A good question!
Color film is basically 3 black & white films sandwiched onto the same film base. Black & white film in its natural state is only sensitive to violet and blue light. To make a color film it is necessity to force one of the film layers to become sensitive to green light and another to red light. The finished film records the three primary colors which are red, green, and blue. Films designed to operate under daylight conditions have reduced sensitivity to blue as daylight is fairly blue (5500K). Films designed to be exposed under studio lighting conditions have increased sensitivity to blue as tungsten lamps are high in red content and deficient in blue content. Studio tungsten lamps are basically ordinary tungsten light bulbs forced to operate at a higher than rated voltage. This higher voltage causes the lamps operate brighter and bluer at the cost of shortening their service life. Two types of photo lamps are produced, one operating at 3200K, and one at 3400K. Film designed to operate with the 3200K lamps is designated as type B and film for 3400K lamps is designated type A. Film designed to operate under daylight conditions is designated type D. Color negative films are all type D, however, during the printing cycle, filters are used to correct mismatched light conditions. Thus it is generally unnecessary to apply color correction filters when using color negative film.
Photographers are generally prepared to take pictures under various lighting conditions and many times they will have the wrong film in their camera. Color correction filters to the rescue. Our gadget bags are loaded with filters that match the film to the light sources.
Wratten filter number that convert:
80A 3200K to 5500K (bluish)
80B 3400K to 5500K (bluish)
85 5500K to 3400K (amber)
85B 500K to 3200K (amber)
The filter designations are from the firm of Wratten & Wainwright, master filter makes of London, were acquired by Kodak in 1912. The name Wratten was retained to honor Frederick Charles Wratten (1840-1926).
The color of light is measured using the Kelvin (K) temperature scale which is the Celsius scale -273⁰C. Potters, metal works and others were able to gauge the temperature of heated objects by observing the glowing color which starts out as cherry red, then red, then white, then blue white. Lamps with metal filaments obey this observation.
Sunlight 5500K
Skylight 12000K thru 18000K
500 watt photo lamp 3400K
500 watt photo lamp 3200K
100 watt general service lamp 2900K