CTO or Color Temperature Orange is a filter that, in it's intended application, converts daylight to tungsten. The gelatin, or gel filters, are placed over the light source to affect it's color temperature. CTO comes in full, 1/2, 1/4, 1/8, and even 1/16 stop filtration power. You can spend hours and hours learning more about CTO, filters, and light at LEE Filters and Rosco.