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Precision thermoforming of optical glass lenses has been an advanced process. It has the advantages of one-shot molding, high efficiency and low cost, suitable for large-scale production of glass lenses, and environmental friendliness. But thermoforming technology is a high temperature optical glass molding process. Due to the uneven temperature in the cooling process and the complex state change of glass in its transition period, the final residual stress is frozen in the glass lens. The existence of residual stress will cause birefringence and refractive index deviation of the glass lens, thus affecting the optical performance of the lens. In order to study the residual stress distribution in the glass lens, we built a circular polariscope, and used two different ways to measure the birefringence of the heat-treated glass.
refractive index of the test lens: 1.76.
Through the stress optical retardation chromaticity diagram (the red part has the largest value and the blue part has the smallest value), we seem to get this conclusion: the center area has smaller stress birefringence, and the edge area has larger stress birefringence).
test lens refractive index: 1.76
immersion liquid refractive index: 1.64.
When the test lens is completely immersed in the immersion liquid, we obtained the following stress optical retardation diagram. We found that the stress distribution was very uniform throughout the glass lens, with no obvious regions of high stress birefringence.
When we further explore which of the above two conclusions is closer to the true state, we need to consider a factor. The test lens is a convex lens, thicker in the center and thinner at the edges. In order to eliminate the influence of this special shape on the stress birefringence, it is recommended to select an immersion liquid with a refractive index close to the test lens for measurement.
