New method of whiteness calibration of spectrophotometers

Abstract A method is described for determining and calibrating the CIE Whiteness Index (WI) of an OBA‐treated specimen without incorporating into the measurement system a mechanically adjustable UV filter. The method requires spectrophotometric measurement of the specimen under two lights: UV‐excluded to infer the reflectance r ( λ ) and UV to infer the fluorescence profile f ( λ ). Also required is a stored illuminant spectrum I in ( λ ) that covers at least the whole visible wavelengths. Only one scalar value k is then needed to mathematically characterize the total spectral radiance factor (TSRF) of the same specimen under I in ( λ ). Calibration uses a reference specimen with known WI, and adjusts k until the computed WI matches the reference WI value. If I in ( λ ) is the illuminant chosen from ASTM E313 to define the WI, the k adjustment reduces to solving a single quadratic equation for k that involves the reference WI. In general, relative to directly measuring a specimen's TSRF, it is more instrument‐independent to use the current method to characterize TSRF. Therefore, if a fluorescent specimen is measured using UV and UV‐excluded illuminants, a complete, instrument‐stable characterization of the specimen's TSRF can be derived by adjusting k , which can be done through a closed‐form relation of k to the WI.