The original number in table 2-1 is also called anantilog. Notice that a number greater than one is apositive log. Any number less than 1, but greater thanzero, is a negative log.Logs are also required between the numbers 1 and10. Since the log of 1 is 0 and the log of 10 is 1, thenumbers 1 through 9 are decimals. (See table 2-2.)Notice the relationships between numbers andtheir logs as follows:When numbers are multiplied, their logs areadded. Example: 8 = 2 4. The sum of log 2 andlog 4 equals log 8: 0.30 + 0.60 = 0.90.When numbers are divided, their logs aresubtracted. Example: 3 = 6 2. Log 3 is thedifference between log 6 and log 2: 0.78 - 0.30 =0.48.The previous discussion is provided to give you ageneral idea on how logarithms are derived. It is notnecessary for you to memorize logarithms, or refer tothe log tables. All scientific calculators have a "log"key that converts numbers to logarithmic form. Youshould become familiar with the functions of yourcalculator before proceeding with the study ofphotographic quality assurance. For more informationon using logarithms, refer to the chapter on logarithmsin Mathematics, Volume 1, NAVEDTRA 10069.One of the main uses of logarithms inphotographic quality assurance is to take the numbersused to indicate exposure in characteristic curves andreduce them to a manageable form. For example, thesensitometer in your imaging facility is set on anexposure time of 1/100 second and provides anilluminance of 80,000 lux (or meter-candles). The logexposure can be calculated easily as follows:E T = H80,000 (lux) 1/100 (set) = 800 lux secondsThe log exposure = the log of 800 or 2.90When you convert exposure to logarithmic form,both density and exposure are on the same scale. Acharacteristic curve indicates how exposure andprocessing differences affect photographic emulsionsby comparing density and the log of exposure.To describe sensitometry, you must becomeacquainted with several new terms and formulas. Asa starting point, you should become familiar with theterms transmission, opacity, and density, or T, O, andD.TRANSMISSIONMost photographic material, even clear film, doesnot transmit all of the incident light that is relevant toit. Transmission is a measure of the light-passingability of a film or other medium. The transmissionof a processed film refers to the fraction, orpercentage, of incident light that passes through thefilm.In a formula, transmission is represented by acapital letter T. The formula for determiningtransmission is as follows:Table 2-2.—Common Logarithms Between 1 and 10T= AmountoftransmittedlightAmount of incident lightThe result is never more than 1/1, or 1.00. Forexample, when 10 meter-candles (mc) of light areincident (or falling) to a film and 5 mc is passingthrough it, the transmission is as follows: T = 5/10 orT = 0.50, or 50 percent. When 2 mc is transmitted,the formula reads T = 2/10 or 0.20, or T = 20 percent.OPACITYOpacity is the ability of a medium to absorb light.The two terms, transmission and opacity, are directlyopposite in meaning. Opacity is indicated in a2-4

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