Selenium is a highly photovoltaic and photoconductive material, that was used extensively in light-metering devices before the 1970s. Selenium light meters had a tendency to lose effectiveness over time as the selenium would "die out" from exposure to light and heat. By the 1970s CdS (Cadmium-Sulfide) meters became the standard, later replaced by Silicon battery powered meters in the '80s, due to more accurate readings and far better longevity. While modern CdS and Sbc meters require battery power, however, the Selenium meters were powered by the very light they metered.
Another photographic use for selenium is the toning of black and white prints. Selenium toning offers both the artistic use of intensifying and extending the tonal range of black and white photos, as well as the archival advantage of increasing the permanence of images.
Selenium is a necessary micronutrient in human health, however it is also considered to be toxic. High blood levels of selenium (greater than 100 μg/dL) can result in a condition called selenosis. (more info) Photographers working with selenium in the dark room should take care to use rubber gloves, and to avoid inhaling or ingesting selenium.
- General info at WikiPedia
- National Institute of Health fact sheet on selenium
- Photoethnography page dedicated to classic light meters