Lens stress is a term that is rarely mentioned, but it is sometimes the main culprit that causes visual discomfort and fatigue. When processing frame glasses, some processors inclines to grind the lenses slightly larger in order to prevent the lenses from being too loose and falling off the frame. If the screws are fixed on the frame, they will be tighter to avoid slipping off. However, this operation is likely to increase the stress on the lens and cause discomfort to wear.
Today we recommend a convenient way to detect lens stress.
Lens stress means that when the ground lens is installed on the frame, the lens surface is not uniformly refraction due to reasons such as too large lens size or too tight fixing of the frame screws. Thereby forming water wave wrinkles, a phenomenon that affects the image quality. The stress change of the lens cannot be directly observed by the naked eye, and it can be detected only by using a special optical instrument - a glass polariscope.
There are two ways to generate lens stress showing as below:
One is that the compression density around the lens increases due to the tightening of the lens. Meanwhile, the refractive index changes, forming a "birefringence" phenomenon.
The second is uneven extrusion to form "wrinkles" on the surface of the lens, causing deflection and scattering.
If it is a short-term stress deformation, it can be relieved or even recovered after the external force is removed. However, if it is the stress deformation caused by long-term extrusion, it may not be able to recover even if it is reinstalled. It can only be re-customized.
Lens stress is relatively more common in full-rim glasses. In half-rim glasses, if the drawing hook is too tight, it will also exist. It usually occurs in peripheral parts and has little effect on human visual quality. So it is not easy to be perceived. But if the residual stress is too large, it will affect the central optical zone, causing blurred vision and visual fatigue.
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