Black Glass calibration values and traceability statement

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Question:

I was asked during a recent audit to provide a calibration and traceability certificates for the black glass I use to calibrate my 45:0 instrument. I said that none were provided when I purchased the instrument. Why were they not provided?

Answer:

We follow the ASTM E1164 Standard Practice for Obtaining Spectrophotometric Data for Object Color Evaluation. The full text of ASTM E1164 is available for purchase from the www.ASTM.org

Referring to section 10.2.1 which states that the Full Scale Standardization shall be done using a White Reference Standard calibrated relative to the perfect Reflecting diffuser. This implies that the White Instrument Standard has unique calibrated values which would need to be substantiated by a Calibration and Traceability statement.

Referring to section 10.2.2.1 for 0:45 and 45:0 Zero Scale Standardization shall be done with "a highly polished black glass standard with an assigned reflectance factor of zero." This implies that for any highly polished black glass the reflectance factor is zero at all measured wavelengths. There is no calibration required to assign the reflectance factor at each wavelength. The reason why no calibration is required is that except for a few very expensive instruments maintained by the NMI's of the world the true reflectance of polished black glass is many magnitudes below a commercial instruments ability to measure it.

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