or print. The color sensitivity determines how the film is   classed.   There   are   four   general   classes   of black-and-white  film  emulsions.  The  four  classes  are  as follows: colorblind (monochromatic), orthochromatic, panchromatic,  and  infrared.  Some  of  these  emulsions respond to a wide range of wavelengths of light. Others respond  to  only  a  narrow  band  of  wavelengths. Light-sensitive   emulsions   are   sensitive   to   all wavelengths of ultraviolet radiation. For all practical purposes,   the   general   classes   of   emulsions   are considered  insensitive  to  the  shorter  wavelengths  of ultraviolet (UV) radiation. This is because glass lenses and the gelatin in most film emulsions completely absorb the shorter wavelengths of ultraviolet radiation. When UV is to be used for photography, a special film with a thin emulsion is required. Color-blind Emulsions.–Black-and-white   color- blind  emulsions  are  sensitive  only  to  UV  radiation, violet, and blue light. Green and red objects record only as clear areas in the black-and-white negative and reproduce as dark areas in the print. Color-blind films are  used  primarily  for  copying  and  graphic  arts photography and may be assigned three or more ISO values;  for  example,  ISO/50  for  daylight,  ISO/8  for tungsten  light,  ISO/20  for  white-flame  arcs,  and  ISO/12 for pulsed xenon. Orthochromatic   Emulsions.–Orthochromatic emulsions are sensitive to ultraviolet radiation, violet, blue, and green light. The sensitivity to green light is gained by the addition of a sensitizing dye to the color-blind silver halides. The emulsions provide an approximate  correct  reproduction  of  blue  and  green objects as corresponding tones of gray in a print; however, red objects record as clear areas in the negative and black areas in the print. since the emulsion is not sensitive  to  red.  Various  orthochromatic  films  with different degrees of contrast, color sensitivity, and emulsion speed are available. Their trade names usually contain the word ortho. Orthochromatic emulsions are used   primarily   for   copying   and   graphic   arts photography. Orthochromatic emulsions that may be used in either daylight or tungsten light are assigned two separate  ISO  film  speeds.  This  is  because  these emulsions are highly sensitive to the predominantly blue colored daylight and less sensitive to the tungsten light that has a higher content of red light. Panchromatic  Emulsions.–Panchromatic   emulsions are sensitive to UV radiation, violet, blue, green, and red. The emulsion spectral sensitivity to green and red light  is  gained  by  adding  sensitizing  dyes  to  the color-blind  silver  halides.  Panchromatic  film  of  varying degrees  of  contrast,  color  sensitivity,  and  emulsion speed is available. Panchromatic emulsions are used for copying, portraiture, and general black-and-white photography. Panchromatic emulsions are assigned only one IS0 film  speed.  This  is  because  panchromatic  emulsions  are sensitive to red light and have an almost equal response to   predominately   blue-colored   daylight   and predominately  red-colored  tungsten  light. Infrared Emulsions.–Infrared (IR) emulsions are sensitive to UV radiation, violet, and blue light, with little or no sensitivity to yellow-green light but with additional sensitivity to red and IR radiation. The sensitivity to infrared radiation is gained by adding a sensitizing dye to the color-blind silver halides. Infrared emulsions are commonly used for aerial and medical photography  as  well  as  forensic  photography (photography  used  for  evidence).  For  best  results  a black-and-white UV film should be exposed only with IR radiation. To prevent any IR radiation or visible light from affecting the infrared emulsion during exposure, you must use a dark, red filter over the camera lens. Since infrared radiation does not focus at the same point  as  visible  light,  a  lens  focus  adjustment  is necessary for critical focusing. Most lenses have a calibrated infrared focusing position on the focusing scale. This position is usually marked by a small, red dot or the letter R in red. Determining a useful exposure index becomes a problem with infrared film, because exposure meters are calibrated  for  visible  light  and  similar  light  sources  can emit different amounts of infrared radiation. When using infrared  film,  you  should  make  trial  exposures  for  each particular  film  and  photographic  situation. Contrast In the development process, the silver halide grains in a black-and-white film exposed to light remain in the film. These grains form the image of the original scene. The colors of the scene are recorded in the negative as densities of gray instead of appearing as their original colors. These densities of gray can range from very dense to very thin. This depends upon the brightness of the objects in the scene, their color, and the color sensitivity of the film. The ratio of the maximum to the minimum brightness of objects in a scene is referred to as  the  scene  brightness  range.  Most  long  scale black-and-white  films  are  capable  of  recording  scene brightness  ranges  up  to  128:1.  In  a  negative,  a 2-3