What are the requirements for the quality of optical glass lenses?Apexeloptic
The importance of optical glass lens has become an essential part of the machine vision system, which directly affects the quality of imaging. It is very important to choose optical glass materials, because different glass types have different characteristics. The following Xiaobian to share with you the basic properties of optical glass lenses:
I. Refractive index
The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in a given material – this is an indication of how the speed of light slows down as it passes through an optical material. The refractive index of the optical glass, ND, is specified at 587.6nm (helium D line). Materials with a low refractive index are often called “coronal glass”, while materials with a high refractive index are called “flint glass”. The typical refractive index tolerance of the components in our catalog is ±0.0005.
Dispersion is the phenomenon that the refractive index varies with wavelength. Using the Abbe value, VD is represented, which is defined as (nD-1)/(NF-NC); NF and nC are the refractive index at 486.1nm (hydrogen F line) and 656.3nm (hydrogen C line), respectively. A low Abbe value indicates a high dispersion. Corona glass usually has a lower dispersion than flint glass. The typical Abbe tolerance for the components in our catalog is ±0.8%.
Standard optical glass lenses provide high transmission across the visible spectrum and in the near-ultraviolet outer and near-infrared range. Corona glass usually has stronger transmission in NUV than flint glass. Due to the high transmittance of flint glass, which leads to high Fresnel reflection loss, it should always be specified for use with antireflection film.
What are the requirements for the quality of optical glass lenses?
1. The specific optical constants and the same batch of glass optical constants consistency
Each kind of optical glass has a standard refractive index value for different wavelengths of light, which is the basis for optical designers to design optical systems. Therefore, the optical constants of the optical glass produced in the factory must be within a certain range of allowable deviation of these values, otherwise the actual image quality will not match the expected result in the design and affect the quality of the optical instrument.
At the same time, because the same batch of instruments are often made of the same batch of optical glass, the allowable deviation of refractive index of the same batch of glasses should be more strict than the deviation between them and the standard value in order to facilitate the unified correction of instruments.
2. High transparency
The image brightness of the optical system is proportional to the transparency of the glass. The transparency of the optical glass to a certain wavelength light is expressed by the optical absorption coefficient kλ. When light passes through a series of prisms and lenses, part of its energy is lost to reflection at the interface of the optical component and part is absorbed by the medium (glass) itself.
The former increases with the increase of the refractive index of glass, and this value is very large for high refractive index glass, such as the loss of light reflection on a surface of heavy flinklock glass is about 6%. Therefore, for the optical system with multiple thin lenses, the main way to improve the transmittance is to reduce the reflection loss on the lens surface, such as coating the surface anti-reflection film.
For large optical parts, such as the objective lens of an astronomical telescope, the transmittance of the optical system is mainly determined by the optical absorption coefficient of the glass itself due to its large thickness.
By improving the purity of glass raw materials and preventing any tinting impurities from mixing in the whole process from ingredients to melting, the light absorption coefficient of glass can be generally less than 0.01.