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.