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Special Applications in Copying
Kelvin  Temperature  of  Illumination

Photography (Basic) - Introduction to photography and other graphic techniques
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Figure 8-3.–Lights positioned at a 45-degree angle. original without moving the camera or the original. When the camera is not designed with a rising, falling, and sliding front, the camera or original must be moved to align the original with the optical axis of the lens. LIGHTING  THE  ORIGINAL One of the most important elements in copying is proper,  even  illumination  of  the  original.  Originals  that are not properly and evenly lighted yield negatives with uneven density, which are hard to print. This is true whether the original is illuminated by daylight or artificial light. Uneven illumination can be caused by improper  placement  of  artificial  lights  in  relation  to  the copyboard or by improper placement of the original in daylight. Artificial lights are normally placed on two sides of the copyboard. A 45-degree angle is recommended for general use (fig. 8-3). At this angle, a minimum of unwanted  reflections  from  the  surface  of  the  original occurs. However, depending on the type of surface of the original, the best angle for the lights may change. For example, an original artwork may have brush strokes that produce reflections. These reflections may be reduced somewhat by placing the lights at an angle greater  or  lesser  than  45-degrees. Do  not  position  artificial  lights  too  close  to  the copyboard. The circles of illumination will not cover the original completely. Movable lights should not be positioned so far from the copyboard that the intensity of  the  illumination  falling  on  the  original  is  greatly reduced. The  evenness  of  illumination  on  an  original  can  be checked with an exposure meter. Do this by placing a gray card on the original and taking a reflected light meter reading from the card. Do not allow the shadow of the meter or your hand to influence the reading. When a gray card is not available, a white card (the back of a sheet of photo paper) can be used, but you must compensate your exposure by two f/stops. Reflected light from the four corners and center area of the original should read the same light values. Daylight  provides  two  choices  of  illumination: direct sunlight and diffused daylight, such as a cloudy, bright  day  or  open  shade.  Evenness  of  daylight illumination is controlled by ensuring that the original is  completely  in  direct  sunlight  or  in  diffused  daylight, and no shadows are cast on the original. Although  an  original  is  uniformly  illuminated  over its entire surface, apparent unevenness in illumination may still appear in the copy if a wide-angle lens is used. This is caused when the light transmitted through the center  of  the  lens  is  more  intense  than  the  light transmitted near the edges of the lens. When the entire angle of view of the wide-angle lens must be used, more illumination to the edges of the original is needed to compensate for the falloff of light at the edges of the lens. This can be achieved by turning the lights slightly toward the edges or by moving the lights close to the edges. The amount of light increase necessary for the edges of the original is best determined by conducting exposure tests with the type of film being used. Lighting  control  is  more  critical  when  using  an extremely high contrast film, compared to a high, moderate, normal, or low contrast film. Uneven lighting is more visible in a copy produced with an extremely high contrast film because of the limited exposure latitude of the emulsion. 8-14







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