ASTM E1341-96
(Practice)Standard Practice for Obtaining Spectroradiometric Data from Radiant Sources for Colorimetry
Standard Practice for Obtaining Spectroradiometric Data from Radiant Sources for Colorimetry
SCOPE
1.1 This practice prescribes the instrumental measurement requirements, calibration procedures, and physical standards needed for precise spectroradiometric data for characterizing the color and luminance of radiant sources.
1.2 This practice lists the parameters that must be specified when spectroradiometric measurements are required in specific methods, practices, or specifications.
1.3 This practice describes the unique calculation procedures required to determine basic colorimetric data of luminous sources.
1.4 This practice is general in scope rather than specific as to instrument, or object, or material.
1.5 The values stated in SI units are to be regarded as the standard.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 1341 – 96
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Practice for
Obtaining Spectroradiometric Data from Radiant Sources
for Colorimetry
This standard is issued under the fixed designation E 1341; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The fundamental procedure for characterizing the color and absolute luminance of radiant sources
is to obtain the spectroradiometric data under specified measurement conditions, and from these data
to compute CIE chromaticity coordinates and luminance values based on the CIE 1931 Standard
Observer. The considerations involved and the procedures to be used to obtain precision spectrora-
diometric data for this purpose are contained in this practice. The values and procedures for computing
CIE chromaticity coordinates are contained in Method E 308. This practice includes minor
modifications to the procedures given in Method E 308 that are necessary for computing the absolute
luminance of radiant sources.
1. Scope of Ultraviolet, Visible, and Near-Infrared Spectrophotom-
eters
1.1 This practice prescribes the instrumental measurement
E 284 Terminology Relating to Appearance
requirements, calibration procedures, and physical standards
E 308 Practice for Computing the Colors of Objects by
needed for precise spectroradiometric data for characterizing
Using the CIE System
the color and luminance of radiant sources.
E 387 Test Method for Estimating Stray Radiant Power
1.2 This practice lists the parameters that must be specified
Ratio of Spectrophotometers by the Opaque Filter Method
when spectroradiometric measurements are required in specific
E 925 Practice for the Periodic Calibration of Narrow Band-
methods, practices, or specifications.
Pass Spectrophotometers
1.3 This practice describes the unique calculation proce-
E 958 Practice for Measuring Practical Spectral Bandwidth
dures required to determine basic colorimetric data of luminous
of Ultraviolet-Visible Spectrophotometers
sources.
2.2 NIST Publications:
1.4 This practice is general in scope rather than specific as
NIST Technical Note 594-1 Fundamental Principles of Ab-
to instrument, object, or material.
solute Radiometry and the Philosophy of the NBS Pro-
1.5 The values stated in SI units are to be regarded as the
gram (1968–1971)
standard.
NIST Technical Note 594-3 Photometric Calibration Proce-
1.6 This standard does not purport to address all of the
dures
safety concerns, if any, associated with its use. It is the
2.3 CIE Publications:
responsibility of the user of this standard to establish appro-
Publication CIE No. 15.2 Colorimetry, 2nd ed., 1986
priate safety and health practices and determine the applica-
Publication CIE No. 38 Radiometric and Photometric Char-
bility of regulatory limitations prior to use.
acteristics of Materials and their Measurement, 1977
2. Referenced Documents
Publication CIE No. 63 Spectroradiometric Measurement of
Light Sources, 1984
2.1 ASTM Standards:
2.4 IES Standard:
E 275 Practice for Describing and Measuring Performance
Annual Book of ASTM Standards, Vol 03.06.
Annual Book of ASTM Standards, Vol 06.01.
Available from National Institute of Standards and Technology (NIST),
This test method is under the jurisdiction of ASTM Committee E-12 on
Gaithersburg, MD 20899–0001.
Appearance and is the direct responsibility of Subcommittee E12.06 on Appearance
Currently available through the U.S. National Committee of the CIE, c/o Mr.
of Displays.
Thomas M. Lemons, TLA-Lighting Consultants, Inc., 7 Pond Street, Salem, MA
Current edition approved June 10, 1996. Published August 1996. Originally
01970-4819.
published as E 1341 – 91. Last previous edition E 1341 – 91.
E 1341
IES Guide to Spectroradiometric Measurements, 1983 6.1.3 The spectral parameters, including the spectral region,
2.5 ANSI Standard: wavelength measurement interval, and spectral bandwidth.
ANSI/IES RP-16-1980 Nomenclature and Definitions for 6.1.4 The type of standard used to calibrate the system, a
Illuminating Engineering standard lamp, a calibrated source, or a calibrated detector, and
the source of the calibration.
3. Terminology
7. Apparatus
3.1 Definitions:
3.1.1 The definitions of appearance terms in Terminology
7.1 The basic instrument requirement is a spectroradiomet-
E 284 are applicable to this practice.
ric system designed for the measurement of spectral radiance
or irradiance of light sources. The basic elements of a spectro-
4. Summary of Practice
radiometric system are calibration sources with their regulated
4.1 Procedures are given for selecting the types and oper-
power supplies, a light detector, electronics for measuring the
ating parameters of spectroradiometers used to produce data
photocurrents, a monochromator with control equipment for
for the calculation of CIE tristimulus values and other color
computer interfacing, receiving optics, and a computer as
coordinates to describe the colors of radiant sources. The
described in CIE 63 and IES Guide to Spectroradiometric
important steps of the calibration of such instruments, and the
Measurements. The computer is listed as an integral part of the
standards required for these steps, are described. Parameters
system since the required precision is unobtainable without
are identified that must be specified when spectroradiometric
automated control. The characteristics of each element are
measurements are required in specific methods or other docu-
discussed in the following sections.
ments. Modifications to Practice E 308 are described in order
7.2 Calibration Sources—The standard calibration lamp for
to account for the differences between objects and radiant
spectroradiometry is a tungsten-filament lamp operated at a
sources.
specified current. Such lamps are available from many stan-
dardizing laboratories. Typical of such standards is the tungsten
5. Significance and Use
filament, 1000 W, halogen cycle, quartz-envelope FEL-type
5.1 The fundamental method for obtaining CIE tristimulus
lamp recommended by the National Institute of Standards and
values or other color coordinates for describing the colors of
Technology (NIST). (See NIST Tech Note 594-1, and 594-3.)
radiant sources is by the use of spectroradiometric measure-
Uncertainties in the transfer of the scale of spectral radiance or
ments. These measurements are used by summation together
irradiance are about 1 %. It is preferable to have more than one
with numerical values representing the CIE 1931 Standard
standard source to permit cross-checks and to allow calibration
Observer (CIE publication No. 15.2) and normalized to K , the
m
at a range of illuminance levels. Such sources can be con-
maximum spectral luminous efficacy function, with a value of
structed from lamps operating at any color temperature and
683 lm/W.
spectral nature that have been characterized against a standard
5.2 This practice provides a procedure for selecting the
lamp. Monochromatic emission sources, such as a low-
operating parameters of spectroradiometers used for providing
pressure mercury arc lamp or tunable laser, should also be
the desired precision spectroradiometric data, for their calibra-
available for use in calibrating the wavelength scale in accor-
tion, and for the physical standards required for calibration.
dance with Practice E 925. Multiline lasers, such as continuous
5.3 Special requirements for characterizing sources of light
wave (cw) argon-ion and helium-neon, are preferred since they
possessing narrow or discontinuous spectra are presented and
can be tuned to a small number of lines of well known
discussed. Modifications to the procedures of Practice E 308
wavelengths.
are given to correct for the unusual nature of narrow or
7.2.1 Calibration Source Power Supplies—The electrical
discontinuous sources.
supplies for the calibration sources should be of the constant
current type. The supply should be linear and not a switching
6. Requirements When Using Spectroradiometry
supply. Current regulation should be maintained to better than
6.1 When describing the measurement of radiant sources by
0.1 %. This level of regulation is required to maintain a
spectroradiometry, the following must be specified.
constant flux across the entrance to the spectroradiometer.
6.1.1 The radiometric quantity determined, such as the
7.2.2 A standard for the measurement of length (such as a
2 2
irradiance (W/m ) or radiance (W/m -sr), or the photometric
high-quality metric rule) should also be available since abso-
quantity determined, such as illuminance (lm/m ) or luminance
lute irradiance calibrations must be performed at exact dis-
2 2
(lm/m -sr or cd/m ). The use of older, less descriptive names or
tances from the filament of the standard lamp.
units such as phot, nit, stilb (see ANSI/IES RP-16-1980) is not
7.3 Detectors:
recommended.
7.3.1 Photomultiplier Tubes—Photomultiplier tubes are the
6.1.2 The geometry of the measurement conditions, includ-
traditional detectors in spectroradiometers. This is due to their
ing whether a diffuser was used and its material of construc-
superior performance in low-light-level conditions such as are
tion, the distances from the source of irradiation to the entrance
encountered at the exit slit of a low-efficiency monochromator.
to the spectroradiometer, and the presence of any special
The photocathodes of photomultipliers are sensitive to tem-
intermediate optical devices such as integrating spheres.
perature, polarization, and magnetic fields. Light levels on the
photocathode should never be allowed to generate photocur-
−6
rents in excess of 10 A. The high-voltage supply should be
Available from American National Standards Institute, 1430 Broadway, NY,
NY 10018–3308. stabilized to better than 0.01 % since the gain of the multiplier
E 1341
tube is controlled by the voltage across the dynodes. The CIE recommends the use of a rotatable integrating sphere
as the input optics (CIE No. 63). The entrance port of the
7.3.2 Silicon Photodiodes—Recently, silicon photodiodes
sphere is rotated to view first the calibration source and then to
have superseded photomultiplier tubes in radiometric instru-
view the test source. Since the efficiency of integrating spheres
ments. Photodiodes are less sensitive to temperature, polariza-
tend to be rather low, this method is only useful for bright
tion, and magnetic fields than photomultipliers, but care should
sources.
still be taken to control these variables.
7.6 Computer System:
7.4 Monochromators—The monochromator is the wave-
7.6.1 There are no special requirements for the computer.
length dispersive element in the system. The region of the
Any minicomputer or microcomputer should suffice. The
monochromator should be 360 nm to 830 nm for highest
program should control or monitor as many of the instrument
accuracy, but a region of 380 nm to 780 nm should suffice for
parameters as possible. Included in the computer system is the
most characterizations. The bandwidth should be kept constant
analog to digital conversion process, which changes the pho-
across the region of measurement at between 85 % and 100 %
tocurrents to voltages, amplifies the voltages, and digitizes the
of the measurement interval, but no greater than 5.0 nm. The
voltages into computer-readable signals. A 3 ⁄2 digit autorang-
CIE recommends a 1.0 nm bandwidth and measurement
ing digital ammeter with a computer interface is suitable for
interval for highest accuracy, and suggests 2.0 nm as a
this purpose. Alternatively, an autoranging electrometer with a
compromise for characterizing radiation sources with spectra
computer interface can be used, but shielding and guarding of
that contain both continuous and line emissions (CIE No. 63).
the low level signals becomes more critical. This is equivalent
The precision of the wavelength setting should be 0.1 nm with
to a twelve bit ADC (analog to digital converter) with variable
an absolute accuracy of better than 0.5 nm. The size and shape
gains on the input signal. The use of a detector housing with a
of the entrance and exit slits of the monochromator should be
built-in current to voltage amplifier is recommended since the
chosen to provide a symmetric bandshape, preferably triangu-
photocurrents are very small and can be affected by stray
lar. The entrance slit should be completely and uniformly filled
electromagnetic fields and capacitances. Amplification and
with light. Specialized versions of the general spectroradiom-
conversion to voltage at the detector package minimizes these
eter may be constructed and used for specific applications
effects and will provide the voltage signal necessary for
where the instrument can depart from the above guidelines. For
common ADC converters that are available for mini and
example, a source with little or no radiant energy in the far red
microcomputers.
end of the visible spectrum may be correctly characterized by
measurements to 700 or 710 nm rather than 780 nm. 7.6.2 The data that is acquired by the computer should be
converted and displayed as real number values. The raw
7.4.1 Scanning Monochromators—The newer technology
readings should be corrected for dark current by subtraction of
of holographically reproduced gratings has made possible the
the measured signal with no light impinging on the entrance to
production of single- and double-grating monochromators with
the monochromator, and then scaled to the absolute values of
very high throughputs and very low stray-light levels. Second-
the calibration source measurements. The results can be
order spectra need to be eliminated through the use of either a
displayed on any appropriate device, though a printed copy is
predisperser or a long-pass filter. A drive mechanism and
a desirable option. The corrected values should be stored on the
position encoder should be attached to the scanning monochro-
computer’s storage media for later processing. The generation
mator drive to allow the monochromator to scan the wave-
of a plot of normalized radiance (irradiance) versus wavelength
length region under control of a computer. Prism-based scan-
is also desirable, since a skilled operator will be able to obtain
ning monochromators can also be used though the drive
much useful information for both diagnostic and analytic
mechanism is more complex and the slit width m
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