that averages approximately 5400 K. Tungsten films are balanced for use with illumination of 3200 K without filtration. The Kelvin temperature of the exposing light for reversal films is much more critical than the color of the exposing light for color negative films. When an exposing light is used other than that for which color negative  film  is  balanced,  adjustments  to  the  filter  pack can be made during printing to achieve proper color balance. With color reversal film, however, a slide is usually the final product. When the color of the exposing light is other than that for which the film is balanced, the transparencies are off-color. You should strive to expose all color films with the color light for which the films are  balanced. Although color films have three separate emulsions, only one ISO film speed is assigned. An ISO film speed for color film is most accurate when the illumination used is the one for which the film is balanced. Amateur  and  Professional  Color  Films Much  of  the  color  film  used  in  the  Navy  is manufactured  by  Eastman  Kodak  Company.  Kodak markets color films for both professional and amateur photographers. Color films intended for use primarily by professionals are identified by the word  professional in  the  name;  for  example,  Kodak  Vericolor  III Professional Film, Type S (VPS). Both professional and amateur films have similar color  quality,  sharpness,  and  granularity  characteristics. Also, they have emulsions made up of many different chemicals that tend to change slowly with time. Starting from the day they are made, all color films begin to change;  and  as  the  films  age,  their  color  balance changes. Amateur films are manufactured to age and reach a peak color balance much later than professional films. The manufacturer allows for the time amateur film will be in storage, on the store shelf, and in the camera before it is developed. The ISO speed assigned is adequate for calculating   exposure   for   normal   picture-taking situations. Professional  films  are  manufactured  so  they  are very near their optimum color balance at the time they are shipped from the factory. These films should be kept refrigerated   or   frozen   until   shortly   before   use. Refrigeration keeps film near the optimum point until used and provides the photographer with confidence in consistent results. Precise film speeds are provided for professional  films.  The  film  is  intended  for  prompt processing to prevent any significant shift in color balance after exposure. If you require optimum color balance and precise film speed within about 1/6 f/stop professional film is appropriate for your work; however, when you intend to be away from home base for an extended period of time without  refrigerated  storage  or  processing  facilities, amateur  film  should  be  your  choice. Instant  Picture  Film Currently, the only manufacturer of instant picture film is Polaroid. There are two basic types of instant picture film: peel apart and integral. After exposure and removal from the camera, peel-apart film must be timed while the film develops. After it has developed for the specified  period  of  time,  the  negative  backing  is  peeled away and discarded. Integral films develop outside the camera  and  have  no  negative  backing  to  be  removed. Instant picture color films are tripack materials with built-in processing. Peel-apart film uses a system whereby the exposed silver halides develop to a metallic silver negative. When no metallic silver is present, dyes pass to form the color image. Integral films use a reversal  process  in  which  the  areas  of  unexposed  silver halides are the locations where the dyes are produced to form  the  positive  image. Instant picture film is a very useful medium in an imaging facility, particularly when still electronic technology is not available. Instant picture film is used commonly  for  identification  and  passport  photographs, but it is also valuable in determining test exposures. Before  you  make  your  final  exposures  on  conventional film, a Polaroid photograph can be taken to confirm composition,  lighting,  and  exposure. FILM  SIZES There are two types of film formats used commonly in photography. They are roll film and sheet film. Both formats come in a variety of sizes. Roll Film All roll film is packaged so the film can be loaded and unloaded from a camera in daylight. Number 120 film has a paper backing that prevents the film from being exposed in daylight; 35mm film is wound in a lighttight  cassette  that  prevents  the  film  from  being exposed  by  ambient  light. 2-7