The difference between achromatic lens and other glasses is that as a part of optical system, it must meet the requirements of optical imaging. Therefore, the determination of the quality of achromatic glue mirror also includes some special and stricter indexes. The following requirements are required for achromatic lens:
1. High transparency
The brightness of optical system imaging is proportional to the transparency of glass. The transparency of achromatic lens for a certain wavelength of light is expressed in terms of light absorption coefficient K_ lambda. When light passes through a series of prisms and lenses, part of its energy is dissipated by the reflection of the interface of the optical parts and the other part is absorbed by the medium (glass). The former increases with the increase of refractive index of glass, and the value of high refractive index glass is very high. For example, the light reflection loss on a surface of heavy flint glass is about 6%.
Therefore, for optical systems with multi-thin lenses, the main way to improve the transmittance is to reduce the reflection loss of the lens surface, such as coating the antireflective film on the surface. For large optical components such as astronomical telescopes, the transmittance of the optical system depends mainly on the optical absorption coefficient of the glass itself because of its large thickness. By improving the purity of glass raw materials and preventing any coloring impurities from mixing in the whole process from batching to smelting, Generally, the light absorption coefficient of glass can be less than 0.01 (that is, the light transmittance of glass with thickness of 1 cm is more than 99%).
2. The consistency of achromatic lenses, specific optical constants and optical constants of the same batch of glasses
Each kind of achromatic lens has a standard refractive index value for different wavelength light, which can be used as the basis for optical designer to design optical system. Therefore, the optical constants of the achromatic lens manufactured in the factory must be within the allowable deviation range of these values, otherwise the actual imaging quality will be inconsistent with the expected results in the design and the quality of the optical instruments will be affected. At the same time, because the same batch of glasses are often manufactured with the same batch of achromatic lens, in order to facilitate the uniform calibration of the instrument, the tolerance deviation of refractive index of the same batch of glasses is more stringent than that of their standard values.