Apochromatic

Explanation

Apochromatic designs require optical glasses with special dispersive properties to achieve three color crossings. This is usually achieved using costly fluoro-crown glasses, abnormal flint glasses, and even optically transparent liquids with highly unusual dispersive properties in the thin spaces between glass elements. The temperature dependence of glass and liquid index of refraction and dispersion must be accounted for during apochromat design to assure good optical performance over reasonable temperature ranges with only slight re-focusing. In some cases, apochromatic designs without anomalous dispersion glasses are possible.