Light diffusion member generally includes a light diffusion member including a light-transmitting substrate having present therein fine particles or gas bubbles having a refractive index different from that of the substrate, and a light diffusion member including a light-transmitting substrate, such as a ground glass, having provided on the surface thereof fine irregularities by a treatment such as sandblasting or etching. Those light diffusion members scatter or diffuse light such as ultraviolet rays by fine particles in the substrate or fine irregular shape of the surface thereof.
In a light diffusion member containing fine particles having a refractive index different from that of a substrate, a degree of light diffusion can be changed by the refractive index of the particles, shape of the particles or concentration of the particles. However, light transmittance thereof is generally about 40 to 60%, and transmission loss of light is large. Furthermore, in a light diffusion member, like a ground glass, including a substrate having provided on the surface thereof fine irregularities, ultraviolet rays can be diffused, but a diffusion angle is narrow and it is difficult to obtain sufficient diffusibility.
As the light diffusion member, for example, Patent Document 1 discloses a silica bonded body formed by bonding plural silica porous bodies to each other or bonding a silica porous body to a silica dense body such as a quartz glass through a silica powder. Patent Document 1 discloses that to bond the silica porous body to the silica dense body, a silica powder having the same quality as those materials is used, and thus the silica bonded body in which those are bonded with high bonding strength is obtained without clogging the whole pores of the porous body.
However, in the silica bonded body described in the Patent Document 1, a silica dense body being a quartz glass is bonded to the silica porous body after producing each of those. Therefore, lead time of processing is long, costs are increased, and productivity was not said to be sufficient. Furthermore, an adhesive layer using a silica powder is present in the vicinity of an interface between the silica dense body and the silica porous body. Therefore, in the silica bonded body, transmission efficiency of ultraviolet rays tends to be deteriorated, and additionally the thickness of the adhesive layer in which a silica powder intervenes tends to be heterogeneous. Thus, in-plane uniformity of the intensity of ultraviolet rays emitted from a spot light source was not sufficient.