Human eyes are far more sophisticated than video cameras. When you look at a white T-shirt outdoors, it looks white. In an office lit with fluorescent tubes, it still looks white. Even in your living room at night, it looks white. This is because your eye and brain will adjust for the changes in white colour temperature.
On the other hand, if a camera set for use indoors videos the T-shirt outdoors, it will look light blue. In an office the T-shirt will look light green. If the camera is set outdoors - a video of a white T-shirt under a regular living room light will look orange.
Cameras need to change their "white balance" depending on the type of light they are shooting in order for white to look truely white. What we are doing here is adjusting the combinations of red and blue in relationship with green signals coming from the camera sensor. The unit "K" is for temperature Kelvin.
- Outdoor light, usually referred to as daylight, ranges from 4500K to 6200K with the average being 5600K (however, on clear blue days, the color temperature could be as high as 12,000K.)
- Indoor incandescent light, like that of the old glowing wire light globes, usually lies in the 2800K to 3400K range with the average being 3200K.
- Indoor fluorescent lights range between 4000K and 7000K with the average being 4300K.
- LED white lights can often 6000K. You would set your white balance to be 6000K to get the correct colour balance.
It can be quite confusing when there are a mix of light sources - as the white balance can change depending on the position of the subject in the shot. This is when we would use light gels to match all the light sources.
A CTB gel (Colour Temperature Blue) is a light blue gel which woudl be placed on Incandesecent lights to emulate daylight.
A CTO gel (Colour Temperature Organge) is an organge gel used to make windows light (at 5600K) look similar to a lamp light (3200K).