Measurement of sixty-degree specular gloss

Abstract

Specular gloss is the attribute next to color that is most often used in the evalua(,ion of the appearance of obj ect s. For 10 years t he American Society for T estin g N[a(,erials has used a method of test for the 60 0 specular gloss of pain t fini shes based on research done at this Bureau. The ASTM method prescribes the illuminator and receiver apertu res that are to be used. However, in order to determine the uncertain ties involved in the calibra-tion of gloss standards, the rate of change of gloss reading with change of aperture was investigated throughout the gloss scale. Thi s was done by modifying an existing gonio-photometer so that it became in effect a versatile glossmeter with provis ion for accuratcly controllin g all of the known geometrical vari ables involved. Data obtained on the varia-tion of gloss readin g with aperture show that standards with widely different distr ibutions of re flected ligh t can be used to check t he adjustment of glossmeters and the conformance of thei r apertures to ASTM specifications in the in creasingly importan t medium-to high-glos range. The problem of obtaining the d ir-ru e correction, which in vol ves sou ree polari za-tion and depart ure of the specimen from a pe rfect diffuso r, has been investigated. The appearance of an object depends upon several factors; the illuminant, the reflection char-acteristics of the material, the surface texture, the illuminating and viewing geometry, and the observer . The color and glossiness of a specimen are determined by the spectral composition and geometrical distribution of the incident light and upon the transformations that take place upon reflection from the specimen. These two appear-ance attributes, color and gloss, arc somewhat related. Color measurement may require cor-rection for surface glossiness. Specular gloss measurement may require correction for diffuse reflectance. FIGURE 1. T ypical goniophotometric curve for a paint spe6men , with defining terms.