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Chapter 2 Light-Sensitive Material
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Photography (Basic) - Introduction to photography and other graphic techniques
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302.258 Figure 2-1.–IS0 of film. The  ISO  assigned  to  a  film  is  labeled  on  the packaging material and on the film cassette or paper backing  (fig.  2-1).  Some  types  of  black-and-white  films are assigned one ISO number; others are assigned two or more. Whether one or more film speeds are assigned depends on whether the film responds differently to different colors or color temperatures of light. Often, this results in a film having one speed for daylight and another for tungsten light. The ISO for a particular film is valid for calculating correct exposure only when that film is developed as recommended by the manufacturer. An exposure index is another numerical value assigned to some films for exposure calculation. The exposure index is a more accurate method of film speed, because  it  is  determined  by  processing  a  particular  film through the actual process in an imaging facility. Exposure indexes are generally assigned to films used for  copying  or  for  technical  applications. SPECTRAL  SENSITIVITY.–The response of an emulsion to specific colors of light or radiant energy is termed  coIor  or  spectral  sensitivity.  You  already  know, from our earlier study of light in chapter 1, that the visible spectrum is made up of violet, blue, green, and red light. When speaking of film or emulsion sensitivity, you are also referring to its sensitivity to ultraviolet and infrared radiation. Spectral sensitivity of an emulsion simply means that the emulsion is sensitive to some energy   of   the   electromagnetic   spectrum.   Also, emulsions  have  color  sensitivity,  which  means  they  are sensitive  to  one  or  more  colors  of  the  visible  spectrum. In this chapter, the terms color sensitivity and color response  are  used  interchangeably.  Color  sensitivity  or color  response  refer  to  ultraviolet  and  infrared  radiation as well as visible light. All  silver  halides  are  sensitive  to  ultraviolet radiation, violet, and blue light. Color-blind emulsion is the  term  given  to  emulsions  sensitive  only  to  these radiations. The addition of sensitizing dyes to silver halides can increase the sensitivity of emulsions and extend their sensitivity to green and red light and infrared  radiation.  Increasing  the  color  sensitivity  of  an emulsion to other than ultraviolet, violet, and blue is called  dye,  color,  or  optical  sensitization. The color sensitivity of a black-and-white film is an important characteristic, since it controls the way colored objects record as tones of gray in the negative 2-2







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