CHAPTER 2 LIGHT-SENSITIVE  MATERIALS Many substances are affected in some way by light. The light-sensitive substances used in photographic film to record an image are silver salts and are called silver halides.  The  silver  halides  react  to  ultraviolet  radiation, violet, and blue light only; however, they can be made sensitive to other colors of light and infrared radiation by the addition of dyes. Depending on the amount of light and the type of silver halide, the light produces a visible or invisible change in the halides of a film or printing paper. An invisible change is made visible by development. Photographic films and papers are composed of two basic parts: the emulsion and the base, or support. The emulsion is the light-sensitive portion of a film or paper that records the image. The emulsion contains the silver halides and any special sensitizing dyes suspended in a binder of gelatin. The gelatin holds the silver halides evenly dispersed and prevents action by a developer until the silver halides have been made developable either by exposure to light or chemical action. Also, the gelatin acts as a sensitizer for the silver salts. In photographic films and papers, the primary purpose of the base is to support or hold the emulsion in place. The base, or support, may be transparent or opaque, depending upon how the recorded image is to be used. A transparent base is used for transparencies that are viewed by transmitted light and for negatives that are 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. Video disks do not contain an emulsion or a base. When electronic 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 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 depend  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 type differs somewhat from the others. You  should  become  acquainted  with  the  characteristics of films. This knowledge is helpful in selecting the film most  suitable  for  each  photographic  assignment. EMULSION  SENSITIVITY  TO  LIGHT The silver halides and sensitizing dyes of most film emulsions are very sensitive to small amounts of light. This  light  causes  invisible  changes  to  the  emulsion  and is called the  latent image.  The latent image can be physically  made  visible  by  the  chemical  step  of development. The extent of the reaction to the light of the emulsion is affected greatly by the size of the silver halide grains and the amount of light reaching the film. The inherent property of a film emulsion to respond to light  is  termed  film speed. Film  Speed Film speed is important, since it is related to the amount of exposure required to produce an acceptable image. Emulsions are rated as slow, medium, or fast, depending on the amount of light required to produce an image  satisfactorily.  Fast  emulsions  require  less  light  to produce  an  acceptable  image  than  slow  emulsions. To calculate the exposure for a film emulsion accurately and consistently with a light meter, the manufacturer has developed a system of rating emulsion speed. The rating system used is the IS0 film speed system. ISO film speed is a numerical value assigned to an  emulsion  used  for  determining  exposures. The International Standards Organization (ISO) is a federation of all the national standard bodies of the world. It has approved a uniform set of film speed standards.  The  standards  call  for  a  universal  expression of both arithmetic and logarithmic speed values with the ISO designation. The ISO designation generally looks like the following: ISO  100/21° 2-1