The stress size of optical materials is one of the seven indicators to evaluate the quality of optical materials (refractive index, dispersion, index consistency, optical absorption coefficient, optical uniformity, stress, stripes, bubbles or stones).The stress of optical materials on optical instruments is mainly shown in the following three aspects:
(1) when the stress is large, it is easy to cause material cracking during optical processing such as cutting and grinding.Even a small amount of stress can deform a high quality optical surface.
(2) due to birefringence caused by stress, the imaging beam of a point will no longer converge at a point, which will have similar effects with stray light, resulting in deterioration of image quality.
(3) the uneven surface stress distribution of optical materials shall lead to the non-uniformity of the refractive index everywhere, which will lead to the deformation of the optical wave wave surface after passing through the optical materials, which will also cause the deterioration of the image quality.
Therefore, stress birefringence testing of optical materials is very necessary.With the development of infrared technology, various kinds of infrared materials and infrared equipment are increasingly used.As far as the infrared optical lens is concerned, most of the optical materials used are the sapphire crystal and other infrared crystal materials, and the stress birefringence index also has an important influence on the imaging quality of the infrared system, which must be tested strictly.