As a method for inspecting a surface of a product, generally known conventional method is a method wherein the light is irradiated on the product from upward by the use of a lighting device having a luminous section on its lower surface and inspection is conducted by means of monitoring or filming the reflecting light at a position near the lighting device. At this time, if the luminous intensity on the surface to be inspected is uneven, there might be a case of failing in detecting a minute flaw on a surface to be inspected or a defect of finishing. As a result, multiple luminous bodies such as multiple LEDs are used as the luminous section wherein the multiple luminous bodies are arranged on the under surface of the lighting device thoroughly, the luminous bodies emit light downward and the luminous intensity on the surface to be inspected is kept even and constant.
However, with the lighting device of the above-mentioned arrangement, since the luminous bodies are arranged on the under surface of the lighting device thoroughly, a large number of the luminous bodies is required, thereby taking considerable time for assembling. In addition, since the luminous intensity on the surface to be inspected becomes uneven to some extent and the unevenness of the luminous intensity on the surface to be inspected might be further promoted due to fluctuation of the direction in which each luminous body is mounted when assembling the lighting device, precise assembly accuracy is also required. Furthermore, in case that the surface to be inspected is a gloss surface, the luminous body itself is reflected on the gloss surface, thereby failing in detecting the surface state of the specimen.
As a result, in order to solve the problems of the above-mentioned conventional lighting device, the present claimed inventor has developed a lighting device (patent document 1) wherein multiple luminous bodies are arranged on an outer surrounding surface of a plate-shaped transparent body having a center bore and light is emitted toward the center bore from the luminous bodies arranged on the outer surrounding surface so that a lower surface of the transparent body surface-emits the light. With this lighting device (hereinafter called as a prior lighting device), an effect can be produced that the luminous surface emits the light with an even luminous intensity. As a result of this, the luminous intensity on the surface to be inspected also becomes even and constant so as to preferably detect a defect such as a microscopic flaw or a defect of finishing on the surface to be inspected.
Patent document 1: Japan Patent Laid-open number 10-21717