In this paper, we examined the physical meaning of the widely used gloss evaluation. Gloss is one of the important qualities for materials. A visual test of the sharpness of a specular reflection image, i.e. gloss, on the surface of materials is often performed to estimate the gloss of materials. If the specular reflection image is sharp, we estimate that it has high gloss. This means that gloss can be estimated by measuring the sharpness of the specular reflection image. In this case, a line light source such as a fluorescent lamp is often used. Here we call it the line light source observation method. We developed a measurement apparatus based on the line light source observation method. As a result of experimental verification, it is confirmed that the Line Spread Function of Specular Reflection (SR-LSF) can be derived from the line light source observation images. However, the reflected image of line light source observation method is considered to be strictly different from SR-LSF. On the other hand, it is clarified that the line source observation method is an approximate SR-LSF measurement method. We also introduce the reconstruct method of gloss unevenness image by following visual observation conditions and image processing in human brain.