much whiter light, but still with a characteristic orange-pink cast. New color-corrected versions producing a whiter light are now available, but some efficiency is sacrificed for the improved color. Like fluorescent lamps, HID lamps require a ballast to start and maintain their arcs. The method used to initially strike the arc varies: mercury-vapor lamps and some metal-halide lamps are usually started using a third electrode near one of the main electrodes, while other lamp styles are usually started using pulses of high voltage. Replacements for the toxic mercury in the HID lamps have been investigated and are a matter of ongoing research. Experiments show promising results and widespread future applications are expected. Radioactive substances Some HID lamps make use of radioactive substances such as krypton-85 and thorium. These isotopes help start the lamps and improve lamp operating characteristics. Krypton-85 is a gas and is found mixed in with the argon, which is in the arc tube of the lamp. The thorium, which is a solid, is used in the electrodes. These isotopes produce ionizing radiation of alpha and beta type. This radiation causes high ionization inside the lamp without being able to escape from the lamp. High ionisation makes arc starting via Townsend avalanche much easier. Moreover, the presence of thorium in electrodes reduces the work function which again results in easier arc starting and sustaining. The amount of gamma radiation produced by the isotopes that can escape from the lamp is negligible. Applications HID lamps are typically used when high levels of light over large areas are required, and when energy efficiency and/or light intensity are desired. These areas include gymnasiums, large public areas, warehouses, movie theaters, football stadiums, outdoor activity areas, roadways, parking lots, and pathways. More recently, HID lamps have been used in small retail and even residential environments because of advances in reduced lumen bulbs. Ultra-high performance (UHP) HID lamps are used in LCD or DLP projection TV sets or projection displays as well. HID lamps have made indoor gardening practical, particularly for plants that require high levels of direct sunlight in their natural habitat; HID lamps, specifically metal-halide and high-pressure sodium, are a common light source for indoor gardens. They are also used to reproduce tropical intensity sunlight for indoor aquaria. Most HID lamps produce significant UV radiation and require UV-blocking filters to prevent UV-induced degradation of lamp fixture components and fading of dyed items