The reduced amount of reflection enhances the appearance of the spectacles, and is particularly effective in improving the appearance of higher powered lenses and high refractive index materials. Photochromatic lenses Photochromatic lenses have a tint which automatically adjusts for the amount of light present. They do this by incorporating a chemical which becomes darker when it absorbs ultra-violet light. The more light is present, the darker the lenses become. Photochromatic lenses allow one pair of spectacles to function as prescription sunglasses, as well as regular spectacles.
Photochromatic lenses are available in grey and brown tints, and in both glass and resin materials.
They generally take a few cycles of darkening and fading to reach their full efficiency, so that after some use they often become darker and change more quickly than when they are brand new. Photochromatic lenses are temperature sensitive, although this is generally noticeable only if they are left on the dashboard of a car in summer, for example, and photochromatic resin lenses may also fade after a few years. The cost depends on many factors, such as the lens design, lens material, coatings, tints, and the prescription. Generally speaking, more complex lens designs are more expensive, so that progressive lenses cost more than bifocals, which in turn cost more than single vision lenses. High refractive index materials are more expensive than materials with lower refractive indices, so that for a given prescription, thinner lenses will cost more than thicker lenses.