光敏電阻器

光敏電阻的主要物料為硫化鎘，硫化鎘化學式為 CdS，因此光敏電阻通常簡稱 CDS 。特性是其電阻值隨光的強度變化，光愈強，電阻值愈低；光愈暗，電阻值愈高。
『光敏電阻器的阻值以非線性的遞減。光敏電阻器多以硒化鎘(CdSe)或硫化鎘(CdS)所製成，把這些材料摻入銅或氯以控制光敏電阻器的阻值對光的變化有正確的反應。如光敏電阻器處於強光環境下，它的阻值會減低；在黑暗時，它的阻值會提高。』
http://www.ats.edu.hk/eca/eeclub/Web_Study%5CCh16%5CCh16_sum.htm

『The more light that strikes the cell, the lower the resistance. Although not accurate, even a simple CdS cell can have a wide range of resistance from less than 100 Ω in bright light to in excess of 10 MΩ in darkness. Many commercially available CdS cells have a peak sensitivity in the region of 500nm - 600nm (green light). The cells are also capable of reacting to a broad range of frequencies, including infrared (IR), visible light, and ultraviolet (UV). They are often found on street lights as automatic on/off switches. They were once even used in heat-seeking missiles to sense for targets.』
http://en.wikipedia.org/wiki/Photoresistor#Cadmium_sulphide_cells

不過，由於主要物料含鎘，不符合目前歐洲的 RoHS 禁用有害物質要求，不少生產商已經停產有關產品，而製造商多以光敏二極管或光敏晶體管等代替。
『The Restriction of Hazardous Substances in Electrical and Electronic Equipment (ROHS) Directive will restrict the import of cadmium containing products to the European Union. As a direct result of this, Hamamatsu will cease its production of photocells containing cadmium in the very near future.』
http://www.sales.hamamatsu.com/en/products/solid-state-division/compound-semiconductors/cds.php