Three direction measurements for characterization of a surface containing metallic particles

DETAILED DESCRIPTION OF THE INVENTION In optically characterizing surfaces containing metallic particles such as metallic paints and films, it was recognized that directional reflectance had to be considered.
Metallic paints contain light-reflecting flakes or platelets of such material as aluminum, bronze, coated mica and the like.
These flakes or platelets function much like little mirrors, reflecting light directionally rather than in a diffuse manner.
The directional reflectance characteristic of a metallic paint film results in a phenomenon known as goniochromatism, which is defined as the variation in color of a paint film as a function of the directions of illumination and viewing.
This phenomenon is also sometimes described as "two-tone", "flop", "flip-flop", "flash", "side-tone", etc.
In sum, the color of a metallic paint will appear different at different viewing angles.
To account for this directional or angular reflectance, i.
e, goniochromatism, spectrophotometrically determined reflectance factors must be taken multiangularly.
The reflectance factor of a paint film is the ratio of the light flux reflected from the film sample to the light flux reflected from a perfect reflecting diffuser when the sample and perfect diffuser are identically irradiated.
A perfect white reflector has a value of 1.
A perfect black nonreflector has a value of 0.
The reflectance factors are used to calculate color descriptor values used to specify color and color difference.
The tristimulus values (X, Y, Z) of a color are calculated by combining the reflectance factor data (R) with data on the sensitivity of the human eye (x y z and the irradiance of a light source (E) all as functions of wavelength (.
lambda.
) in the visible spectrum.
The defining equations for tristimulus values are: ##EQU1## The tristimulus values can be used to calculate color descriptors which relate to visual perception of color and color difference.
One of many sets of descriptors which can be used are the CIELAB perceptual color scales recommended by the International Commission on Illumination ("Recommendations on Uniform Color Spaces, Color Difference Equations, Psychometric Color Terms", Supplement No