Color Temperature Blue (CTB) correction gels are calibrated to raise color tempreature to 5500 Kelvin (Daylight). They are availble in several strengths and are primarily used to correct Tungsten (3200 Kelvin)to Daylight. CTB (Full Blue) raises 3200 Kelvin to 5500 Kelvin. 3/4 CTB (Three-Quarter Blue) raises 4700K to 5500K. 1/2 CTB (Half Blue) raises 4100K to 5500K. 1/3 CTB (Third Blue) raises 3800K to 5500K. 1/4 CTB (Quarter Blue) raises 3500K to 5500K. 1/8 CTB (Eighth Blue) raises 3300K to 5500K.