indium gallium selenide is a material used in solar cells. Photoconductors Amorphous selenium (α-Se) thin films have found application as photoconductors in flat panel x-ray detectors. These detectors use amorphous selenium to capture and convert incident x-ray photons directly into electric charge. Rectifiers Selenium rectifiers were first used in 1933. Their use continued into the 1990s. Other uses Small amounts of organoselenium compounds have been used to modify the catalysts used for the vulcanization for the production of rubber. The demand for selenium by the electronics industry is declining. Its photovoltaic and photoconductive properties are still useful in photocopying, photocells, light meters and solar cells. Its use as a photoconductor in plain-paper copiers once was a leading application, but in the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers switched to organic photoconductors. Though once widely used, selenium rectifiers have mostly been replaced (or are being replaced) by silicon-based devices. The most notable exception is in power DC surge protection, where the superior energy capabilities of selenium suppressors make them more desirable than metal-oxide varistors. Zinc selenide was the first material for blue LEDs, but gallium nitride dominates that market. Cadmium selenide was an important component in quantum dots. Sheets of amorphous selenium convert X-ray images to patterns of charge in xeroradiography and in solid-state, flat-panel X-ray cameras. Ionized selenium (Se+24) is one of the active mediums used in X-ray lasers. Selenium is a catalyst in some chemical reactions, but it is not widely used because of issues with toxicity. In X-ray crystallography, incorporation of one or more selenium atoms in place of sulfur helps with multiple-wavelength anomalous dispersion and single wavelength anomalous dispersion phasing. Selenium is used in the toning of photographic prints, and it is sold as a toner by numerous photographic manufacturers. Selenium intensifies and extends the tonal range of black-and-white photographic images and improves the permanence of prints. 75Se is used as a gamma source in industrial radiography. Pollution In high concentrations, selenium acts as an environmental contaminant. Sources of pollution include waste materials from certain mining, agricultural, petrochemical, and industrial manufacturing operations. In Belews Lake North Carolina, 19 species of fish were eliminated from the lake due to 150–200 μg Se/L wastewater discharged from 1974 to 1986 from a Duke Energy coal-fired power plant. At the Kesterson National Wildlife Refuge in California, thousands of fish and