To assess the quality of finished surfaces various schemes have been devised to measure some parameter and assign a figure of merit to the finish. For evaluating automotive paint or other finishes, two of the most important and generally accepted measurements are the distinctness of image and gloss. These are not used each to the exclusion of the other but rather they are employed together since they measure different surface characteristics. As a consequence, two separate meters are employed, each dedicated to a single parameter measurement. The extra cost and inconvenience of using two separate instruments is evident.
Surface reflectivity is a function of material properties such as index of refraction. Of the total light reflected some will be reflected at a specular angle and the remainder will be scattered at other angles. The specularly reflected portion is a measure of gloss. Thus rough surfaces and matte finishes tend to scatter the light and result in low gloss. Conventional gloss meters measure the light reflected through a small aperture at the specular angle and are calibrated to a standard index. The distinctness of image of a finish is a measure of the sharpness of images reflected from the surface. For example the edge of an image may be sharp or blurred to some degree. It depends on the scattering properties of the surface and the paint pigments and flakes. My previous patent, U.S. Pat. No. 4,527,898, disclosed a distinctness of image meter projecting a light onto a test surface and sensing the light reflected from the surface over a large angle and focused through an aperture. The light is interrupted by a moving blade near the source and the sharpness of the image at the aperture determines how rapidly the detected light level changes. The rate of change is a measure of the distinctness of image and depends on the amount of scattering while the maximum detected amplitude depends on the total reflected light. Thus all the information for determining the surface gloss is available in the detected light signal in the distinctness of image instrument. I have discovered that the gloss measurement can be made by my distinctness of image meter with a modification of the signal processing at substantially no additional cost.