Since it would be impossible to judge whetherdensities are reproduced accurately by viewing yournormal production products, special test strips, orcontrol strips, are prepared. The test strips areexposed accurately with varying amounts of light.These test strips are developed in your process, andthe resulting densities are read on a densitometer andaveraged together (fig. 2-1). The densities are thenplotted on a graph. The plot of the data establishesthe standard. Similar test strips are then processed atregular intervals and compared with the standard toensure that the processing is under control. This,basically, is sensitometry.Another term used in conjunction withsensitometry is densitometry. Densitometry is anintegral part of sensitometry. These two terms areoften used together or interchangeably. Technically,however, there are differences between them. Thedifferences are as follows:Sensitometry, or measurement of photographicsensitivity, is the science of determining thephotographic characteristics of light-sensitivematerials.Densitometry, or measurement of densities, isthe method whereby data are obtained forsensitometric calculations.Measurements of densities are done on alogarithmic scale. To understand sensitometry, youmust become acquainted with logarithms.COMMON LOGARITHMSComplex problems can be calculated easily andaccurately by means of logarithms. You can addlogarithms to achieve multiplication, subtract them toachieve division, and divide them to derive squareroots.In photographic quality assurance, logarithms areused for the following:Determining densityPlotting characteristic curvesDetermining contrastDetermining log HReading the densitometer scaleA common logarithm (log _{10}) is an exponent to abase number of 10. The base 10 is used because ournumerical system is based on units of 10. This can bedemonstrated easily by using scientific notation, or"powers of 10" For example, the logarithm of 100 is2, because 10^{2} equals 10 times 10, or 100. Thelogarithm of 1,000 is 3, because 10^{3}equals 10 times10 times 10, or 1,000. Table 2-1 shows how somecommon logarithms are computed. Notice therelationship between the exponent (superscript) andthe common log.Table 2-1.—Examples of Some Common Logarithms2-3