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Sensitometry - 14208_40
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Photography (Advanced) - Advanced manual for photography and other graphic techniques
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Since it would be impossible to judge whether densities are reproduced accurately by viewing your normal production products, special test strips, or control   strips,   are   prepared.   The   test   strips   are exposed accurately with varying amounts of light. These test strips are developed in your process, and the resulting densities are read on a densitometer and averaged together (fig. 2-1). The densities are then plotted on a graph. The plot of the data establishes the standard. Similar test strips are then processed at regular  intervals  and  compared  with  the  standard  to ensure  that  the  processing  is  under  control.  This, basically, is sensitometry. Another   term   used   in   conjunction   with sensitometry  is  densitometry.  Densitometry  is  an integral part of sensitometry. These two terms are often  used  together  or  interchangeably.  Technically, however, there are differences between them. The differences  are  as  follows: Sensitometry,  or  measurement  of  photographic sensitivity, is the science of determining the photographic  characteristics  of  light-sensitive materials. Densitometry, or measurement of densities, is the method whereby data are obtained for sensitometric  calculations. Measurements   of   densities   are   done   on   a logarithmic  scale.  To  understand  sensitometry,  you must  become  acquainted  with  logarithms. COMMON  LOGARITHMS Complex  problems  can  be  calculated  easily  and accurately  by  means  of  logarithms.  You  can  add logarithms to achieve multiplication, subtract them to achieve division, and divide them to derive square roots. In photographic quality assurance, logarithms are used for the following: Determining   density Plotting  characteristic  curves Determining   contrast Determining log H Reading  the  densitometer  scale A common logarithm (log 10) is an exponent to a base number of 10. The base 10 is used because our numerical system is based on units of 10. This can be demonstrated  easily  by  using  scientific  notation,  or "powers of 10" For example, the logarithm of 100 is 2,  because  102  equals  10  times  10,  or  100.  The logarithm of 1,000 is 3, because 103 equals 10 times 10 times 10, or 1,000. Table 2-1 shows how some common   logarithms   are   computed.   Notice   the relationship  between  the  exponent  (superscript)  and the common log. Table  2-1.—Examples  of  Some  Common  Logarithms 2-3







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