CTO is CT Orange or Color Temperature Orange. A CTO is a color-correcting gel for daylight bulbs to make them appear like tungsten lights. They are available in varying densities of 1/4 CTO, 1/2 CTO, and FullCTO to warm up a light. They can be used individually or added together, for instance, a 1/4 CTO and 1/2 CTO will effectively be the same as a 3/4 CTO.