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An optical polariscope is a benchtop device used in optical laboratories and shops. It consists of a light source and two polarized filters. Its purpose is to make invisible stress patterns within an eyeglass lens visible to the human eye. It is a standard tool for quality control and safety testing.
read more about optical polariscope tool ...The tool works by passing light through two polarizing filters that are set at right angles to each other. Normally, this blocks all light, making the view dark. However, when you place a lens between the filters, any physical stress or strain in the lens material bends the light. This twisted light passes through the second filter, appearing as bright streaks or rainbow patterns against the dark background.
One of the most important jobs of a polariscope is to check glass lenses. In the United States, glass lenses must be heat or chemically tempered to be impact-resistant. A polariscope confirms this process was done. A heat-tempered glass lens will show a distinct "Maltese Cross" pattern of light and shadow. If this pattern is missing, the lens is not safe to wear.
For plastic lenses, the polariscope is used to check the fit of the frame. If a lens is cut slightly too large for the frame, the frame rim squeezes the lens. Under the polariscope, this pressure shows up as bright, spiky white or rainbow flares coming from the edges. This tells the optician that the lens is under too much tension and could chip or crack later.
Using a polariscope requires skill to read the patterns. A lens with no stress looks dark or gray. A lens with safe, uniform stress (like tempered glass) has a symmetric pattern. A lens with bad stress has random, jagged, or intense bright spots. These bright spots are "stress risers," which are weak points where the lens is most likely to break.
A polariscope is also a handy trick for identifying lens materials. Polycarbonate material has natural internal stress called birefringence. When you put a polycarbonate lens (even an unmounted one) under a polariscope, it will light up with dark rainbows or black crosses. Other materials like glass or standard plastic (CR-39) will appear relatively clear or gray unless they are being squeezed by a frame.
Most optical laboratories have them. Retail optical shops that finish their own lenses in-house will also have one. Shops that send all their work to a central lab might not have one on site.
A tiny amount of uniform tension is needed to keep a lens from falling out of the frame. However, uneven or excessive stress is bad because it can cause the lens to warp or break.
No. The test is non-destructive. It simply uses light to look inside the material. It does not touch or harm the lens or the frame.
You do not need to ask for this test, as it is part of the manufacturing process. However, if your lenses develop mysterious cracks near the edges or if they seem to warp and distort your vision over time, you can ask your optician to check them. They can use a polariscope to see if the frame is too tight and causing stress on the lenses.