A Research On Color Measurement Methods And Classification Of Porcelain

In its hundreds years of development,porcelain has always fulfilled human's pursuit of art and beauty with its kaleidoscopic colors and graceful shininess.This is why colorimetry,an objective measurement of visual features,is now widely used in color research of porcelain.Conventional color measuring methods for porcelain take measurements only in a certain geometric condition,but as a truth of fact,porcelain demonstrates different colors in different illumination or viewing angles,plus that the gloss of porcelain,another surface reflection feature,can't be effectively described in a single geometric condition,it can be said that conventional color measuring methods can not represent the characteristics of porcelain surface thoroughly.As a solution,bidirectional reflectance distribution function(BRDF),a function of angle of incidence and angle of emergence,is adopted to express the reflecting features of porcelain.Firstly,according to the changing rate of radiance,7 polar angles of incidence,11 azimuthal angles and,corresponding to different angle of incidence and azimuthal angle,6 to 15 polar angles of emergence have made up 633 different angle pairs of illumination and viewing,in which spectral BRDF of a real porcelain is measured.The measured data with wavelength of 532 nm are then fitted with several common BRDF parametric models,their differences analyzed.After that,the measured data are interpreted from 3 aspects including spectra,color and gloss.How a porcelain may look different in distinctive geometric conditions and the law in it are explained.The experiment has given the result that common BRDF parametric models are not good at describing color changes in diffuse directions as they use Lambertian as the diffuse component.The main variance in specular directions is irrelevant to color,but a function of the porcelain's refraction index.In diffuse directions,the spectral components responsible for its color also change,effecting the final perceived color.As result,in CIELAB color space,lightness has its maxima in mirror direction,and decreases slower with azimuthal angle than with polar angle;as for chroma,it's the faintest in specular directions,and increases with the angle between angle of viewing and mirror direction,and decreases after a maxima.The radiance of highlight of the porcelain increases with the incidence angle,as well as absence of bloom gloss.