action. The gelatin also acts as a sensitizer for the silver salts. In  photographic  films  and  papers,  the  main  purpose of the base is to support or hold the emulsion in place. Depending on how the recorded image is to be used, the base   or   support   may   be   transparent   or   opaque.   A transparent  base  is  used  for  transparencies  viewed  by transmitted   light   and   for   negatives   printed   with transmitted light. An opaque base is used for prints that are viewed by reflected light. The latest state of the art in light-sensitive materials used in photography is the use of the electronic medium. Still video disks do not contain an emulsion or a base. When  video  mediums  are  used,  light  is  converted  to electrical   impulses,   and   these   impulses   are   stored magnetically on a tape or disk. Since it is the camera itself that converts the light to electrical impulses, the recording  medium  and  all  stages  of  the  photographic process can be carried out in normal room light. Black-and-White  Film The characteristics and use of black-and-white film depends   largely   on   the   actual   construction   of   the emulsion. These characteristics include the following: the  degree  of  sensitivity  to  light,  response  to  various colors of light (color or spectral sensitivity), contrast, exposure   latitude,   emulsion   latitude,   and   emulsion definition. There  are  many  types  of  black-and-white  films available.  Each  differs  from  others  in  one  or  more characteristics.  You  should  become  acquainted  with  the characteristics  of  films.  This  knowledge  is  helpful  in selecting the film most suitable for each photographic assignment. Color Negative Film A  color  negative  film  records  a  scene  in  image densities  opposite  to  the  brightness  of  objects  in  the scene,  the  same  as  a  black-and-white  negative  film. Color films can be recognized because they contain the suffix  “color,”  such  as  Vericolor,  Kodacolor  and Fujicolor. These color films are used when a print is the final  product.  Most  color  negatives  (except  for  color film used for aerial photography) has an orange mask incorporated init. This orange mask increases the color separation which reproduces colors more accurately in the final print. During   development,   colors   that   are   comple- mentary to the color in the original scene are formed in the emulsion. For example, a red object in the scene is recorded  as  cyan  in  the  negative.  A  combination  of yellow, magenta, and cyan record all the other colors that  we  see  in  the  scene.  Color  dyes  in  the  emulsion layers  control  the  colors  of  light  passing  through  the color  negative. To produce color prints or color transparencies, you can print color negative film images on color positive materials such as color paper and color print film. Color negatives  also  can  be  printed  on  a  special  panchromatic black-and-white   paper   to   produce   black-and-white prints. Amateur and Professional Color Films Much  of  the  color  film  used  in  the  Navy  is manufactured by the 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. They also have emulsions made up of many different chemicals that tend to change slowly with time. 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  the  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. MEASURING  EXPOSURE The  quantity  of  light  can  be  measured  in  several ways. The most accurate method is the use of exposure meters. Photographic exposure meters are designed to be sensitive to light in the same reamer as panchromatic film.  Therefore,  an  exposure  meter  reading  can  be assumed to be valid under any visible lighting condition. Several types of hand-held meters are available. Some 11-22