Optical sensors employing CdS (cadmium sulfide) cells are photosensitive devices used to detect and measure light intensity levels. CdS cells change resistance in response to variations in incident light, making them suitable for applications such as automatic lighting control, photographic exposure meters, and dusk-to-dawn switches. These sensors exhibit a nonlinear response to light, with resistance decreasing as light intensity increases. CdS cells offer simplicity, reliability, and cost-effectiveness compared to other light-sensing technologies. Despite their declining popularity due to environmental concerns associated with cadmium, CdS cells remain in use in certain niche applications where their characteristics meet specific requirements.