• Home
  • Download PDF
  • Order CD-ROM
  • Order in Print
Transmission
Processing  Sensitometric  Strips

Photography (Advanced) - Advanced manual for photography and other graphic techniques
Page Navigation
  26    27    28    29    30  31  32    33    34    35    36  
formula by the capital letter 0 and is the reciprocal of the transmission (T). The formula for opacity is given as follows: O =  Amount  of  incident  light Amount of transmitted light You  can  see  that  opacity  is  the  transmission formula inverted. Again, when a material has 10 mc of light falling on it and 5 mc is being transmitted, you can determine the opacity by the formula O = 10/5, or 2. Putting it a different way, opacity is the reciprocal of transmission, or O = 1/0.50 = 2. Or, when 2 mc is being transmitted, the formula is O = 1/0.20, or O = 10/2; in either case, the opacity is 5. DENSITY Density is indicated by the capital letter D and is another   way   of   expressing   opacity   or   the light-stopping ability of a medium. Density is nothing more than the common logarithm of opacity. For example, when opacity is 2.0, the density is log 2 = 0.30. In sensitometry, density is the term with which you  are  most  concerned.  However,  density  cannot  be disassociated  from  transmission  and  opacity,  because they all are dependent and directly related to each other. When the value of any one of these factors is known, you can calculate the others. When you know the transmission of a film, you can easily determine the  density.  Or  conversely,  you  can  measure  the density   and   then   determine   the   amount   of transmission. While charts are available to provide some conversions directly, you should be capable of determining any of those figures. SENSITOMETERS Sensitometric-densitometric  testing  requires  a method  of  providing  the  exact  same  exposure  to different emulsions, or the same emulsion type that is processed  differently,  and  then  comparing  the  resulting densities. Sensitometers make controlled exposures that are suitable   for   sensitometric-densitometric   testing procedures. Densitometers measure density. For  sensitometric  testing  purposes,  you  must provide a way of exposing sensitized material with a known quantity and quality of light. One of the first requirements for sensitometric control  is  to  have  a  sample  of  the  light-sensitive material  that  has  been  exposed  properly  under measurable   and   reproducible   conditions.   For sensitometric test purposes, it is common practice to expose  a  strip  of  film,  so  a  number  of  varying exposures are made on the same strip. This series of controlled exposures is made with a sensitometer through a series of neutral-density filters. Ideally, a sensitometer should be designed so you can do the following: Predetermine  the  total  amount  of  exposure. Determine the difference in exposures given to various areas. Control the color quality of the light. Reproduce   or   duplicate   the   exposure consistently. Obtain a wide enough range of exposures to produce densities ranging from very light to dark. Scenes that might be photographed include a wide range of brightness values that are represented on a negative  as  areas  of  varying  amounts  of  density; however,   these   different   densities   are   scattered throughout  the  picture  area  and  are  difficult  to measure. For the sake of simplicity, uniformity, and reproduceability in the application of sensitometry, the exposures produced by a sensitometer are arranged in a series of gradually increasing steps. These steps correspond  to  the  relative  brightness  values  of  a normal scene. The exposures are made on a length of film  or  paper  of  the  same  type  that  you  are processing.  This  sample  is  called  a  sensitometric strip. "Photographic exposure" is defined as the product of illumination and time. The two important parts of a sensitometer are the light source and the device for controlling the amount of light transmitted to the emulsion. Since total exposure is the result of the intensity of illumination and the length of time it is 2-5







Western Governors University

Privacy Statement
Press Release
Contact

© Copyright Integrated Publishing, Inc.. All Rights Reserved. Design by Strategico.