy.

Typical values obtained at several wavelengths are listed below:

Wavelen gth Fraction of

(nm) Stray Radiant Energy

400 2.3 x 10" 6

500 2.1 x 10~ 6

600 6.3 x 10" 6

630 69.5 x 10-6

700 8.4 x 10-6

Double-Aperture Linearity Testing

The purpose of this test is to determine a linearity correction,

AT, which can be applied to the measured transmittance, Ta , such that

the true transmittance, T, is:

T = T + AT

a

Since a complete discussion of linearity testing using a double aperture

has been published elsewhere [11] , this description will deal only with

practical details.

12
Studies were made of the observed signal as a function of time.

Three typical cases are shown in Fig. 19. In all cases, the drift was approximately linear over a 30 minute time interval. The slope of the curve was not always negative.

Three quantities are measured in the double aperture method: 1(A),

the intensity reading for aperture A alone; 1(B), the reading for

aperture B; and I(A+B), the reading for both apertures at once. These

determine a quantity cx(A,B) defined by

1 + °<A > B) - I(l)tl(B) < 6 >

If a(A,B) is not zero, the system is not linear.

Defining <j> as the total flux when both apertures are open, a(cf>) is determined in the following manner. The data acquisition and control

system is programmed to actuate the valves operating the shutters such

that 27 readings, 0, A+B, 0, A, 0, B, 0, A+B, 0, B, 0, A, 0, A+B, 0, A, 0,

B, 0, A+B, 0, B, 0, A, 0, A+B, are effected at 20 second intervals and

recorded on punched-paper tape. The signal counter initiates a count

after a 10 second interval, counts for 10 seconds, and then activates

the data acquisition and control system which punches a datum point on

paper tape and then opens or closes the appropriate shutters after which

the cycle is repeated.

Here, "0" is a dark-current reading; and "A", "B", and "A+B" are

the readings obtained with either or both apertures open. A computer

program was written to perform the following calculations. The dark

currents on both sides of "A", "B", or "A+B" are averaged and subtracted

from these readings, yielding a sequence of 13 adjusted signal currents,

Il(A+B), Ii(A), Ii(B), I 2 (A+B), I 2 (B),...I 4 (A), I 5 (A+B) .

The averages 1(A), 1(B), and I (A+B) are then used to compute

a(4>) = I (A+B)/ [1(A) + 1(B)] - 1 , (7)

as defined by Eq. 6. In order to determine the standard deviation of

this value of a, the 13 quantities

Il(A+B) Ii(A) Ii(B) 1 2 (A+B) I

2 (B) 14(A) 1 5 (A+B)

Y — (Q\

1

I (A+B) ' 1(A) ' 1(B) ' I (A+B) ' 1(B) ' 1(A) I (A+B)

are fitted by least squares to a linear function of time,

13
\ - P + qt

±

(9)

where q is the average drift for 40 seconds. Typical values calculated

for q ranged in magnitude from 0.001% to 0.008%.

The standard errors in 1(A), 1(B), and I(A+B) are given by

'[Ii (A)-T(A7(p+2q) ] 2+[I2 (A)-T(A7(p+6q) ] 2+. . A

±/2

dl(A) = | ) (10)

12

dl(B) =

[I 1 (B)-I(B)(p+3q)] 2+[I2 (B)-I(B)(p+5q)] 2+...\ 1/2

(11)

12

and

1/2 [Il(A+B)-I(A+B) (p+q)] 2+[I 2 (A+B)-I(A+B) (p+4q)] 2+. .

.

dI(A+B) = I I (12)

20

The uncertainty in a is then given by equation (20) in reference [11] as

[doWV2 =- ———— <[l+a(<j>)] 2 [dI(A) 2+dI(B) 2 ]+dI(A+B) 2

l (13)

+ I(B)1 2 l ' [1(A) + 1(B)]

Since a is a small quantity, the statistical fluctuations must be

sufficiently small to make a meaningful measurement. The quantity which

was used as a criterion for a valid measurement was:

^

% dY. =

13

£

-*

Y, - P

- qt.

i=l 12

1/2

(14)

This quantity is plotted in Fig. 20 versus I. As indicated by the solid

line, the relationship between dl and I is, approximately

dl

I

Vl0/I x 10~7 if I < 10" 7 A

10-k if I > 10~ 7 A

(15a)

(15b)

14
The signal-to-noise ratio, i/dl, being proportional to VI for

the small anode currents below 10~ 7 A suggests that, in this region, the

flux measurements are limited by phot omultiplier shot noise. For a photomultiplier with linear dynode chain the theoretical expression for

the shot noise, due to photon noise of the incident flux and noise con- tributions from the photocathode and the dynode chain, is [12]

Idl

_ /nT<6 - 1)

\ 6 k + 1 _ Jlx(6 - 1)

\ q6

k + 1 (16)

where n = i/q is the average arrival rate of electrons at the anode,

q = 1.6 x 10" 19 As is the electron charge, x is the integrating time,

6 is the gain per dynode, and k is the number of dynodes. In our case

t = 10 s and k = 11, so that eqs. (15a) and (16) may be identified with

each other if 6 12 /(6 - 1) ^ 6 x 106

, or 5 % 4. Since this is the correct

order of magnitude [13] for 6, the measured signal uncertainty dl for

1 < 10-7 A may indeed be interpreted as photomultiplier shot noise. The

limiting value (15b) of dl for signal currents I > 10-7 A is attributed

to lamp noise.

The above method only defines a for one value of the flux <\>. A

plot was made of a versus I for values of <j> yielding anode currents

ranging from 3 x 10-6 to 6 x 10~9 A as shown in Fig. 21. These measurements were made with high voltages on the photomultiplier tube of 750V,

850V, and 950V. It was found that the non-linearity was independent of

which of these three voltages was used; but rather, the non-linearity

depended on the anode current.

It is shown in reference [11] that it is possible to measure a(T<J>)

for evenly distributed values of T rather than attenuating by factors of

2 as is customary. To measure a(T<j>) versus T, a lamp current is chosen

so that the unattenuated flux <j> through both apertures corresponds to

the 100% point of the transmittance scale. After a(cj)) has been measured

as described above, the incident flux is attenuated in four 20% steps,

a(T<f>) is measured for T = 0.8, 0.6, 0.4, and 0.2. This series of

measurements is then repeated four times.

An example of the results is shown in Fig. 22. In all cases, the

dependence of' a on T could be represented in the form of a parabola,

a(Tcj>) = aT + bT2. (17)

A statistical analysis showed that the residuals had no pattern suggesting

higher order terms. As shown in Eq. (19) of reference [11], the nonlinearity correction AT is then given by

15
AT = 2aT (1 - T ) + £ (a 2 + b)T (1 - T ) a a 3 at a_

1 + 2a + |- (a2 + b)

(18)

An analysis was done using Ta , a(T<j>), and da(Tcj)) to compute AT and its

standard error as a function T [14] . The individual data for a were

weighted by the reciprocal square of the da of Eq. 13. The standard

error for AT was computed from a propagation of error formula based on

Eq. 18. A typical curve of AT versus T is shown in Fig. 23 with the

shaded area denoting ± one standard deviation. The non-linearity correction, AT, can be determined to a high degree of precision (see

Table II) . The linearity correction was found to be independent of source polarization, free from interference errors, but slightly dependent on wavelength. Columns headed 0° and 90° in Table III show that

for the electric vector perpendicular and parallel to the grating

grooves, there is no difference in linearity to 1 part in 100,000. The

column headed by ± 45° shows that for two polarizers at the double

apertures at 90° with respect to each other and at 45° with respect to

another polarizer, there is no significant change in linearity as com- pared to columns 2 and 3. This shows that the measurements are not

biased by interference. However, comparison of the columns headed by

525 nm, and with the columns headed by 400 nm and 750 nm shows a slight

dependence on wavelength. This difference was verified by a statistical analysis of the data [14].

C. Transmittance Measurements

The random error dT„ of the measured transmittance Ta is given by

dT

V

dl(TJ)

a

KT<j>)

+

dl((j))

(19)

where <J> is the radiant flux corresponding to the 100% point of the

transmittance scale. This relationship can be used, together with the

values of dl/l from Eqs. (15a, b) to calculate dTa versus I(<f>) for given

values of Ta

. A family of curves thus generated is shown in Fig. 24.

The error is largest if Ta = 1, in which case

dT < a — y/2 x 10_1+ if I(<J>) > 10~ 7 A. (20)

Thus, a single measurement of I(Ta<f>) and I(<j)) may be expected to give

Ta

to within approximately 10_1+ transmittance units as long as the

operating conditions of the spectrophotometer are chosen such that the

anode current corresponding to 100% transmittance is not smaller than

10- 7 A. We chose 3 * 10~ 7 A as the normal 100% signal current for the

spectrophotometer with a photomultiplier voltage of 850V.

16
Measurements based on a series of repeated readings, such as listed

above, will then be reproducible to within a few 0.00001 transmit tance

units. This expectation was verified directly, when separate transmission measurements of neutral-density glass filters with nominal transmit tances of 0.1, 0.2, and 0.3 were found to be repeatable to ± 0.00004

(see Section V) . Transmittance measurements are made in a defined

polarization, e.g., either with the electric vector parallel or perpendicular to the grating grooves.

The filter wheel accommodates three filters plus a clear space so that either one, two, or three filters may be measured in a given run. We will discuss the procedure for a three filter measurement from which

it should be obvious how one would measure one or two filters. Since

it was shown in Section IV. B above that the drift of the system is

predominantly linear, we perform a series of measurements designed to

eleminate the linear drift as in the case of the linearity test. We

chose to record data for 0, F , 0, F 1} 0, F 2 , 0, F 3

, 0, F , 0, F 3, 0,

F2 , 0, F l5 0, F , 0, F lf 0, F 2 , 0, F 3 , 0, F , 0, F 3

, 0, F 2 , 0, Fj, 0,

F where Fg, F^, F2 , F 3 are the signals for the fluxes of a clear

space, filter 1, filter 2, and filter 3; and "0" is the dark current

reading. Since the filter wheel is controlled remotely and the driving

motor is reversible, this sequence of measurements was achievable easily

with the time elapsed between Fg and F 1} and F^ and F 2 , etc. being 1

minute. These measurements are not yet automated so that the counter had

to be activated at fixed intervals after which it integrated over a ten

second interval. However, after terminating its integration, the counter

signaled the control-log to record a datum point on a punched-paper tape.

A computer program was written to analyze the data in a manner

similar to that for the linearity test. The dark current on both sides

of "F are averaged and subtracted from these readings,

yielding a sequence of 17 adjusted signal currents, I 01> Ml* i 21» -LSI'

--02' 1 1 14, Iq5« The averages I 32» ± 22»

then used to compute the transmittance

T. = -1

0' 1> J-2» and Io are

(21)

for filter j. ,The set of 17 Y,

Y. =

l

loi I

•o

11 L 21 J-31 m 05

(22)

0

may be fitted to a straight line, and the coefficient of variation of

the data may be calculated. A typical value for this parameter is found

to be 0.02%.

17
Again assuming

Y

± = P + qt± (23)

the standard errors, dig, dlj, dl 2 , and dl3 may be calculated. Then, the

standard error in the transmittance measurement for filter j is

dT. = T.

3 3

1/2

(24)

This quantity was as large as .007% for a 60% filter; but for Tj less

than 50%, dT_= was typically less than .004%.

The computer program also calculates a linearity correction

AT(X,T) by a numerical interpolation formula,

AT(A,T) = AT(A ,T) [1 + f (X - X ) + g(X - A ) 2

] (25)

using values of f and g in Eq. (25) computed from the linearity data at

400 nm, 525 nm, and 750 nm.

V. Results of a Specific Test

Three filters were obtained from Dr. Radu Mavrodineanu from sets

which are certified and issued as Standard Reference Material 930, as measured with his spectrophotometer [15],

The three filters labeled with their approximate transmittances

(10%, 20%, and 30%) were measured on our high accuracy spectrophotometer.

The spectral bandpass for these measurements was approximately 0.8 nm,

and the temperature was 23.0 °C ± 0.5 °C. The filters were aligned

normal to the light beam by means of the alignment laser. Three sets of

measurements at the wavelengths for which the glass filters are certified were made in collimated light averaging over an area of approximately

2 mm by 12 mm. These measurements were made using polarized light with

the angle of polarization parallel or perpendicular to the grating grooves.

A measurement with parallel or perpendicular polarization consisted of

4 determinations as described above. The average value of parallel and

perpendicular polarization is reported in Tables IV, V, and VI as Runs

1, 2, and 3. The difference between the values furnished by Dr.

Mavrodineanu and our values are listed in Tables IV, V, and VI as well

as the difference between values for the parallel and perpendicular

polarizations.

18
Between Run 1 and Run 2, a realignment of the optics was made so that the area of the filter measured in Runs 1 and 2 were not quite the same indicating that the filters (especially the one designated 10%) are

not uniform. The agreement between Runs 2 and 3, when no changes were

made, attest to the reproducibility of our instrument (approximately

± 0.00004 transmittance units).

Since there is an indication of non-uniformity, the filters were

moved across the 2 mm wide aperture in 1 mm steps scanning a 4 mm wide

section of the filters for parallel polarization at 635 nm in collimated

light. The results are listed in Table VII.

All values of transmittance listed in the Tables have been corrected

for non-linearity of the instrument as a function of wavelength.

It should be noted that non-uniformity is not peculiar to these

filters. Three neutral filters of another type were measured; and they

exhibited similar non-uniformity.

VI . Appendices

A. Discussion of Beam Non-Uniformity

Let T(x) be the transmittance of a filter area element between x

and x + dx, and let <Kx) be the radiant flux incident upon it. Then the

measured apparent transmittance of a filter area of width x,

J

1

2

X

dxT(x)<f>(x)

T = 2

X

(Al)

A

J2 X

1

2

X

dx <f>(x)

is different from the true transmittance,

1

2

X

T = / dxT(x) , (A2)

1/

"

2

X

provided that both filter and beam are non-uniform.

If we assume linear gradients in T(x) and <j>(x) such that

19
T(x) = T (1 + ax) , (A3)

<f,(x) = cj> (1 + gX) , (A4)

then

_ ±J

dx[l + (a + 3)x + a3xz

]

L A 1~ - 2

X

T A = T — (A5)

x J

dx(l + 3x) "

2

X

= T (1 + ^2 a ^x2 ) • <A6 >

which is used in Section II.F. above.

B. Derivation of Efficiency of Averaging Sphere

The efficiency, e, of the averaging sphere will be defined as the

ratio of the flux incident upon the detector, (j)', to the flux entering

the sphere, <f>;

e = <j)'/* (Bl)

A number of parameters will be defined as in Fig. 25. Let

d = inside diameter of the sphere

Ag = ird 2 = total inside area of the sphere

e = distance separating sphere and detector

Aj) = photosensitive area of the detector

A = area of entrance port

A' = area of exit port

A-J- = directly illuminated area of sphere

Ay = area of sphere viewed by the detector

fi = solid angle subtended at detector by A' or Av

With L as the diffuse radiance (W/m2 /sr) of the uniformly illuminated sphere wall, the flux from a surface element dAy of the sphere

wall onto a surface element dAD

of the detector is

dc|>' = LdA

D —

(B2)

Z

2 20
where Z is the distance between dAy and dAp as shown in Fig. 25. Generally, Z is a complicated function of sphere geometry; but for the

present purpose, we can replace Z by its approximate value (d + e) . Then

the total flux into the detector is

io *v

(d + e7, i I

dAjjd^ (B3)

LA,, (B4)

(d + e) 2

According to Fig. 25, provided that I ^ 0,

\ Aj_

(d + e) 2 e

2

therefore,

e

2

(B5)

(B6)

where L is still undetermined.

Except for the directly irradiated area At, the inside sphere wall

is uniformly illuminated by an irradiance E/(W/m2

) given by the following

argument

.

Let r equal the diffuse reflectance of the sphere walls. Then,

the flux distributed inside the sphere after the initial reflection

from Aj is

<j>l = r<|> (B7)

Of this, the fraction (A+A')/As is lost through the holes. Hence, the

flux due to the second reflection is

4>2 = r[l - (A+A')/As] ch , (B8) = r

2 [l - (A+A')/As

] <J) . (B9)

21
Again, a fraction (A+A')/As is lost so that

<j> 3 = r[l - (A+A')/As] <j> 2 , (BIO)

= r

3 [l - (A+A^/Ag] 2

<J> . (Bll)

Generally,

4> n

= rn

[l - (A+A')/As ] n_1

+ (B12)

Therefore, the irradiance of the sphere wall is

^1 "*" $2 "*"

• • • E = t (B13) As

= (rtj)/As ) |l+r[l-(A+A')/As ]+r 2 [l-(A+A»)/As ] 2+...l (B14)

r*/A

E = (B15)

1 - r[l - (A+A')/As ]

This irradiance causes a diffuse radiance

L = rE/ir (B16)

of the inside sphere walls. Combining equations B6, Bl5, and Bl6, we obtain the desired efficiency of the averaging sphere.

(A'AWTre2 A r

2

e = -5 S

( B17)

1 - r[l - (A+A f )/As ]

which was used as Eq. 3, above.

22
We would like to thank Drs. R. P. Madden and J. Reader who initiated this work; and Drs. C. E. Kuyatt and R. Mavrodineanu for many helpful discussions and ideas concerning this instrument. Mr. J. C. Schleter

designed the filter wheel and the shutter for the averaging sphere, and

also built the box for enclosing the instrument. We acknowledge the

construction and discussion of the current-to-frequency converter by

Mr. L. Marzetta, and the design of the circuitry for the stepping motor

of the neutral density wheel by Dr. W. H. Venable. We also thank Mr. C.

H. Popenoe for aid in automating the equipment. The interf erometric

testing of the grating was done by Mr. F. W. Rosberry.

23
References

[I] Reader, J., J. Res. Nat. Bur. Stand. (U.S.), 53, 243 (1969).

[2] These measurements were made by Mr. J. C. Schleter of NBS.

[3] This experiment was done with the aid of Dr. J. Reader of NBS.

[4] Mielenz, K.D., J. Res. Nat. Bur. Stand. (U.S.), to be published.

[5] Mielenz, K.D. , Eckerle, K.L., Reader, J., and Madden, R. P. (to

be published)

.

[6] The light beam in our monochromator travels in the reverse direction, i.e., the entrance aperture became our exit aperture and

vice versa. This was done so that the monochromator controls

would be accessible from the source side. The predisperser drive

(which is coupled to the monochromator) was modified accordingly

so that the monochromator screw and predisperser would be linked

together.

[7] Saunders, J.B., J. Res. Nat. Bur. Stand. (U.S.), 68C , 155 (1964).

[8] Taylor, D.J. , Rev. Sci. Instr., 40, 559 (1969).

[9] Certain commercial instruments or materials are identified in this

paper in order to specify adequately the procedure. In no case does such identification imply endorsement or evaluation by the

National Bureau of Standards.

[10] Estimating Stray Radiant Energy, 1970 Annual Book of ASTM Standards

Part 30, p. 1154 (1970).

[II] Mielenz, K.D., and Eckerle, K.L., (scheduled to be published in

Appl. Optics in October, 1972).

[12] Photomultiplier Manual (RCA Electronic Components, Harrison, N.J.,

07029, 1970), p. 65.

[13] The manufacturer quotes the "typical" gain of the dynode chain as

6 11 = 2 x 106

, hence, 6 = 3.7. He also states that, for CsSb

dynodes, 6 has the empirical form 6 = 0.2 V

0,7

, where V is the

interdynode voltage. Hence, for a total applied voltage of 850 V,

V = 850/12 = 70 V and 6 = 3.9.

[14] This analysis was done with the aid of Mr. J. M. Cameron, of NBS.

[15] Menis, 0., and Schultz, J.I., (Editors), Nat. Bur. Stand. (U.S.),

Tech. Note 584 (1971).

24
Table I

Item Model Number Manufacturer

Lamp 18A/T10/1 SR8 General Electric

Lamp Power Supply KS18-50(M) Kepco

Neutral wedge A6020 Eastman Kodak

Predlsperser Model 608 McPherson

Monochromator Model 2051 McPherson

Parabolic mirrors Special Ferson

High Voltage Power 412B Fluke

Supply

Counter 1192/z General Radio

Data Acquisition and Special Compumetries

Control System

Photomultiplier 9558QA EMI

Shunt (0.1ft, 50A) 4360 Leed and Northrup

25
Table II

Final Values of o and AT Obtained by Repeated

Measurements (A = 525 nm, I(cf>) = 3 x 10-7 A)

T

First Me<asurement Second Measurement

o x 10^ AT x lO 4 a x 10^ AT x lO4

0.1 0.07 0.72 0.19 0.77

0.2 0.25 1.38 0.45 1.46

0.3 0.52 1.96 0.79 2.02

0.4 0.91 2.40 1.21 2.43

0.5 1.39 2.67 1.70 2.66

0.6 1.97 2.73 2.27 2.68

0.7 2.66 2.53 2.92 2.45

0.8 3.45 2.04 3.65 1.95

0.9 4.34 1.21 4.45 1.14

1.0 5.34 0.00 5.33 0.00

26
Table III

Measured Non-Linearity Correction AT

for Different Operating Conditions

for Spectrophotometer (I(<j>)=3xlO~ 7A)

AT x 10 4

T

525 nm

0°

525 nm

90°

525 nm

±45° 400 nm 750 nm

0.1 0.75 0.73 0.75 0.80 0.59

0.2 1.42 1.41 1.40 1.53 1.17

0.3 1.98 1.99 1.92 2.15 1.70

0.4 2.37 2.45 2.28 2.62 2.14

0.5 2.67 2.72 2.48 2.90 2.43

0.6 2.70 2.78 2.47 2.94 2.52

0.7 2.49 2.58 2.25 2.74 2.37

0.8 1.99 2.08 1.78 2.18 1.94

0.9 1.18 ' 1.23 1.04 1.28 1.12

27
Table IV

Transmittance of Nominal 10% Filter

R. Mavrodineanu (Lab 1)

Mielenz and Eckerle (Lab 2)

Alignment 1 - Run 1

Alignment 2 - Run 2

Alignment 2 - Run 3

(Lab 1) - (Lab 2)

Run 1

Run 2

Run 3

Change in transmittance

between parallel and

perpendicular polarization

Run 1

Run 2

Run 3

440 nm 465 nm 590 nm 635 nm

.1159 .1356 .1037 .1136

.11448 .13391 .10244 .11217

.11516 .13467 .10297 .11275

.11511 .13462 .10295 .11278

.00142 .00169 .00126 .00143

.00074 .00093 .00073 .00085

.00079 .00098 .00075 .00082

-.00008 -.00001 .00000 «-

-.00005 +.00001 -.00001 +.00001

-.00005 +.00010 -.00014 -.00016

28
Table V

Transmittance of Nominal 20% Filter

R. Mavrodineanu (Lab 1)

Mielenz and Eckerle (Lab 2)

Alignment 1 - Run 1

Alignment 2 - Run 2

Alignment 2 - Run 3

(Lab 1) - (Lab 2)

Run 1

Run 2

Run 3

Change in transmittance

between parallel and

perpendicular polarization

Run 1

Run 2

Run 3

440 nm 465 nm 590 nm 635 nm

.1980 .2259 .1916 .2060

.19795 .22587 .19161 .20608

.19811 .22602 .19170 .20618

.19805 .22595 .19168 .20617

+.00005 +.00003 -.00001 -.00008

-.00011 -.00012 -.00010 -.00018

-.00005 -.00005 -.00008 -.00017

+.00001 -.00003 +.00001 +.00007

-.00009 -.00004 -.00003 +.00006

+.00018 +.00017 +.00006 +.00004

29
Table VI

Transmittance of Nominal 30% Filter

R. Mavrodineanu (Lab 1)

Mielenz and Eckerle (Lab 2)

Alignment 1 - Run 1

Alignment 2 - Run 2

Alignment 2 - Run 3

(Lab 1) - (Lab 2)

Run 1

Run 2

Run 3

Change in transmit tance

between parallel and

perpendicular polarization

Run 1

Run 2

Run 3

440 nm 465 nm 590 nm 635 nm

.3287 .3553 .3113 .3255

.32893 .35566 .31154 .32581

.32901 .35577 .31154 .32593

.32902 .35566 .31153 .32592

-.00023 -.00036 -.00024 -.00031

-.00031 -.00047 -.00024 -.00043

-.00032 -.00036 -.00023 -.00042

-.00001 +.00003 +.00009 -.00004

-.00022 +.00001 -.00003 +.00006

-.00002 .00000 -.00008 -.00003

30
Table VII

Range of Transmittance of Filters When

Area Measured is Varied at 635 nm with a

2 mm by 12 mm Aperture and Parallel Polarization

, mm .. -1 mm . . +1 mm

(Central Position)

Nominal 10% Filter .11247 .11270 ,11301

Nominal 20% Filter .20601 .20619 .20641

Nominal 30% Filter .32593 .32591

Range for 10% Filter = 0.00054

Range for 20% Filter = 0.00040

Range for 30% Filter = 0.00009

.32600

31
(A) Z

Ulo

<

I0.O

440 450

WAVELENGTH (nm)

460

Fig. 1. Measured spectra of a holmium oxide glass filter,

32
.2 .5 1.0 2.0

SPECTRAL BAND PASS (nm)

5.0 10.0

Fig. 2. Variation of measured optical density with spectral bandpass of

the holmium oxide glass filter.

33
is.

Li.

z

o

o

CO

r

B

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co ^

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35
Fig. 5. Exit Arrangement and Detector of Spectrophotometer.

36
Fig. 6. Control Panel and Electronics,

37
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39
LASER

PARABOLIC

MIRROR

MIRROR

2

40 X MICROSCOPE

OBJECTIVE

40 X MICROSCOPE

OBJECTIVE

SHEARING

INTERFEROMETER

(a)

LASER

MIRROR

SHEARING

INTERFEROMETER

lb)

Fig. 9. Use of Shearing Interferometer to Check Off -axis Parabolic Mirrors,

(a) Individually

(b) Both mirrors in "z" arrangement

40
POSITION (mm)-

Fig. 10. Nonuniformity of spectrophotometer light beam; scan parallel to

grating grooves at 632.8 nm.

i i i n r~r 1 r i i i i i O X = 632.8 nm

• X=725 nm A X = 410 nm

I I I I

I -10 -8-6-4-2 2 4 6 8 10 POSITION (mm)—

Fig. 11. Nonuniformity of spectrophotometer light beam; scan perpendicular to grating grooves at 410 nm, 632.8 nm, and 725 nm. 41
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43
Fig. 14. Double Aperture Linearity Device.

44
ILLUMINATED

AREA

PHOTOMULTIPLIER

CATHODE

Fig. 15. Geometry of Averaging Sphere and Incident Light,

45
1

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QC

u

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650

Fig. 16.

750 850

DYNODE VOLTAGE (V)

Dark Current of Photomultiplier Tube.

46

950
GROOVES

a

GROOVES

b

Fig. 17. Twyman-Green Interferometer Photographs of Grating,

(a) Parallel to the grooves

(b) Perpendicular to the grooves

47
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48
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43400

43350

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44900

45600

45550

45500

dl

= 0.0061%

DRIFT = -0.045%/min

— dl = 0.0052%

DRIFT = -0.032%/min

= 0.0125% — DRIFT = -0.0294 %/min

t 1 1 10 20

TIME (MINUTES) —

ry

j_JT

30

Fig. 19. System Drift as a Function of Time,

49
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51
Fig. 22. Measured dependence of a(Tcf>) on T for I(cj>) = 3 x 10~7 A and

X = 525 ran. 52
1

o

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Fig. 23. Nonlinearity correction AT computed from measured data in

Fig. 22.

53
tob

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55
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS TN-729

2. Gov't Accession No.

4. TITLE AND SUBTITLE

DESIGN, CONSTRUCTION, AND TESTING OF A NEW HIGH

ACCURACY SPECTROPHOTOMETER

3. Recipient's Accession No.

5. Publication Date June 1972

6. Performing Organization Code

7. AUTHOR(S)

K. D. Mielenz and K. L. Eckerle

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. Project/Task/Work Unit No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same as No. 9.

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A new spectrophotometer is described which has an accuracy of approximately

0.0001 transmittance units. The spectrophotometer utilizes a collimated beam in

the sample area. This is accomplished by means of off-axis parabolic mirrors

in the monochromator and sample compartment. Also, circular holes are used as entrance and exit apertures in the monochromator. All components of this spectrophotometer were chosen to achieve maximum accuracy. The result of this work

is a "state of the art" instrument. The instrument was tested to evaluate its

performance. Systematic errors such as detector non-linearity, stray radiant

energy, and beam non-uniformity are measured. A correction for non-linearity of

the photomultiplier and electronics is applied.

17. key words (Alphabetical order, separated by semicolons) Beam non-uniformity ; circular entrance

and exit apertures; high accuracy spectrophotometer; linearity; parabolic mirrors;

stray radiant energy, transmittance.

18. AVAILABILITY STATEMENT

'

CE] UNLIMITED.

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

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21. NO. OF PAGES

60

22. Price

60 cents

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