Sensor Shading
Most camera sensors used for vision applications include microlenses that gather more of the available light and focus it onto the active pixel area.
However, under certain circumstances, when the incident light falls on the sensor at a very shallow angle, some of the light is refracted such that it no longer falls on the active area of the pixel. This effect is called 'microlens shading' and it can be particularly noticeable on sensors with asymmetric pixels.
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