Gloss Evaluation Method for Metal Surface with Hairline Using Patterned Area Illumination

Gloss evaluation of a metal surface is important for the quality as the product. In general, gloss of a metal surface is influenced by specular reflection of light on the surface. Therefore to evaluate the specular reflection is almost the same as the evaluation of the gloss. We propose the method to evaluate the specular reflection using patterned area illumination. In the specular reflection, the angle of incidence is equal to the angle of reflection. Consequently when lights (rays) of the area illumination which has a stripe pattern are projected to a glossy metal surface, the stripe pattern occurs on the reflection image by the mirror-like reflection and the sharpness of the stripe pattern of the image has information of the specular reflection on the surface. This character is applied for the metal surface with hairline. The metal surface with hairline has different gloss feeling at different point of view. When the direction of the stripe pattern of the illumination is changed, the sharpness of stripe pattern on the reflection image varies owing to the surface roughness of hairline. Therefore we can evaluate the glossiness for various directions by rotating the stripe pattern. This paper shows the availability of the proposed method through the experiment.