allowed  to  act,  the  exposure  may  be  varied  by changing either the intensity or the time. Light sources for a sensitometer must be carefully chosen, and their characteristics must be precisely known.  The  intensity  of  the  light  must  also  be known.  The  time  of  exposure  should  correspond closely  with  actual  photographic  practice  and  must remain constant over long periods of time. In addition to  this,  the  color  temperature  must  be  known,  must remain constant, and must correspond closely to the quality of light that is likely to be used in practical photography. These qualities allow the test strip to be exposed under conditions as close as possible to those that occur in practical use. When you base sensitivity measurement on a consistent factor that is known, the response  of  light-sensitive  emulsions,  under  practical conditions, is predictable. Thus various emulsions may  be  compared  to  each  other.  You  know  that emulsions often change in apparent sensitivity (speed) with a change in the spectral composition (color) of the  exposing  light.  Processing  also  has  an  effect  on the speed of emulsions. When   incandescent   lamps   are   used   in sensitometers, they operate at a color temperature of about  3200  K.  The  color  temperature  of  sunlight  is about 5400 K. Therefore, you may need a filter to alter the color of the lamp, so it is equivalent to the spectral energy of sunlight. Differences in time can also lead to errors due to reciprocity law failure. The EG&G sensitometer, used commonly in Navy imaging facilities, permits you to change the exposure times as well. The exposure can be  changed  from  1/100  second  to  1/10,000  second. This allows you to test the reactions of emulsions to various exposure times. A     sensitometer     is     used     to     produce     a logarithmically graded series of exposures with values that are already known. These are generally arranged in steps from low to high. A part of the strip is left unexposed, so the gross density of the material itself may be determined. The device or method used to vary the amount of exposure  must  also  meet  certain  requirements.  The device should be able to produce an exposure range that conforms closely to that found in actual practice, to  be  accurate,  to  be  consistent,  and  to  have  no significant effect on the color quality of light. A step tablet is used in a sensitometer for this purpose. Step  Tablet The   11-   and   21-step   step   tablets   are   most commonly used in sensitometers. These step tablets are comprised of a series of neutral-density filters with densities that range approximately from 0.05 to 3.05. On  an  11-step  step  tablet,  one  f/stop  exposure difference (or 0.30 density difference) exists between each step of the tablet. On a 21 -step step tablet, a 1/2 f/stop exposure difference (or 0.15 density difference) exists between each step of the tablet. Selection of a step tablet should be based on the emulsion latitudes and  contrast  differences  in  pictorial  and  copy  films and papers. A 21-step step tablet is used normally for long-scale films, and a 11-step step tablet is used for short-scale films and printing papers (fig. 2-2). Processing  Sensitometric  Strips Sensitometric strips (scnsi-strips) are made by processing  under  controlled  conditions  of  time, temperature, agitation, and chemical activity. This holds true whether the material is black and white or color. Black-and-white control strips are normally made in the lab, while color control strips are obtained from  the  manufacturer  of  the  material  being  processed. When you are machine processing control strips or sensi-strips, they can be attached to either a roll of film or a leader. To reduce the likelihood of bromide drag, ensure the leading edge of the sensi-strip has received   the   least   amount   of   exposure   in   the sensitometer. Following this procedure, the strip goes through the processing machine tanks with the rest of the roll and receives the same processing. Remember that all steps in the processing of control strips and film are important if the process is to maintain high standards and uniformity of results. Each step in the entire process must be carried out as carefully and as systematically as possible with uniform times and handling techniques in each step from exposure to drying. Refer to figure 2-3 for the relationship between the sensitometer, step tablet, and sensi-strips. CHARACTERISTIC CURVES The  characteristic  curve  shows  the  relationship between the exposure and the density, resulting from 2-6