Photosensitive glasses that produce color upon UV irradiation followed by thermal treatment have been known for many years.1–3 A typical example of a photosensitive image in glass is the so-called gold ruby as shown in Figure 2.1. The "ruby" notation comes from the color and is actually produced from gold nanoparticles, as will be shown. A typical photosensitive glass composition is provided in Table 2.1.