Glass beads are a new type of silicate materials with good reflective properties of the regression between when the refractive index of 1.9 to 2.1. That is, when a beam of light within a certain range in any angle onto the surface of the beads, the beads due to the refraction of light to be condensed on the rear reflective layer of microbeads particular, the reflective layer reflecting the light back along a direction parallel to the light source, Figure 1 is a diagram of retroreflective glass beads. (FIG. 1 as an optical axis OP, C is the angle of incidence, b is the angle of refraction, n is the refractive index of the glass beads, no is the refractive index of air.)
Figure 3. Retroreflective glass beads
When the angle of incidence C ≠ 0, the condition can occur in accordance with the refractive retroreflective theorem: nosin C = nsin b, due to no = 1, so that sin C = nsin b, obtained by optical theory C = 2b, so that the refractive index of the glass beads n = 2cos b. We know from the optical processing, C≤90°, which is b≤45°. Thus only when n≥√2 when retroreflective glass beads phenomenon will occur, but this time as a result of the reflection of light can not be concentrated in a small cone angle, retroreflective glass beads phenomenon is not clear, but close to the optical axis beam by the refractive index of refraction after n≥√2 glass beads with different refractive indices have three kinds of focusing situation:
1. The refractive index √2 <n <1.93, the focus falls outside the beads;
2.n = 1.93, the focus falls on the inner surface of the beads;
3. The refractive index n> 1.93, the focus falls on the microbeads.
We can also thick lens equation (1) is applied to glass beads up on the retroreflective glass beads, chiefly of the analysis
F = nr√2 (n-1) (1) where, F the focal length; the refractive index n of the glass; r the diameter of the glass beads. For the light passes near the center of the microbeads for, n = 时 2, F = r, so that the focus is located on the back of glass beads, in which case it presents a complete retroreflective characteristics. However, the spherical aberration due to the influence of the surrounding light is refracted by the very large, so the light from the back side through 37°at the center, in the n = 1.9, the microbeads gather in its focus, then in order to present a complete retro-reflective properties.