It's important at this point to understand how gels work. Gels are optical filters that act by absorbing certain wavelengths (colors) and transmitting others freely. A CTB gel transmits blue frequencies but absorbs red, orange, and yellow frequencies. A gel works by removing certain colors. It's a common misconception that gels add color to the light. Gels can't add color. If the desired frequency isn't present in the light in the first place, or is present in low amplitude, a gel won't add more of the color. The sodium vapor lights used in parking lots, for example, emit only a narrow frequency of orangish light. Placing a CTB gel over one of these will not produce blue light; it will absorb the orange and pass almost no light.


Ah, but what about the rooms that have florescent overheads, tungsten lamps and big, daylight windows?


What you have to do is to choose your dominant light color and go with it. keep in mind that tungsten and daylight mix much better than do florescent and everything else.

Each fluorescent tube transmits a different range of colors depending on the age and composition of the tube. Generally, it transmits light in the yellowish green range.

Furthermore, it's a pulsating light source. It's regulated by alternating current (60 cycles per second in the U.S.). This means any shutter exposure faster than 1/50th is still unpredictable after all other variables are removed.