By subtractive mixing of the three scrolls any colour can be obtained. Expressed as an index, where natural daylight equals 100. Using sources with higher colour rendition indices gives more accurate renderings of objects. By rotating the disc the gel presented to the lens can be ch anged, and thus a remote colour change achieved. The disc is usually about 3 times the lens diameter.
Many units also allow continuous rotation.
See also semaphore unit , scroller . It sits next to the lamp, before any other lenses in the optical path. See also convex , plano-convex . A type of discharge lamp used in some followspots, with a colour temperature of 4000K, and poor colour rendition . They are the first type of discharge lamp to be used in stage applications.