In the early days, photo films sported low sensitivity to light. Indoor photography was first practiced in studios equipped with large bay windows and skylights. Color films where initially fabricated to operate under daylight conditions. These films are type D for work in ambient daylight conditions. The Kelvin temperature scale was the favorite of physicists. It is the Celsius scale however; zero is set at absolute zero – 273. This scale uses only positive value thus no confusion as to plus or minus. Daylight, film is balanced for 5500 K.

In an effort to get more light from ordinarily tungsten, in studios, these lamps were commonly over-voted. Thus studio type photo light bulbs were made to be used at voltages below household normal. However the studios operated them at normal, usually 117 volts. This procedure outputted a brighter and bluer light however the life of the light bulb was reduced to about 3 hours.

Type A lamps outputted 3400 K and color film for these studio lamps was designated Tungsten A. Type B lamps outputted 3200 K and the matching color film was Tungsten B.

Now flash bulbs, a one-time-use lamp outputted 4200 K. Special color films for these flash bulbs was Type F. Color films suffered color shifts when used for long exposure tasks. Special color films for use when the exposure time exceeded 1 second were made, Type L. A color shift occurred if the color film was to be used for super short exposures, 1/1000 of a second or shorter. These films were Type S.

Many types of black & white film were tested for sensitivity under various types of lighting conditions. Their sensitivity, now ISO, in that time period ASA or DIN, was published. As an example a film rated at 100 ASA could also be rated at 80 ASA when used with studio tungsten light bulbs.