When the illuminator and receptor axes of goniophotometer are fixed orthogonally and a glossy surface of specimen is so rotated that its normal approaches the latter axis from the initial position of specular reflection, the reflected flux I from the specimen decreases rapidly at first and then slowly as the angle of rotation θ increases.
Using a photometer capable of measuring the relative brightness, the slowly decreasing part of I-θ curve is found to be expressed approximately in the form of I2 exp(-βθ2) for many papers. The gloss number GNF1) is defined as the ratio of I0/I2 where I0 is the value of I at the initial position θ=0. The values of GNF for a series of 14 papers, white and coloured, are compared with the visual gradings of Japanese observers. And it is found that, the logarithms of GNF are linearly correlated with the visual gradings of 13 observers except for three white papers, but the correlation holds good for all the papers when the gradings of those two observers who have participated in similar tests several times in the past are taken as the values of visual gloss.
