APOchromatic): it can be imagined that if a material with the change of the refractive index of the value of the wavelength can be arbitrarily controlled, then we can design a completely without the color of the lens. It is a pity that the dispersion of the material can not be controlled arbitrarily. We step back to imagine, if we can be divided into the visible spectrum of blue - green, green - red two ranges, and these two ranges can be applied to eliminate color difference technology, two levels of spectrum can be basically eliminated.

However, after calculation shows: if the green and red light to eliminate color, then the blue color will become very big; if the blue light and green color, the red color will become great. Theoretical calculation for apochromatic find a way, if the low refractive convex lens manufacturing rate of blue light emitting materials of green part of relative color exactly and manufacturing a concave lens of high refractive rate part of the material relative color the same, then the realization of blue and red achromatic after, green color just eliminate.

This theory points out that the realization of apochromatic right way, is to look for a kind of special optical materials. Its blue to red light relative dispersion should is very low, and blue to green part of the relative dispersion should be very high and a high dispersion material with the same. Fluorite is such a kind of special material, the dispersion is very low (Abbe number up to 95.3) and relative partial dispersion and many optical glass close. Fluorite (CaF2, molecular formula CaF2) refractive index is relatively low ND=1.4339, slightly soluble in water (0.0016g/100g water), processing and poor chemical stability, but due to its excellent achromatic properties, making it become a kind of precious optical materials. Fluorite was first used only in microscope, since the fluorite crystallization process to achieve, the high long focal lens in fluorite is almost an indispensable material.

Because of the fluorite is expensive, difficult processing, the optical company has spared no effort to the search of fluorite substitutes, fluoro crown glass is one of the. The company called AD glass, ED glass, UD glass, often is this kind of substitute.