3/24/10 11:19 AMefg's Tech Note: USAF 1951 and Microcopy Resolution Test ChartsPage 1 of 9http://www.efg2.com/Lab/ImageProcessing/TestTargets/#USAF1951Image ProcessingTech Note USAF 1951 and MicrocopyResolution Test Chartsand Pixel Profilesby Earl F. GlynnUSAF 1951 3-Bar Resolving Power Test ChartMilitary StandardFrom MIL-STD-150A, Section 5.1.1.7, Resolving Power Target:"The resolving power target used on all tests shall be as follows: The target shall consist of a series ofpatterns decreasing in size as the , , , with a range sufficient to cover the requirements of thelens-film combination under test. The standard target element shall consist of two patterns (two sets of lines)at right angles to each other. Each pattern shall consist of three lines separated by spaces of equal width.Each line shall be five times as long as it is wide." (See Figure 1)Figure 1. Standard Resolving Power Test Target Element.From MIL-STD-150A: "The patterns of lines are parallel lines 2.5 x millimeters long and 0.5 x millimeterswide with space 0.5 x millimeters wide between the parallel lines, where x equals the number of lines permillimeter.""The target contrast (the difference in photographic density between the lines and spaces) shall be eitherhigh, medium, or low contrast, as specified.""High contrast target. A high contrast target is one in which the density difference between the light anddark areas is greater than 2.00.""Medium contrast target. A medium contrast target is one in which the density difference between the lightand dark areas is equal to 0.80 ± 0.05."3/24/10 11:19 AMefg's Tech Note: USAF 1951 and Microcopy Resolution Test ChartsPage 2 of 9http://www.efg2.com/Lab/ImageProcessing/TestTargets/#USAF1951"Low contrast target. A low contrast target is one in which the density difference between the light anddark areas is equal to 0.20 ± 0.05."The combination of a single black bar and a single white bar, with combined size 1/x as shown in Figure 1,is known as a cycle. Resolutions are often specified in cycles/millimeter. Since one cycle contains a single(black) line, lines per millimeter and cycles per millimeter are numerically the same.Selwyn's "Rule of Thumb"MIL-STD-150A, Section 5.1.2.12, Resolving Power quotes a "rule of thumb" by Selwyn that the numericalvalue of the magnification should approximately equal the number of lines resolved per millimeter. (Basedon Selwyn's experiments from National Bureau of Standards C526, 219, 1954 and Photographic Journal88B, 46, 1948). This is a useful approximation for determining what test target should be resolved for agiven magnification.USAF 1951 3-Bar Resolving Power Test ChartThis test target shows several groups of test target elements, which conform to MIL-STD-150A. Eachgroup consists of six elements, which are progressively smaller. The elements within a group are numberedfrom 1 to 6. Odd-numbered groups appear contiguously, 1 through 6, at the upper right corner. The firstelement of even-numbered groups is at the lower right, with the remaining five elements, 2 through 6, at theleft. Each even-odd pair make up a layer, with the next smaller even-odd pair near the center. See Figure 2.Figure 2. USAF 1951 Test Target.(Test Target T20 from Applied Image, Rochester, NY)Per MIL-STD-150A, the size of each target element within a group is progressively smaller. Table 1 showsthe scaling factors for the elements of a group given the size of the first element:Element of Group Factor(mathematical)Factor (numerical)1201.0000022-1/60.8909032-2/6=0.7937042-3/6 =0.7071152-4/60.6299662-5/60.56123Table 1. Scaling Factor for Each Element Within a Group.3/24/10 11:19 AMefg's Tech Note: USAF 1951 and Microcopy Resolution Test ChartsPage 3 of 9http://www.efg2.com/Lab/ImageProcessing/TestTargets/#USAF1951Table 1. Scaling Factor for Each Element Within a Group.For the Applied Image T20 Target, The starting point of the largest group (labeled "-2") is a test targetelement with lines that are 10 mm long. The 1st element of the "-2" group is at the lower right of the testtarget with the labels "-2" and "1". The elements 2 through 6 of this group are at the far left. The lengths ofthe lines of the various elements in the "-2" group are in Table 2 (using the factors from Table 1 androunding to the nearest 0.01 mm):Element of "-2" Group Line Length [mm]1 10.002 8.913 7.944 7.075 6.306 5.61Table 2. Size of Elements in "-2" Group.The largest element of the second group (labeled "-1" and "1") is at the upper right corner. This firstelement of the "-1" group is half the size of the first element of the "-2" group. As you can see from thetable above, Scaling Factor for Each Element Within a Group, the factor for a seventh element in a groupwould have been 0.5 if one existed. So, mathematically the first element of a group continues the sameprogression from the last element of the previous group.The group number gives the power of 2 that is the "x" value in the MIL-STD-150A Figure caption shownabove. This figure caption indicates lines are 2.5/x mm long. So for the "-2" group, we haveOr, x = 2-2 = 0.25, which means there are 0.25 lines resolved per mm. Table 3 summarizes the test targetgroup "-2" to "5":GroupLine Length ofFirst Element of Group[mm]Lines ResolvedPer Millimeterx = 2Group-2 10.000 0.25-1 5.0000 0.500 2.5000 11 1.2500 22 0.62500 43 0.31250 84 0.15625 165 0.078125 32Table 3. USAF 1951 Test Target Group Summary.A general formula for the length or width of any target element line can be expressed as the following:Line Length[mm] =3/24/10 11:19 AMefg's Tech Note: USAF 1951 and Microcopy Resolution Test ChartsPage 4 of 9http://www.efg2.com/Lab/ImageProcessing/TestTargets/#USAF1951Since, Line Width[mm] = Line Length[mm] / 5, the following expression holds:Line Width [mm] = Table 4 shows the length in millimeters of target element lines for the various groups:ElementGroup 1 2 3 4 5 6-2 10.00000 8.90899 7.93701 7.07107 6.29961 5.61231-1 5.00000 4.45449 3.96850 3.53553 3.14980 2.806160 2.50000 2.22725 1.98425 1.76777 1.57490 1.403081 1.25000 1.11362 0.99213 0.88388 0.78745 0.701542 0.62500 0.55681 0.49606 0.44194 0.39373 0.350773 0.31250 0.27841 0.24803 0.22097 0.19686 0.175384 0.15625 0.13920 0.12402 0.11049 0.09843 0.08769Table 4. Line Length[mm] as Function of Group Index and Element Index.Let's reconsider Selwyn's "rule of thumb" that says the numerical value of the magnification shouldapproximately equal the number of lines