The status of each of these products is elucidated under "Standard Reference Materials Issued," including those that have been discontinued. The image shown on this page illustrates a number of SRMs produced and supported by this program over the last two decades. A more important concept is that traceability should not be limited to a particular national metrology institute (NMI), inasmuch as the equivalence of numerous national transmittance scales is assured by interlaboratory comparisons. Secondary standards producers may provide (NIST) transmittance traceability through the use of our SRMs or through the Transmittance Calibration Program supported by the NIST Optical Technology Division.
However, we continue to support recertification of all of our individually-certified solid filters.
The extended range companion SRMs 19 have been discontinued for the same reason. In particular, the oldest NIST transmittance standard, SRM 930 (neutral density glass filters for spectrophotometry), was recently discontinued in favor of readily available equivalent products. The production of spectrophotometric SRMs may be discontinued as fit-for-purpose traceable reference materials are introduced by the commercial secondary standards sector. NIR transmittance (absorbance) is traceable through the NIST Calibration Program. Wavelength traceability is through recognized atomic transitions utilized as secondary standards of length. UV/visible transmittance traceability is to the second-generation High Accuracy Spectrophotometer (HAS II) patterned on the original instrument with modern automation.
The original objective of supporting accuracy in spectrophotometry has evolved with the world metrology system to encompass the provision of traceability to references of the highest order in transmittance (absorbance) and wavelength.
More recently, wavelength SRMs have been developed and produced for the near infrared (NIR) spectral region, and transmittance (absorbance) SRMs are under development for the NIR. This program, sometimes referred to as the Optical Filters program, has supported the development, certification, and (as specified) recertification of Standard Reference Materials (SRMs) for the verification of the transmittance (absorbance) and wavelength scales of spectrophotometers in the ultraviolet (UV) and visible spectral regions for several decades.
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