There are various methods for strengthening glass, such as hy- drofluoric acid etching [19], heat treatment [20], crystallization [21], and ion exchange [22,23]. In general, the optical and me- chanical properties are affected by the quality of the glass surface. In the case of hydrofluoric acid etching, flaws and cracks on the glass surface are eliminated, thus improving the mechanical properties, but the resulting increase of surface roughness will decrease the transmittance of light owing to the increase of reflectance. Crystallization forms crystal-phase precipitates inside the glass, thus reinforcing the glass by increasing its density. However, increased light scattering may occur as a result of the increased crystal size, thus decreasing transmittance in the visible range. The ion exchange method, in which the glass is dipped in a potassium nitrate powder and tempered in order to exchange po- tassium ions having a large atom radius with sodium ions inside the glass, thus creating compressive stress inside the glass, is the most reasonable method of strengthening glass because the resulting glass exhibits excellent mechanical properties with a short process time. In addition, it is possible to control the penetration depth of the potassium ions, thus decreasing optical transmittance loss after forming the compressive layer.