NAT'L INST. OF STAND & TKH RXC NIST

PUBUCATIOHS

AlllDS DM7b75

Nisr United States Department of Commerce

Technology Adm nistration

National Institute of Standards and Technology

NIST Technical Note 1421

A National Measurement System for Radiometry,

Photometry, and Pyrometry Based Upon

Absolute Detectors

A. C. Parr

NIST Technical Note 1421

A National Measurement System for Radiometry,

Photometry and Pyrometry Based Upon

Absolute Detectors

A. C. Parr

Optical Technology Division

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

September 1996

^^ATES O*

'^

U.S. Department of Commerce

Michael Kantor, Secretary

Technology Administration

Mary L. Good, Under Secretary for Technology

National Institute of Standards and Technology

Arati Prabhakar, Director
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent of and Technology Washington: 1996 Documents

Technical Note 1421 U.S. Government Printing Office

Natl. Inst. Stand. Technol. Washington, DC 20402

Tech. Note 1421

35 pages (Sep. 1996)

CODEN: NTNOEF
Table of Contents

Page

I. Introduction 2

II. Electrical Substitution Radiometry 3

III. Detector Spectral Comparators 8

IV. Filter Detector Systems and Use 12

a. Photometry and Colorimetry 12

b. Spectral Radiometry 19

c. Pyrometry 25

V. Conclusion and Suggestions 25

References 27

111

A National Measurement System for Radiometry, Photometry, and

Pyrometry Based upon Absolute Detectors.

Albert C. Parr

Optical Technology Division

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD USA 20899-0001

Advancements in the performance and ease of use of absolute photodetectors based upon electrical

substitution principals offers the possibility of a simplified and more accurate way to transfer the

fundamental radiometric, photometric, and pyrometric units from NIST to the U.S. technical

establishment. The history of electrical substitution radiometers at NIST is briefly reviewed and

the implementation of the latest generation absolute cryogenic radiometer is discussed. Procedures

to maintain fundamental units based upon a detector approach are reviewed in a general way with

references to the technical literature for detailed discussion. It is proposed that NIST customers

consider adopting the suggestions made in this technical note to simplify and improve their optical

radiation based calibrations.

Keywords: cryogenic radiometers, photometry, pyrometry, radiometry
I. Introduction

The National Institute of Standards and Technology's (NIST) measurement procedures employed

to characterize the spectral characteristics of light sources have been based upon the well

established physics of blackbody sources [1]. The photometric units were traditionally established

using optical source based methods and utilizing the accepted International Commission on

Illumination (CIE) defined response for ordinary human vision, the spectral luminous efficiency,

V{X) [2]. This Technical Note will outline the new NIST methods of maintaining radiometric,

photometric, and pyrometric units based upon the use of absolute detector systems and will

advocate a shifting of methodology for these measurements that takes advantage of the new

opportunities afforded by detector based measurements.

The radiometric and photometric units and quantities usually referenced in technical discussions are shown in Table 1 . The left side of the table identifies the radiometric quantity, the usual symbol

utilized to describe it, and the corresponding units in the International System of units (SI) [3].

The right side describes the photometric units, their symbols and the SI units associated with them.

In the table, the SI unit of the candela (cd) is replaced with the unit of the lumen (Im) using the

defined relationship, cd = Im/sr. Representing the quantities in terms of the lumen, the

photometric equivalent of the watt, instead of the candela lends symmetry to the table and a convenience for understanding the often confusing radiometric and photometric terms.

Table 1. This table gives the radiometric and photometric quantities, their usual symbols and their

metric unit definitions

Radiometric Quantity symbol units units symbol Photometric Quantity

Radiant Energy

Radiant Flux (power)

Irradiance

Radiance

Radiant Intensity

Q J Im s Qv Luminous energy

Luminous Flux

Illuminance

Luminance

Luminous Intensity

P, <P W Im

(Pv

E W/m^ Im/m^ Ev

L W/(m2 sr) lm/(m2 sr) Lv

I W/sr Im/sr h

J=joule, W=watt, In

sr=steradian,

i=lumen, m=r

s=second

neter

The NIST maintenance of irradiance, radiance, and the photometric units has been discussed in a

series of special publications available from NIST or the U.S. Government Printing Office [4,5,6]. In order to assess the importance of the NIST photometric and radiometric work in the technical

community, Kostkowski reviewed the industrial and commercial impact of these activities and the
Council for Optical Radiation Measurement (CORM) has produced detailed reports outlining

industrial and technical requirements for the U.S. scientific community [7,8].

The NIST unit of radiation temperature is based upon the radiation output of blackbody sources and is described in detail in a NIST special publication [9]. In 1990 the Comite International des

Poids et Mesures (CIPM) decided that temperature measurement for temperatures above the

freezing point of silver, T= 1234.93 K, were to be maintained using optical techniques [10]. This

change results in the temperature unit for temperatures above the silver freezing point to depending

upon similar optical measurement technology that used to define the radiometric and photometric

units. As a consequence the NIST radiation temperature unit for a well characterized blackbody

source can be referenced to absolute detectors.

In 1979 the Conference Generale des Poids et Mesures (CGPM) adopted the 1977 CIPM

recommendation for the redefinition of the candela. The new definition for the candela is a source of monochromatic radiation at a frequency of 540 x 1012 Hz (about 555 nm) and with an intensity

of 1/683 W/sr. This optical power based definition, and the recognition of the CLE V{X) function,

provided the opportunity to base photometric units on detector measurements [11]. Many national

laboratories have implemented this approach for developing their photometric units and after a

substantial development program NIST recently completed its efforts to place the calibrations for

the candela and the lumen directly upon an absolute detector [12,13]. An impetus for the change

to a detector approach for radiometry and photometry is the advent of the availability of high

accuracy cryogenic electrical substitution radiometers. These devices can have a relative combined

standard uncertainty of less than 0.01 % for optical power measurements and can be conveniently

utilized in a well equipped modern radiometric laboratory*. A brief review of these devices will be

given in a following section of this Technical Note.

Users of optical radiation measurement devices require improved accuracy to meet competitive

demands in the marketplace and to improve the quality and efficiency of production facilities. Optical sensor systems for space based Earth observation have an increasing demand for more

accurate measurements. These place a burden on the national metrology system to provide

calibrations support with reduced uncertainties. This Technical Note will review NIST's efforts to improve the accuracy and stability of its radiometric, photometric, and pyrometric units to accommodate these expressed needs of its customers. General aspects of radiometry and

photometry will not be reviewed as this task is effectively and broadly covered by the technical

literature and recent books [14].

II. Electrical Substitution Radiometry

Electrical Substitution Radiometers (ESRs) are devices that measure optical radiation by

comparison to an equivalent amount of electrical power. ESRs are sometimes referred to as Electrically Calibrated Radiometers (ECRs) or simply as absolute radiometers. The fundamentals

of an ESR can be understood by reference to Fig. 1. The radiant flux (optical power) <r>p is incident upon a receiving cavity designed to optimize the collection of radiation. Upon absorption

of the radiation, the cavity will experience a temperature rise. The receiver is coupled to a constant

temperature heat sink at a reference temperature Tq with a thermal conductor of conductance G.

Ignoring losses due to radiafion, convection and stray thermal conductance, the equilibrium (long

time) temperature rise is given by T - Tq = 0p IG. When the shutter is imposed to intercept the

light beam, electrical power to the cavity is increased a sufficient amount to maintain the cavity

temperature determined by the temperature sensor system at the shutter open level. Ignoring
corrections and losses mentioned earlier, the optical power is given by 0p = /h^ R where i\^ is the

increased current applied through the heater of resistance R required to maintain temperature

stability. The practical challenge when utilizing ESRs is to carefully characterize the potential loss

mechanisms in order to apply appropriate corrections to the power equivalence relationship.

Power Supply

Shutter

Radiant flux, (Pp

Heater of

resistance R

Temperature

sensor

Receiving cone

of temperature T

and heat capacity H

Thermal link of

conductance G

Heat Sink at Tq

at steady state, T-Tq z ^p/Q

Figure 1. Schematic diagram of the essential components of an electrical

substitution radiometer. The total light flux ^p measured in watts is collected

by a receiving cavity usually shaped in the form of a cone. The temperature of

the cavity, T, and the temperature of the heat sink, Tq , are monitored by

temperature sensor systems. When the shutter is closed, electrical power

equivalent to the optical power is applied by the power supply system thereby

establishing the optical power.

ESRs have been in use for 75 years or more and have had their history and development described

by Hengstberger in considerable detail [15]. Work on HSR technology was pioneered by Coblentz

at NIST, at that time the National Bureau of Standards (NBS), during the early part of this century

[16]. He developed a number of radiometers and used them for diverse purposes in photometry

and radiometry, including an early measurement of the Stefan-Boltzmann constant [17]. For a

variety of technical reasons and the fact that the SI photometric unit, the candela, was defined until

1979 in terms of the output of candles and eventually a fixed point platinum blackbody, radiometry

4
and photometry depended upon characterization of sources for the maintenance of units. The

1979 candela redefinition in terms of optical power helped spur the shifting of radiometric and

photometric measurements to detector based technology.

The incorporation of detectors into photometric and radiometric standards was assisted by several

developments in the 1970's and 1980's. High quality silicon photodiodes became available and

provided a convenient device with which to make optical measurements in the visible wavelength

region [18]. Electrical substitution devices were designed and constructed to operate at cryogenic

temperatures in order to increase the sensitivity of the devices and to reduce the uncertainties due to radiation and conduction losses. The first cryogenic radiometer at NIST was constructed by

Ginnings and Reilly in 1972 to measure thermodynamic temperatures above OC [19]. For a

variety of reasons this project did not achieve the desired results, but by building on the experience

gained by Ginnings and Reilly, in the mid- 1 970' s Yokley built a cryogenic radiometer system at NIST to measure the radiation temperature of low temperature blackbodies used in a low

background environment [20]. This device was used for a number of years to perform specialized

calibrations of low flux sources but was not engineered to perform high accuracy measurements

and as a consequence produced results with relative uncertainties of several percent.

Quinn and Martin at the National Physical Laboratory (NPL) in the UK developed a high accuracy

cryogenic radiometer for use in a radiometric determination of the Stefan-Boltzmann constant [21].

The system that the NPL team developed allowed for the determination of the Stefan-Boltzmann

constant with a relative combined standard uncertainty of 0.013 %. This important benchmark

work contains a detailed analysis of the errors and uncertainties associated with a cryogenic ESR

and has led to the adoption of these devices as fundamental radiometric standards with relative

combined standard uncertainties of less than 0.01 %. NPL staff later developed this device into a radiometer for laser power sources and thereby provided a comparative technique to establish high

accuracy radiometric units with other stable detectors [22]. The technology has evolved and at least two companies are offering commercial versions of absolute cryogenic radiometers [23]. The

availability and accuracy of the instruments has resulted in their employment by a number of

national standards laboratories to provide the basis of radiometric measurement [24].

The NIST high accuracy cryogenic radiometer (HACR) is shown in Fig. 2 and is described in

detail in the technical literature [25]. The heart of the instrument is the absorbing cavity which is connected to a thermally controlled heat sink held at 5 K. The apparatus is evacuated with a vacuum pumping system and has cryogenic fluid reservoirs to provide the low temperature

environment for the cavity absorption and electrical heating system operation. Polarized optical

radiation from a laser system enters the vacuum vessel of the HACR through a window at Brewster's angle.

A typical mode of operation for the HACR is shown schematically in Fig. 3 and is described in the

literature [25,26]. A secondary or transfer standard detector (TSD) is inserted into the laser beam

and intercepts the same beam as measured by the HACR. A TSD is a detector which is calibrated

directly with the HACR and can then be used to transfer the detector response unit to other

calibrations systems. After appropriate corrections for HACR entrance window transmittance and

other systematic effects the absolute response of the TSD is deduced. In Fig. 3 the TSD is depicted as a trap detector, so called because of the arrangement of several silicon photodetectors in a configuration designed to absorb a large portion of the incident light. These silicon devices work

well in the 400 nm to 1000 nm wavelength region and different types of TSDs ai-e used in other

wavelength regions.
Liquid Helium ,^ Reservoir

Germanium Resistance

Thermometer

50 K Radiation Shield

77 K Radiation Shield

Radiation Trap (4.2 K)

Pumping Port

Brewster Angled

Window

Liquid Nitrogen

Reservoir

5 K Reference Block

Thin Film Heater 10 K

Absorbing Cavity

(specular black paint)

Alignment

Photodiodes

100 mm

Laser Beam

Figure 2. Sectional drawing of the NIST high resolution cryogenic radiometer.
Monitor

Photodiode

HACR

S

Laser

Polarizer

Entrance

Window

m

Trap

Detector

Spatial Filter

and Lens

J

Aperture

Beamsplitter

Turning

Mirror

Shutter

Figure 3. Schematic of the HACR optical arrangement used to characterize transfer standard

detectors for use in other optical measurements. Various lasers provide radiation for

wavelengths in the UV to the IR. Each wavelength used requires careful characterization of the

entrance window for its transmittance and scattering properties.

The TSD can be calibrated directly using the HACR at selected laser wavelengths and by

determining the responsivity for regions between the laser wavelengths from a knowledge of the

physics of the detector. In the case of silicon photodiodes a considerable amount of effort has

gone into modeling the responsivity as a function of wavelength with the result that the

responsivity from 400 nm to 1000 nm can be accurately inferred [27]. Gentile and co-workers

used this technique to ascertain the NIST unit of detector spectral responsivity with a relative

combined standard uncertainty of less than 0.04 % in the wavelength range of 406 nm to 920 nm.

Other types of detectors can be calibrated using this technique in different wavelength regions,

including spectrally flat absorptive bolometer detectors and other semiconductor devices [18,28].

The complete calibration for semiconductor devices consists of determining the absolute spectral

responsivity of the device as a function of wavelength based upon the HACR measurements and

appropriate modeling. This information and data on the responsivity spatial uniformity and

temperature stability of the detector allow the device to be used as a TSD in other spectral

radiometric measurement circumstances.
As an alternative to having a semiconductor detector with a limited response range, a bolometer

can be utihzed as a TSD by carefully characterizing the stabihty and the spectral absorption of the

absorbing surface material utilized on the bolometer. A careful characterization of the elements of

the detector that determine its relative spectral response to a high accuracy allow it to be calibrated

with the HACR at visible and near infrared (IR) wavelengths and its response elsewhere inferred

from the relative spectral response of the device. In some wavelength regions lasers may not be

readily available for use in the manner shown in Fig. 3. An approach to avoid this problem has

been the development of cryogenic radiometers that operate with monochromatic light provided by

conventional optical sources [29]. These sources typically have less optical power per unit

wavelength than a laser and hence the use and characterization of the radiometer can require greater care to achieve high levels of accuracy. In all cases, use of a cryogenic radiometer relies upon the

availability of suitable window materials which may present technical challenges in some

wavelength regions.

III. Detector Spectral Comparators

A characterized TSD is employed in Detector Spectral Comparators (DSC) to calibrate a wide range

of detector and detector-filter combinations. NIST has a variety of these instruments that operate

from the vacuum ultraviolet (VUV) through the infrared (IR). A typical instrument is depicted in

Fig. 4(a). The main component of a DSC is a high quality monochromator, usually a double

monochromator to reduce stray light, and an appropriate assortment of light sources to span the

designated wavelength region for the particular instrument [30]. The light exiting the

monochromator is usually focused onto a point in space where a movable stage(s) can impose

detectors destined for calibration. In most cases a portion of the exiting beam is intercepted with a monitor detector system used to normalize the data for fluctuations in the light sources. The

monitor need not be absolutely calibrated but must be characterized for temporal and thermal

stability at uncertainty levels consistent with the measurement objectives of the calibration scheme.

One or more positions of the detector translation stage is occupied by a TSD or a detector calibrated

directly with the TSD. This detector, often called the working standard detector, is used for the

routine transferral of the absolute spectral responsivity units and is the subject of intense quality

and reliability procedures to ensure maximum measurement accuracy.

The typical result of DSC measurements are curves such as shown in Fig. 4(b). These are representative plots of absolute responsivity in units of amperes/watt for several of the solid state detectors in widespread use. The data shown in Fig. 4(b) was taken with the UV and visible to near IR instruments at NIST. The chain of calibration has resulted in electrical power

measurements in the HACR being transferred by a series of steps to determine the electrical

response of a solid state device to optical power. As a result, the response of the photodetector is determined in terms of the electrical watt maintained by NIST in the form of voltage, current, and

resistance standards [31]. Some of the NIST DSCs have a stage for translation of the detector

vertically in the plane perpendicular to the incident light. This feature is labeled x/y scanning

carriage in Fig. 4(a). The spot size of the incident beam is usually on the order of a millimeter but can be varied for other purposes by the focusing optics. The small size allows measurement of the

spatial uniformity of a detector system which is important in circumstances where the detector

system will be employed in an underfilled mode of operation and hence possibly show a sensitivity

to the incident beam position on the detector. Additionally, the spatial uniformity may be a

function of wavelength in certain semiconductor devices, and if left uncharacterized can cause
unwanted irregularity in system calibrations. NIST routinely supplies this information to calibration customers requiring spatial performance characteristics for detectors.

(a)

Light Tight Box

Prism - Gratins

IVIonochromator

u

Baffle I Shutter

Focusing Optics

Sources

Input Optics

(b)

Absolute Spectral Responsivity

1.0

s-> 0.8 >

v^ 0.6 eoc

^ 0.4

Pi

a>s 0.2

0.0

'

1 1 1 1 1 1 1 1 1 j.' -i-i" ' - ^ InGaAs ^

.-^*^^

^u. \ _2 — ^^^ _ ^

'

« 1 — _ ^^•^ . -•' « 1 —

^

.Jy^

Si (IR Enhanced) C m'

^ Ge * 1 % 1 -^

H-NK

1 1

. . . 1 . . . 1 .^K.*. . 1 ill 1 1 1 1 ^

200 400 600 800 1000 1200 1400 1600 1800

AVavelength [nm]

Figure 4. (a) Diagram of a typical detector spectral comparator instrument employed at NIST

to characterize optical radiation detectors and detector systems.The major components of the

instrument include appropriate light sources, a high quaJity monochromator, entrance and exit

optics to convey the light to its intended points, and a mechanical arrangement such as

translation stages to position the TSD or working standard detector and test devices in the

beam.

(b) Typical characterization results of a selection of photodetectors. The vertical scale is absolute responsivity in units of AAV and the horizontal scale is wavelength in nm. Other

types of photodetectors are used in the UV, Far UV and the IR.
A DSC is a versatile optical instrument which can be used for spectral transmittance measurements

and other characterizations requiring a known optical beam. Important to the establishment of

detector based optical units is the characterization of a photodetector coupled with an optical filter restricting the wavelength interval of transmittance. Such a device is often called a Filter

Radiometer (FR). An FR can range from a simple silicon photodiode with a colored glass

broadband filter to a sophisticated temperature controlled device designed for precision

measurements. An example of the latter type of device is shown in Fig. 5 [32]. The FR system

consists of a precision aperture to define the amount of light entering the system, a thermoelectric

(TE) temperature controlled filter, and appropriate absorbing glasses to protect the system against

unwanted radiation and environmental effects. (Depending upon use, the precision aperture may

or may not be the limiting aperture in the optical system.) After propagating through the beam

definition opfics the radiation is collected by an appropriate detector. The device shown in Fig. 5 is modular and can support a wide range of filters and detectors by a simple mechanical interchange.

Where necessary the detector module can be temperature controlled. Devices like this require

considerable care in design and assembly to provide elimination of unwanted reflections and scatter of the radiation entering in the system.

Sensor Housing Filter

Aperture

Silicon Photodiodes or appropriate sensor system j,^ Cooler Heat Absorbing Glass

Figure 5. Cross section diagram of a typical filter radiometer system consisting of a precision

aperture, filter, and sensor system. The NIST design is modular to provide the opportunity to use

various filters and detector systems with some economy of components.

The absolute spectral responsivity of the resultant sensor system can be calibrated using the DSC

facility. Figure 6 show some typical examples of FRs calibrated at NIST for use in photometry

and for implementing a detector based spectral radiance and irradiance unit. FR#2 with a peak

response around 550 nm is a photometer and the 380 nm and 910 nm systems were designed for

radiance temperature measurement research. FRs can be sensitive to beam divergence and

polarization as well as exhibiting interference anomalies if a coherent radiation source is used.

Consequently considerable care, calibration, and study must be directed toward the use of a FR in

radiometry and photometry. Depending upon the light source to be measured by the FR, the out of

10
band rejection of the filter system must be characterized in detail and with sufficient accuracy to provide the measurement accuracy desired. As an example, if the system shown as FR#2 in Fig.

6 has a poor near IR rejection where a silicon detector has good sensitivity, large errors can result

when used with an incandescent source which has large near IR output.

Filter Radiometer absolute response

0.14 I T^^^.,..^

^^^^^^^

300 400 500 600 700 800 900 1000 1100

Wavelength [nm]

Figure 6. Examples of the calibration of filter detector systems in the DSC instrument. The

vertical scale is absolute responsivity in AAV and the horizontal scale is wavelength in nm.

These examples represent a UV filter, FR#1, a photopically corrected filter radiometer, FR#2,

and a near IR system, FR#6.

11
A calibration scheme based upon the HACR has allowed the accuracy of DSC performed

calibrations to be significantly improved over that reported in Ref. 30 when the instrument was

first constructed [25]. The relative combined standard uncertainty for the detector response

measurements now are in the 0. 1 % range in the visible (400 nm to 900 nm) using silicon detectors

and 0.2 % to 1.3 % in the near IR for germanium devices. In the UV region (200 nm to 400 nm),

the relative expanded uncertainty is 1 .75 % to 0. 1 % with the higher uncertainty in the shorter

wavelength region. As our understanding of the physics of these devices improves, the

uncertainties associated with interpolating the spectral response between the laser wavelengths will

continue to diminish with a corresponding decrease in the uncertainty of calibrated detectors that

NIST provides.*

An alternate method of calibrating an FR was developed by Schaefer and his collaborators at NIST

[33]. This technique utilized a dye laser system which featured wavelength tunability and provided

the capability of scanning over the wavelength range of the filter's transmittance. The laser power

could be direcdy determined with a calibrated detector and the absolute response of the FR

inferred. It was found that care had to be exercised to account for artifacts of the measurement

introduced by the coherent properties of the laser radiation. While the coherence aspect adds an

extra dimension to the practical utilization of the FR calibrated using a dye laser system, it is a manageable matter and this technique offers the possibility of high accuracy direct calibration of FR

systems based upon a cryogenic radiometer. Schaefer and his colleagues reported a relative

combined standard uncertainty of 0.18 % in their system using an absolute silicon detector system

for the reference.

IV. Filter Detector Systems and Use

a) Photometry and Colorimetry

Photometry is the science of light measurement in a manner proportional to human visual response

according to an accepted average human visual response function. The intemationally accepted

standard in this regard is the CIE spectral luminous efficiency function for photopic vision, usually

designated as y(A) [2, 34]. This function is shown as the curve labeled "green" or y in Fig. 7. The other curves plotted in Fig. 7 are the CIE color matching functions "red" or xand "blue" orZ

used to define points in the CIE color coordinate system [2,14,35]. The definition of the candela

and the V(X) function provide a means of coupling the photometric and radiometric units shown in

Table 1 in the manner described below.

Let 2a represent the general radiometric quantity on the left side of Table I and Q^ represent the

general photometric equivalent function, then,

Q. =^Je,n^)cix . (1)

Km is the luminous efficacy defined by the CIPM to be 683 ImAV for photopic vision [11]. The

range of the integral has practical limits restricted to the region of non-zero values for V(?C). Figure

8 illustrates a typical measurement configuration for a light source and filter detector system. A

source is placed a distance r from the detector system whose input is determined by the amount of

12
1.5

o

o

c

3

Li.

V.

O

o

o

0.5

CIE Color Matching Function

I I I

I I I I II I I I

I I

-Red(x) — Green (y

)

-Blue(z)

300 400 500 600 700 800

Wavelength [nm]

Figure 7. CIE color matching functions. The curve labeled "green" is the spectral luminous

efficiency function for photopic vision, V(A,)

.

optical power entering the precision aperture of area A. In most configurations in photometry and

radiometry one would choose the distance r large compared to the dimensions of the aperture and

the source size. For utmost precision one can consider the appropriate configuration factors for the

total system of source and receiver. For our purposes we shall assume that the limit where the source size is approximated by a point source, the aperture is sufficiently small so that there are no

variations in the light flux distribution over the aperture, and the responsivity is appropriately area averaged in order to avoid integration over the aperture.

13
Light Source

Precision Aperture

Area = A

Photometer

V(X) Filter SiUcon Photodiode

Figure 8. Schematic of a setup for using an absolutely calibrated photometer to perform

luminous intensity and illuminance measurements. The distance, r, is large compared to the size of the lamp and the dimensions of the photometer's aperture. The distance, r, is measured from the lamp filament to the aperture of the photometer.

The photometer is calibrated on the DSC and has an absolute spectral responsivity given by s(X)

[AAV]. The uniform irradiance of the source £'a(A) on the aperture A produces a current /q from

the detector systems,

/o = AJ • E^{X)*s{X)dX . (2)

The filter is designed such that its transmittance combined with the responsivity of a photodetector

produces a response of the FR nearly proportional to the V(/L) function. An example of such a

detector's response is indicated as FR#2 in Fig. 6.

14
The responsivity of the photometer to the hght in terms of luminous flux can be defined as shown

in the equation,

i, =A'R^^K^j E,{X)>V{X)dX , (3)

X

where Rv/is the photometric response of the system to luminous flux measured in terms of A/lm.

Eliminating /q between eqs. (2) and (3) results in

J E^{X)'s{X)dX

X

R , = (4)

^.•J E,{X)-V{X)dX

X

To the extent that s(X) and V(X) have the same functional form, eq. (4) reduces to having the lumen

response determined in terms of the ratio of the absolute response at 555 nm and the luminous

efficacy. This is evident if one writes,

s{X) = s{555)'s„{X) , (5)

where s^ (A) is the spectral responsivity normalized to the value at 555 nm.

15
The luminous flux responsivity then can be written,

s(555).

J E^{X)'s^(k)dX

X

R , = (6)

K^'\ EM'V{X)dX

X

If the functional forms of Sn(X) and V(X) are sufficiently close one can write,

5(555) •(1.0 + corrections)

^., =

-^ K • ^^^

Knowing £'a(A), the correction terms can be calculated for the spectral distribution of various

sources. These matters are discussed extensively in the technical literature and will not be

reproduced here [2,36].

Assuming the luminous flux is uniform over the aperture A, we can write,

Ov =E,A, (8)

where Ey is the illuminance and conclude that the illuminance responsivity R^j is.

R =AR ^

. (9) V,l v,f ^ '

In the approximation that the source is a point source the illuminance is related to the luminous

intensity by well known inverse square law.

v,i v,f

16
We can write the luminous intensity /y expressed in candela as,

IqK

', = j^. (U)

Keeping in mind that Ryj-was determined by a direct measurement on the DSC utilizing eqs. (6)

and (7), it is noted that the SI unit of the candela is derived direcdy from the primary detector

standard and that the auxiliary unit for illuminance in lux or lm/m2 is similarly derived.

Employing the wide dynamic range of photopically corrected detector systems, a single calibrated

instrument can serve to establish luminous intensity and illuminance units over a broad range and removes the necessity of every calibration laboratory maintaining a large inventory of lamps to provide complete calibration coverage [32].

Light sources used for illumination are rated in terms of the total luminous flux output, often

abbreviated to luminous flux, measured in lumens. The measurement of the luminous flux

involves the integration of the luminous intensity of the source over the total solid angle of

illumination, ordinarily 4k sr. This is usually accomplished with a goniophotometer which uses a

calibrated photometer in a mechanical arrangement to move the photometer over the solid angle of

interest [37]. This method lends itself to immediate application of standard detectors because a photometer can be directly calibrated as an illuminance meter and upon geometric considerations

and suitable summation of signals, the luminous flux is directly deduced.

A second frequently used integration method is the use of an integrating sphere for averaging the

output of a particular source and deriving its luminous flux by comparison to a known source in

the same sphere. This method relies upon the availability of a luminous flux standard source and

the ability to make appropriate corrections for source configuration issues. NIST has recently

developed a new method utilizing an integrating sphere but relying upon the comparison to a known externally provided luminous flux introduced into the sphere by a measured illuminance

within a defining aperture [38,39] .

Figure 9 illustrates the main features of the NIST integrating sphere approach to a direct detector

measurement of the luminous flux. A critical aspect of this approach has been the development of

techniques to characterize the integrating sphere for effects from sources that supply luminous flux

in different manners. An illuminance or irradiance standard lamp is used as a source of external

radiation with the only requirements on its performance being stability of output and operation at an

identified color temperature. A precision aperture of area A defines the amount of luminous flux

that will enter the sphere through an opening.

The standard illuminance meter measures the illuminance E^ behind the aperture, and assuming the

flux is homogeneous in space, the total flux C^m entering the sphere is,

0,„ =E^'A [Im] . (12)

17
Window Monitor Photometer or Spectroradiometer

0in=Ev'A Standard

Photometer

(NIST illuminance

scale)

'<"):.V!^

Illuminance /

Irradiance

Standard

Lamp

^

test ~ ^

in X (signal ratio of Monitor Photometer)

Figure 9. Schematic diagram of the integrating sphere method NIST uses to determine

luminous flux based upon the detector candela.

The monitor photometer or spectral radiometer records the signal as a result of supplying the

known flux. The test lamp is then placed into operation and the external source eclipsed with a

shutter. A new reading on the monitor allows for determination of the luminous flux of the test lamp by the comparative equation.

^resi - ^in * (monltor signal ratio) • (corrections) (13)

18
The corrections depend upon angular correction factors for light impinging upon different regions

of the sphere and for the fact that the external light impinges upon the sphere at an oblique angle

whereas the test lamp provides illumination at nearly normal incidence. Other corrections can occur

for color temperature variations between the external source and test lamp. All these corrections

are small, with the largest on the order of 1 % and smallest on the order of a few tenths of a

percent. Ohno has written a detailed analysis of the correction strategy and the reader is referred to the original literature for the details [13,40]. NIST believes that the luminous flux unit can be

maintained in this manner to a relative combined standard uncertainty of 0.3 %. The wide dynamic

range of the photometer allows for a characterization of the sphere and the calibration of a lamp at a given lumen level to be transferred to other lamps over a large range of lumen values.

As a practical matter for calibration purposes it is convenient to calibrate a selection of standard

lamps using the absolute method described above. These standard lamps can be used to calibrate

other lamps by ordinary substitution methods employing the sphere. Periodically the sphere must

be checked and the standard lamp calibrations verified. The characterization of the sphere

described in the references is a straightforward procedure and can be duplicated in most well

equipped photometry laboratories and provides a mechanism for other laboratories to establish a luminous flux unit based upon calibrated photometers. A single calibrated photometer could then

be used to assure the maintenance of a number of photometric quantities with good stability and

accuracy. It might be necessary to have periodic measurement verifications provided by NIST to ensure that the procedures developed in an individual laboratory are correct and maintained over

time. Implementing these procedures could result in considerable savings to calibration

laboratories by not maintaining large selections of calibration artifacts. NIST encourages its major

customers to consider this strategy for incorporation into their long term planning. An update to the NIST SP250 document that describes the photometric calibration program at NIST and which

contains many details of how to establish the detector based photometric program is in preparation

[41].

An extensive discussion of colorimetry is beyond the scope of this Technical Note. Hence the

discussion will be limited to the strategy for measurements based upon detectors. Colorimetry

consists of measuring either direct, reflected, or transmitted radiation with sensor systems whose

responses are weighted as shown in Fig. 7. The result is a set of numbers which define a point in a suitably chosen color coordinate space. Detector filter systems can be constructed that give

relative responses that are proportional to the curves in Fig. 7 and can be calibrated on the DSC in

the same manner as the photometer. In fact the "green" curve in Fig. 7 is the V(X) curve. Appropriate corrections for errors introduced by differing source spectral distributions and other

effects can be determined to provide a strategy to utilize the DSC calibrated detectors to maintain

colorimetric measurements. This project is in its early stages at NIST and will be reported on at an

appropriate time.

b) Spectral Radiometry

The spectral radiance and irradiance units have traditionally been based upon the use of blackbody

sources with temperatures defined in terms of accepted melting or freezing points of materials

[4,5]. The blackbody sources are characterized to ensure their spectral output can be correctly

described by Planck's radiation law. Recently Mielenz, et al. determined the freezing point of gold

using calibrated detectors to determine the absolute spectral radiance within a selected wavelength

band and inferred the temperature by application of Planck's radiation law [42]. Using the ideas

19
developed by Mielenz, et al. NIST has designed a FR based system coupled with a variable

temperature blackbody to maintain the units of spectral radiance and irradiance [43, 44]. This

realization is based upon concepts associated with the conservation of radiance in a nonabsorbing

medium with a constant index of refraction [14, 45]. Figure 10 illustrates the basic geometry and

ideas associated with the method employed to determine the irradiance and radiance units. If A\

and A2 are the areas of aperture 1 and aperture 2 respectively, the flux A0emergent through A2 can be expressed as

AO =

L«A, • COS0, • A-,* cosG-

[W] , (14)

where L is the radiance of the wide aperture source, d is the distance between the midpoints of the

apertures and the 9s are the angles defining the inclination of the apertures with respect to the

central ray passing through the midpoints of the area elements [45]. These concepts can be

generalized for apertures of an arbitrary size by considering the limit of small areas in eq. ( 14) and

summing contributions from the elements of area using integration techniques. In this connotation

the areas Ai and A2 become elements of area of larger apertures at the positions indicated in Fig.

10. Using this idea and letting the elements of area become differentials, the total flux CP exiting

aperture A2 can be determined by integrating the radiance over the two apertures,

O -n

A,A2

L 'dA, 'COsGi •dAj •cos02

7

[W]. (15)

In the general circumstance this is a complicated integration because the radiance may vary over the

aperture and the distance J is a function of the position on the apertures. Our application provides

some simplifying and useful conditions which make the problem tractable in mathematical form.

The sources to be employed for radiance and irradiance measurements are Lambertian sources and

have sufficiently large extent to completely overfill the aperture A]. Additionally the detector has a diameter sufficient to intercept all the radiadon exiting through aperture A2. This field of view is delineated by the extreme rays shown in Fig. 10. With these assumptions, L is constant and can

be removed from the integrals. The result is often conveniently written;

O -J dA, • cosG, • dA2 • cos92

A, A^

[W] (16)

= L•A^ •%• F^_2 '

20
where F\^2 is the configuration factor defined for the two apertures [45,46]. For the

circumstance we are exploring, F\^2 is given by the following:

F,., = l/2[^-[^^-4.(^)^]"^] ,

x= 1+ -, , (17)

D _ !_ p _ £_

where r\ and r2 are the radii of the apertures A] and A2 respectively. It is convenient to rewrite eq. (16) in terms of the variables, r\, rj, and d, to obtain insight into its application.

O = L.^.[(r;+ rl+ d')- [{r]+ r\^ d^- A^r\^r\X''\ (18)

The quantity {r\^ + Y'^}- + d'2) can be factored out and since d is much larger than the radii of the

apertures, the expression can be expanded using the binomial expansion with the following result:

r ^ 2 „ 2 2 2 L •K •r^ •7t ^2 r, Tj

(rj+r^+d^)

^ {r]+rl+d'y ^

2 •(1 + 6+...) . , (19)

L 'A, •A2

D

2 . __ 2 r * r

>2 / 2 , 2 . j2 \ c

' 1 ' 2

The major functional relation in eq. (19) can be seen to evolve directly from eq. (14) if the angles

are assumed small and the distance between the two apertures is large compared to the dimensions

of the apertures. For this discussion it is assumed that the apertures are large compared to the

wavelength of light being used and hence diffraction and interference effects can be ignored. In some cases in radiometry and photometry this approximation may not be valid.

21
Wide aperture i source

Aperture of area, /A-j

Detector

Aperture of area, A2

Figure 10. Diagram illustrating relationships for the fundamental concepts in the measurement

of radiance and optical power. The vertical dotted lines represent two reference planes

separated by a distance, d. Two apertures, Ai and A2, determine the geometry of the

arrangement and the amount of optical power incident upon the detector from the wide aperture source.

In the NIST implementation of the direct radiance determination, the radii r\ and r2 are 3 mm and 2

mm respectively and d is 500 mm: hence the first correction in the brackets, in eq. (19), is on the

order of 1.4 x 10- 4. For most applications with these types of dimensions the radii squared terms can be neglected with respect to J 2 in the denominator terms. The variable temperature blackbody

source is designed to provide a spatially uniform beam and hence we can write the irradiance in the

plane of aperture 2 as.

E = —- .(1 + 5 + ...) (20)

22
The same equations govern the respective spectral quantities where the radiance L is replaced by

the spectral radiance Lx- Detector systems shown in Figs. 5 and 6 can be used to deduce the

temperature of the wide aperture variable temperature blackbody source and hence establish units

of spectral radiance and irradiance and radiation temperature. This technique relies upon knowing

the aperture areas to at least the intended accuracy of the measurement or as a practical matter,

somewhat better than the desired accuracy. To assist in the achievement of the highest accuracy in

these measurements, NIST has developed a new facility to characterize apertures employed for

radiometric and photometric purposes. The facility features the capability of aperture area measurement with a relative combined standard uncertainty of 0.04 % [47].

The NIST system designed to determine the spectral radiance and irradiance units is shown

schematically (distances not to scale) in Fig. 11. The configuration meets the criteria for overfilling

aperture A\ and having sufficient aperture to collect all the light exiting aperture Ai- The spectral

responsivity of the detector, s{X), is determined and the responsivity spatial uniformity

characterized in the DSC. It is important to understand the spatial uniformity since the detector is underfilled and in some cases detectors have shown position sensitive responsivity. In cases where the response variations are large enough to affect the measurement accuracy, the spatial

response should be appropriately averaged over the area of the detector to be utilized in the

experiment. With these assumptions the output current of the detector system is,

i, = J

In I 0,(^).5(^). ^1 (A) •S{A.} •dX

X

(21)

= ^^

•(1 + 5+...) \ L,{X)^s{X)'dX

X

The spectral radiance L^ (A) and spectral radiant flux 0^ (A) are known functions of wavelength X

and absolute temperature Tand are given by the well discussed Planck radiation law [45,48].

Equation (21) can be numerically solved to find a value of temperature that satisfies the conditions

of the equation. The accuracy of the temperature determination is directly related to the accuracy of

the determination of geometric quantities, the current, and the FR spectral responsivity, s{X). It is important to have the FR characterized over the entire wavelength range of sensitivity of the

photodiode or other photo-conversion device to account for any out-of-band problems in the filter used. For example, if the filter has significant infrared leakage and the detector is a silicon

photodiode, significant errors can result due to the increasing output of thermal sources in the

infrared. These issues have been discussed in the literature [42-44]. NIST expects temperature to be determined to within 0. 10 K [49]. As a check on stability and to provide redundancy of

measurement, several FRs will be used to determine the temperature and to monitor the calibration

procedures.

23
„ . . T^ V ' A 4. Precision Defining

Precision Defining Aperture

Aperture

of area Ai = 28.27 mm^

^f area A2 = 12.57 mm

wvxMaxMgan

w

^SMMMMMM

Wide

Aperture

Source

^2H

d = 500 mnr Filter Detector system

Figure 11. Schematic diagram of NIST apparatus to determine scales of spectral radiance and

irradiance. The linear dimensions are not to scale in order to better show the details of the

measurement. The wide aperture blackbody source has an opening of 17 mm and is placed

approximately 60 mm behind the first aperture. The 10 mm square detector is approximately 43 nmi

behind aperture A2

The spectral irradiance Ex (X) can be obtained by examination of eq. (20) when it is rewritten in the

spectral form,

E.W =

^,{X) L,{X)'A,

D'

• (1+ 6+ ...) (22)

When the spectral irradiance is measured at NIST, an integrating sphere is used in conjunction with

a known aperture and a monochromator. Hence the aperture A2 designated in eq. (22) is not

necessarily the same one used in the temperature determination but it must meet the same

dimensional requirements discussed above in terms of geometrical constraints. In practice the

known spectral irradiance of the blackbody is used to calibrate the combined integrating sphere and

monochromator system and determine the spectral irradiance of other sources by relative

measurement. The spectral radiance measurements will be made utilizing an imaging system in

conjunction with the monochromator system employed for the irradiance work. In normal

operation a working standard lamp may be used to transfer from the variable temperature

blackbody to calibration test lamps. In this arrangement the FRs can be used to check the stability

of the measurement technique with time.

24
The procedures outlined here provide a mechanism to estabhsh source spectral units in terms of a FRs absolute spectral responsivity. This responsivity is directly traceable to the HACR and

geometric configuration factors which can accurately be determined. The same procedures can be

carried out by other laboratories to establish and maintain their source spectral units and could

result in better stability of the calibration endeavor and perhaps a savings in the purchase and

maintenance of source standards.

c) Pyrometry

The accurate radiometric determination of the gold point was the first step in establishing the

temperature unit based upon absolute detectors [42]. The procedures oudined in Ref [44] and in

the solution of eq. (21) in the previous section define the temperature unit based upon absolute

radiometry. The increasing availability of high quality infrared and near infrared detectors afford

the opportunity to extend this methodology to even lower temperatures. It is important to characterize the quality of the blackbody sources used in these calibration strategies in order to ensure uniformity, high emissivity, stability, and account for any diffraction effects. Efforts are underway at NIST to develop control techniques for blackbody sources that have a radiometric sensor for the input to the feedback system and thus stabilize the system for the parameters of

concern. The Special Publication [9] describing pyrometry at NIST is being rewritten and should

be available in the near future.

The discussions in these sections have focussed on the use of FR systems using interference filters or other bandpass limiting mechanisms. The concepts developed do not depend on a particular

technology for providing the bandpass limiting mechanism and in fact could include well

characterized dispersive instruments using gratings or prisms. As the technology develops it may

be possible to consider dispersive instruments with array detectors that capture a portion of the

spectra and can define temperature directly as well as provide increased speed in determining

spectrum distributions. Fourier transform spectroscopic devices offer some hope for application in

this area but will require further development of the procedures to characterize these instruments

for full spectroradiometric use.

V. Conclusions and Suggestions

The advent of cryogenic electrical substitution radiometers with unprecedented accuracy has

provided the opportunity to recast the procedures and techniques utilized to maintain radiometric

and photometric units. In the case of photometry the impetus for change is further assisted by the

development of stable, reliable, and inexpensive silicon photodetectors for use in photometric

measurements. The availability of high quality solid state current amplifiers allows silicon detector

systems to provide measurement over many decades of optical or photometric power. These

detector systems can be calibrated at NIST or other calibration laboratories and, with the

measurement strategies outline above, photometric and radiometric units can be defined and

maintained for the calibration of many types of optical sensing equipment. As pointed out in the

photometry section of this Technical Note, the maintenance of a single photometer by an

organization interested in photometric measurement could suffice to define all the calibration

parameters ordinarily required and thereby obviate the need for the care and maintenance of a large

number of lamp standards.

Obtaining the national optical measurement units from an accurate cryogenic radiometer allows for

their simultaneous improvement as the techniques for better utilization of the cryogenic radiometer

are developed. While improvement in the accuracy of the cryogenic radiometer itself would not

25
necessarily be of immediate benefit, the ability to perform improved transfer measurements to other

devices can produce corresponding improvements to the measurement chain. Improvements in

transfer standard detectors in all wavelength regions will assist the efforts to propagate the benefits

of the high accuracy of the HACR to radiometric enterprises in general. NIST has programs in

these areas to develop better mechanisms for calibrations in the infrared and the ultraviolet. An

additional effort is underway to improve the accuracy of the NIST synchrotron ultraviolet radiation

facility, SURF II, by relating its spectral output to the HACR through a FR system. The SURF II facility provides a source of radiation from about 1 nm to the far infrared for use in a variety of

scientific endeavors including far UV radiometry. This will assist the effort to unify the NIST

radiometric units maintained in differing ways by various technologies.

NIST customers of radiometric and photometric services are encouraged to renew their thinking on

calibration strategies for the future to take advantage of the savings and increased accuracy

suggested by the developments outlined in this note. Using a well characterized detector system

offers the possibility of a significant reduction in the number of calibration artifacts necessary to maintain a direct traceability to NIST.

26
References

* Uncertainties in this technical note follow the nomenclature and suggestion in B.N. Taylor

and C.E. Kuyatt, "Guidelines for Evaluating and Expressing the Uncertainty of NIST

Measurement Results," NIST Technical Note 1297, U.S. Government Printing Office,

Washington, DC 20402-9325, 1994.

[1] See the collection titled "The Theory of Heat Radiation," volume 1 1 of the American

Institute of Physics collection in the History of Modern Physics, 1988.

[2] CIE Proceedings 1924, Cambridge University Press, Cambridge, 1926,

"The Basis of Physical Photometry," Publication CIE number 18.2 , CIE , Kegelstrasse

27, A- 1030, Vienna, 1983.

[3] B.N. Taylor, "Guide for the Use of the International System of Units (SI)," NIST Special

Publication 811, 1995, U. S. Government Printing Office, Washington, DC 20402-9325,

1995.

[4] J.H. Walker, R.D. Saunders, J.K. Jackson, and D.A. McSparron, "Spectral Irradiance

Calibrations," NBS Special Publication SP250-20, U. S. Government Printing Office,

Washington, DC 20402-9325, 1987.

[5] J.H. Walker, R. D. Saunders, and A.T. Hattenburg, "Spectral Radiance Calibrations,"

NBS Special Publication SP250-1, U. S. Government Printing Office, Washington, DC

20402-9325, 1987.

[6] R.L. Booker and D.A. McSparron, "Photometric Calibrations," NBS Special Publication

SP250-15, U. S. Government Printing Office, Washington, DC 20402-9325, 1987.

[7] H.J. Kostkowski, "The National Measurement System for Radiometry and Photometry,"

NBSIR 75-939, National Bureau of Standards, Gaithersburg MD, 1977.

[8] The CORM report can be obtained from the CORM Secretary,

Dr. Arthur Springsteen

Labsphere, Inc. P.O. Box 70

North Sutton, NH 03260-0070

[9] W.R. Waters, J.H. Walker, and A.T. Hattenburg, "Radiance Temperature Calibrations,"

NBS Special Publication SP250-7, U. S. Government Printing Office, Washington, DC

20402-9325, 1987.

[10] "Techniques for approximating the international temperature scale of 1990," BIPM,

Pavilion de Brueteuil, F-92310, Sevres, France, 1990.

27
[11] "Le Systeme International d'Unites (SI),"6e Edition, BIPM, Pavillion de Breteuil, F-923 10

Sevres, France.

[12] C.L. Cromer, G. Eppeldauer, J.E. Hardis, T.C. Larason, and A.C. Parr, "National

Institute of Standards and Technology detector based photometric scale," Appl. Opt. 32,

2936, 1993.

[13] Y. Ohno, "Realization of NIST Luminous Flux Scale using and Integrating Sphere with an

External Source," CIE Publication 1 19, 23rd Session, New Delhi, 1995, Central Bureau of

the CIE, Kegelstrasse 27, A- 1030, Vienna, Austria, 1989;

a related paper is accepted for publication in J. lES 1996, see Ref. 40.

[14] See for example the following and references therein:

W.R. McCIuney, "Introduction to Radiometry and Photometry," Artech House, Boston,

1994, ISBN 0-89006-678-7;

R.W. Boyd, "Radiometry and the Detection of Optical Radiation," John Wiley, New York,

1983, ISBN 0-471-86188-X;

C.L. Wyatt, "Radiometric Calibration: Theory and Methods," Academic Press, Orlando,

1978, ISBN 0-12-766150-6;

C.L. Wyatt, "Radiometric System Design," Macmillian Publishing Company, New York,

1987, ISBN 0-02-948800-1.

[15] F. Hengstberger, "Absolute Radiometry," Academic Press, Inc., Boston, 1989, ISBN 0-

12-340810-5.

[16] W.W. Coblentz, "Various modifications of Bismuth-Silver thermopiles having a continuous absorbing surface," Bulletin of the Bureau of Standards 11, 131, 1914, also

issued as Scientific Papers of the Bureau of Standards #229, March 1914;

W.W. Coblentz, "Studies of Instruments for Measuring Radiant Energy in Absolute Value:

An Absolute Thermopile," Bulletin of the Bureau of Standards 12, 503, 1915, also issued

as Scientific Papers of the Bureau of Standards #261, March 1916.

[17] W.W. Coblentz, "Present Status of the Determination of the Constant of Total Radiation

from a Black Body," Bulletin of the Bureau of Standards 12. 553. 1915, also issued as

Scientific Papers of the Bureau of Standards #262, February 1916.

28
[18] The literature on silicon detectors is too extensive to review here as it is not the central

theme of this Technical Note. The reader is referred to the following and the extensive

references contained therein:

W. Budde, "Physical Detectors of Optical Radiation," Volume 4 of the Optical Radiation

Measurement series, Academic Press, New York, 1983, ISBN 0-12-304904-0;

S. Middelhoek and S.A. Audet, "Silicon Sensors," Academic Press, London, 1989, ISBN

0-12-495051-5;

For a review of early NBS (NIST) efforts see,

J. Geist, M.A. Lind, A.R.Schaeffer, and E.F. Zalewski, "Spectral Radiometry: A new

approach based upon electro-optics," NBS Technical Note 954, U.S. Government Printing

Office, Washington, DC 20402-9325, 1977.

[19] D.C. Ginnings and M.L. Reilly, "Calorimetric measurement of thermodynamic

temperatures above OC using total blackbody radiation," Temperature; Its Measurement

and Control in Science and Industry, Vol 4, Instrument Society of America, Pittsburgh,

1972.

[20] C.R. Yokley, "Long Wave Infrared Testing at NBS," SPIE Vol 416-Apphcations of

Optical Metrology-Techniques and Measurements II, 1983;.

C. R. Yokley, "A Radiometric Calibration Facility for Low Temperature Blackbodies,"

Final Report to the NASA Lyndon B. Johnson Space Center, contract T-9550C,

November 23, 1976.

[21] T.J. Quinn and J.E. Martin, "A radiometric determination of the Stefan-Boltzmann constant

and thermodynamic temperatures between - 408 oC and + 1008 oC," Phil. Trans. Roy.

Soc. London, A316, 85, 1985.

[22] J.E. Martin, N.P. Fox and P.J. Keys, "A cryogenic radiometer of absolute radiometric

measurements," Metrologia 21, 147, 1985.

[23] The author knows of only two companies offering a commercial product, although other

laboratories have developed cryogenic radiometers for their own use.

In the United States,

Cambridge Research and Instrumentation, Inc

2 1 Erie Street Cambridge, MA 02139

In the United Kingdom,

Oxford Instruments, Ltd.

Eynsham

Oxford 0X8 ITL

UK

29
[24] At the 1994 CCPR meeting in Paris approximately 10 nations indicated that they had

purchased a cryogenic radiometer or were in the process of purchasing one and would use

it as the basis of the radiometric measurements in their respective nations.

[25] T.R. Gentile, J.M. Houston. J.E. Hardis, C.L. Cromer, and A.C. Parr, "National

Institute of Standards and Technology high-accuracy cryogenic radiometer," Appl. Opt.

35, 1056, 1996.

[26] T.R. Gentile, J.M. Houston, and C.L. Cromer, "Realization of a scale of absolute spectral

response using the NIST High Accuracy Cryogenic Radiometer," accepted by Applied

Optics 1996.

[27] There is considerable literature on the quantum efficiency of silicon and a review of this

topic is beyond the scope of this Technical Note. Some early NIST efforts in this regard

are summarized in a paper by Zalewski and Geist titled, "Silicon photdiode absolute

spectral response self-cahbration," Appl. Opt. 19, 1214, 1980;

See also a chapter by Geist titled, "Silicon Photodiode Self-Calibration," in Theory and

Practice of Radiation Thermometry, edited by D.P. DeWitt and G.D. Nutter, WileyInterscience Publications, Inc., New York, 1988, ISBN 0-471-61018-6.

[28] G.H. Rieke, "Detection of light: from the ultraviolet to the submillimeter," Cambridge

University Press, Cambridge, 1994, ISBN 0-521-41028-2.

[29] Private Communication, P. Boivin, NRC Ottawa.

[30] E.F. Zalewski, "The NBS Photodetector Spectral Response Transfer Program," NBS

Special Publication 250-17, U.S. Government Printing Office, Washington, DC 20402-

9325, 1988;

T.C. Larason, S.S. Bruce, and C.L. Cromer, "The NIST High Accuracy Scale For

Absolute Spectral Response From 406nm to 920 nm," J. Res. Natl. Inst. Stand. Technol.

101, 133, 1996.

L. Canfield, "Far Ultraviolet Detector Standards," NBS Special Publication 250-2, U.S.

Government Printing Office, Washington DC, 20402-9325, 1987.

[31] Electrical standards are maintained in the Electronics and Electrical Engineering Laboratory

at NIST.

[32] G. Eppeldauer and J.E. Hardis, "Fourteen-decade photocurrent measurements with large- area silicon photodiodes at room temperature," Appl. Opt. 30, 3091, 1991.

G. Eppeldauer, "Temperature monitored/controlled silicon photodiodes for

standardization," SPIE, 1479, 71, 1991.

30
[33] A.R. Schaefer and K.L. Eckerle, "Spectrophotometric tests using a dye-laser-based

radiometric characterization facility," Appl. Opt. 23, 250, 1984;

A. R. Schaefer and R. D. Saunders, "Intercomparison between silicon and blackbodybased radiometry using a sihcon photodiode/filter radiometer," Appl. Opt. 23, 2234, 1984.

[34] CIE Publication No. 15.2, "Colorimetry," Central Bureau of the CIE, Kegelstrasse 27, A1030, Vienna, Austria, 1986.

[35] "The Science of Color," Committee on Colorimetry, Optical Society of America,

Washington, DC, 1963.

[36] CIE Publication No. 69, "Methods of characterizing illuminance meters and luminance

meters," Central Bureau of the CIE, Kegelstrasse 27, A-1030, Vienna, Austria, 1986.

[37] CIE Publication No. 84, "The measurement of luminous flux," Central Bureau of the CIE,

Kegelstrasse 27, A-1030, Vienna, Austria, 1989.

[38] Y. Ohno, "New Method for Realizing a Total Luminous Flux Scale using an Integrating

Sphere with an External Source," J. lES. 24, 106, 1995.

[39] Y. Ohno, "Integrating Sphere Simulation-Application to Total Flux Scale Realization,"

Appl. Opt. 33, 2637, 1994.

[40] Y. Ohno, "Realization of NIST 1995 Luminous Flux Scale using an Integrating Sphere

Method," To appear in, J. lES. 1996.

[41] This document will be available in late 1996 as an update to NIST SP250-15 describing the

NIST photometric calibration program.

[42] K.D. Mielenz, R.D. Saunders, and J.B. Shumaker, "Spectroradiometric determination of

the freeing temperature of gold," J. Res. Natl. Inst. Stand. Technol. 95, 49, 1990.

[43] B.C. Johnson, C.L. Cromer, R.D. Saunders, G. Eppeldauer, J. Fowler, V.I. Saprtisky,

and G. Dezi, "A method of realizing the spectral irradiance based on an absolute cryogenic

radiometer," Metrologia 30, 309, 1993.

[44] B.K. Tsai, B.C. Johnson, R.D. Saunders and C.L. Cromer, "Comparison of filter radiometer spectral responsivity with the NIST spectral irradiance and illumnace scales,"

Metrologia 32, 1, 1995.

[45] F.E. Nicodemus, "Self-Study Manual on Optical Radiation Measurements," NBS

Tehcnical Note 910-1 through 910-8, U.S. Government Printing Office, Washington, DC

20402-9325. On a limited basis some of these documents are available from the

Radiometric Physics Division, NIST, Gaithersburg, MD 20899-0001.

31
[46] R. Siegel and J.R. Howell, "Thermal Radiation Heat Transfer," Third Edition, Hemisphere

Publishing Corp., Washington, 1992 ISBN 0-891 16-271-2.

[47] J. B. Fowler and G. Dezsi, "High Accuracy Measurement of Aperture Area Relative to a Standard Known Aperture," J. Res. Natl. Inst. Stand. Technol. 100, 277, 1995.

[48] D.P. DeWitt and G.D. Nutter, "Theory and Practice of Radiation Thermometry," John

Wiley and Sons, New York, 1988, ISBN 0-471-61018-6

[49] Private Communication, R.D. Saunders. The Special Publications, SP250-1 and

SP250-20 (Refs. 4 and 5), are being rewritten and will feature a complete uncertainty

discussion for the determinations indicated here.

32
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