ASTM E387-04
(Test Method)Standard Test Method for Estimating Stray Radiant Power Ratio of Dispersive Spectrophotometers by the Opaque Filter Method
Standard Test Method for Estimating Stray Radiant Power Ratio of Dispersive Spectrophotometers by the Opaque Filter Method
SCOPE
1.1 Stray radiant power (SRP) can be a significant source of error in spectrophotometric measurements, and the danger that such error exists is enhanced because its presence often is not suspected (). This test method affords an estimate of the relative radiant power, that is, the Stray Radiant Power Ratio (SRPR), at wavelengths remote from those of the nominal bandpass transmitted through the monochromator of an absorption spectrophotometer. Test-filter materials are described that discriminate between the desired wavelengths and those that contribute most to SRP for conventional commercial spectrophotometers used in the ultraviolet, the visible, the near infrared, and the mid-infrared ranges. These procedures apply to instruments of conventional design, with usual sources, detectors, including array detectors, and optical arrangements. The vacuum ultraviolet and the far infrared present special problems that are not discussed herein.Note 1
Research () has shown that particular care must be exercised in testing grating spectrophotometers that use moderately narrow bandpass SRP-blocking filters. Accurate calibration of the wavelength scale is critical when testing such instruments. Refer to Practice E 275.
1.2 These procedures are neither all-inclusive nor infallible. Because of the nature of readily available filter materials, with a few exceptions, the procedures are insensitive to SRP of very short wavelengths in the ultraviolet, or of lower frequencies in the infrared. Sharp cutoff longpass filters are available for testing for shorter wavelength SRP in the visible and the near infrared, and sharp cutoff shortpass filters are available for testing at longer visible wavelengths. The procedures are not necessarily valid for "spike" SRP nor for "nearby SRP." (See Annexes for general discussion and definitions of these terms.) However, they are adequate in most cases and for typical applications. They do cover instruments using prisms or gratings in either single or double monochromators, and with single and double beam instruments.Note 2
Instruments with array detectors are inherently prone to having higher levels of SRP. See Annexes for the use of filters to reduce SRP.
1.3 The proportion of SRP (that is, SRPR) encountered with a well-designed monochromator, used in a favorable spectral region, typically is 0.1 % transmittance or better, and with a double monochromator it can be less than 110-6, even with a broadband continuum source. Under these conditions, it may be difficult to do more than determine that it falls below a certain level. Because SRP test filters always absorb some of the SRP, and may absorb an appreciable amount if the specified measurement wavelength is not very close to the cutoff wavelength of the SRP filter, this test method underestimates the true SRPR. However, actual measurement sometimes requires special techniques and instrument operating conditions that are not typical of those occurring during use. When absorption measurements with continuum sources are being made, it can be that, owing to the effect of slit width on SRP in a double monochromator, these test procedures may offset in some degree the effect of absorption by the SRP filter; that is, because larger slit widths than normal might be used to admit enough energy to the monochromator to permit evaluation of the SRP, the stray proportion indicated could be greater than would normally be encountered in use (but the net effect is still more likely to be an underestimation of the true SRPR). Whether the indicated SRPR equals or differs from the normal-use value depends on how much the SRP is increased with the wider slits and on how much of the SRP is absorbed by the SRP filter. What must be accepted is that the numerical value obtained for the SRPR is a characteristic of the particular test conditions as well as of the performance of the instrument in normal use. It is an indication of whether high absorbance measuremen...
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Designation: E387 – 04
Standard Test Method for
Estimating Stray Radiant Power Ratio of Dispersive
1
Spectrophotometers by the Opaque Filter Method
This standard is issued under the fixed designation E387; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope gratings in either single or double monochromators, and with
single and double beam instruments.
1.1 Strayradiantpower(SRP)canbeasignificantsourceof
error in spectrophotometric measurements, and the danger that
NOTE 2—Instruments with array detectors are inherently prone to
such error exists is enhanced because its presence often is not
having higher levels of SRP. SeeAnnexes for the use of filters to reduce
2
SRP.
suspected (1-4). This test method affords an estimate of the
relative radiant power, that is, the Stray Radiant Power Ratio
1.3 TheproportionofSRP(thatis,SRPR)encounteredwith
(SRPR), at wavelengths remote from those of the nominal
a well-designed monochromator, used in a favorable spectral
bandpasstransmittedthroughthemonochromatorofanabsorp-
region, typically is 0.1% transmittance or better, and with a
-6
tion spectrophotometer. Test-filter materials are described that
double monochromator it can be less than 1310 , even with a
discriminate between the desired wavelengths and those that
broadband continuum source. Under these conditions, it may
contribute most to SRP for conventional commercial spectro-
be difficult to do more than determine that it falls below a
photometers used in the ultraviolet, the visible, the near
certain level. Because SRP test filters always absorb some of
infrared, and the mid-infrared ranges. These procedures apply
theSRP,andmayabsorbanappreciableamountifthespecified
to instruments of conventional design, with usual sources,
measurement wavelength is not very close to the cutoff
detectors, including array detectors, and optical arrangements.
wavelength of the SRP filter, this test method underestimates
The vacuum ultraviolet and the far infrared present special
the true SRPR. However, actual measurement sometimes
problems that are not discussed herein.
requires special techniques and instrument operating condi-
tions that are not typical of those occurring during use. When
NOTE 1—Research (3)hasshownthatparticularcaremustbeexercised
absorption measurements with continuum sources are being
in testing grating spectrophotometers that use moderately narrow band-
pass SRP-blocking filters.Accurate calibration of the wavelength scale is made, it can be that, owing to the effect of slit width on SRP
critical when testing such instruments. Refer to Practice E275.
inadoublemonochromator,thesetestproceduresmayoffsetin
some degree the effect of absorption by the SRP filter; that is,
1.2 These procedures are neither all-inclusive nor infallible.
because larger slit widths than normal might be used to admit
Because of the nature of readily available filter materials, with
enough energy to the monochromator to permit evaluation of
afewexceptions,theproceduresareinsensitivetoSRPofvery
the SRP, the stray proportion indicated could be greater than
short wavelengths in the ultraviolet, or of lower frequencies in
wouldnormallybeencounteredinuse(buttheneteffectisstill
the infrared. Sharp cutoff longpass filters are available for
more likely to be an underestimation of the true SRPR).
testing for shorter wavelength SRP in the visible and the near
Whether the indicated SRPR equals or differs from the
infrared, and sharp cutoff shortpass filters are available for
normal-use value depends on how much the SRP is increased
testing at longer visible wavelengths. The procedures are not
with the wider slits and on how much of the SRP is absorbed
necessarily valid for “spike” SRP nor for “nearby SRP.” (See
by the SRPfilter. What must be accepted is that the numerical
Annexesforgeneraldiscussionanddefinitionsoftheseterms.)
valueobtainedfortheSRPRisacharacteristicoftheparticular
However, they are adequate in most cases and for typical
test conditions as well as of the performance of the instrument
applications. They do cover instruments using prisms or
in normal use. It is an indication of whether high absorbance
measurements of a sample are more or less likely to be biased
1
This method is under the jurisdiction ofASTM Committee E13 on Molecular
by SRP in the neighborhood of the analytical wavelength
SpectroscopyandisthedirectresponsibilityofSubcommitteeE13.01onUltraviolet
where the sample test determination is made.
and Visible Spectroscopy.
Current edition approved Feb. 1, 2004. Published March 2004. Originally 1.4 The principal reason for a te
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