Introduction to Test Targets
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A resolution target, also called a test target, is a standard pattern used to evaluate the performance of an optical or electro-optical system, or to compare the quality of one system against another. Test targets usually consist of groups of patterns, graduated in size from large to small in precise increments. When an optical device can no longer distinguish the pattern, it is said to have reached its “limit of resolution.” Naturally, in order to discover the limit of resolution, it is essential to use a target whose range of resolution is greater than that of the system being tested. In other words, the optical device must fail to resolve at least the smallest pattern in the range for the test to be valid.
Resolution targets are analogous to the series of tests that optometrists use to prescribe corrective lenses. The optometrist relies on standard sets of patterns—lines of type, rows of numbers or arrangements of abstract designs—to measure the ability of each eye to resolve the image. When the patient reports he or she cannot read the next-smallest row of characters on a chart, for example, the optometrist knows approximately what the limit of resolution is for the eye being tested. That information indicates which set of test lenses will yield the exact prescription that increases the patient’s visual acuity. The test patterns help speed up the process of elimination.
There are many types of targets, each of them useful in an array of applications. Ronchi Rulings and Sayce targets, for instance, are ideal for scanning with a recording microdensitometer or for assessing the resolving capability of imaging materials. Star patterns and annulars accentuate lens aberrations and errors in focus and alignment. Three-bar and five-bar targets enable engineers to assign numerical tolerances to optical systems and photographic processes. Microcopy test charts, consisting of five-bar patterns, are useful in evaluating the output image of cameras, lenses, microfilm systems and enlargers, and are widely recognized as standards in those industries.
Targets are available in a variety of materials including chrome on glass, quartz, and white opal. Some targets are also available on photographic paper. JML offers Ronchi Rulings, star targets, microcopy test charts, USAF test targets, Sayce logarithmic charts, dot distortion targets, PIMA/ISO contrast, stage micrometers and resolution charts.
Figure 1: Line grid pattern.
Figure 2: Enhanced digital camera resolution chart.
Figure 3: Stage micrometer
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Figure 4: Dual cross stage micrometer
Dot distortion targets are excellent for measuring distortion in optical systems, especially for machine vision applications.
Figure 5: Dot array target.
Other targets such as PIMA/ISO contrast work well for digital camera calibration requirements. This camera contrast chart conforms to International Standard ISO 14524, “Photography-Electronic still-picture cameras – Methods for measuring opto-electronic conversion functions (OECFs).”
Figure 6: Pima ISO camera contrast
The IEEE chart conforms to standard STD 208-1995, “Measurement of Resolution of Camera Systems.” This standard is used to test frequency response characteristics, performance of the lens, camera, and display device.
Figure 7: IEEE resolution chart
The ISO camera chart conforms to ISO Final Draft International Standard 12233, “Photography – Electronic still-picture cameras – Resolution measurements.”
This target’s active area measures 20cm high and has features of 0.1mm. This target is available in the following sizes:
Figure 8: ISO camera resolution chart
The Sayce chart is used primarily when scanning with a recording microdensitometer. Applications include assessing the resolving capability of imaging materials and determining optical system performance. The resolution obtained in a test situation may be determined by actual line count or linear distance into the pattern. The pattern covers a frequency range of 0.25 to 5.0 cycles/mm. Progression tables are included.
Figure 9: Sayce logarithmic test chart
Rulings consist of 50/50 bar-to-space ratios (duty cycle) centered on a 2" x 2" substrate. Overall image size is 50mm x 50mm. Line width tolerance is ±10%.
Figure 10: Ronchi Ruling
Figure 11: Sector Star Target
The Sector Star target, designed to check resolution in all directions, consists of high and low density wedges of equal width, spaced at 2.5° intervals. The design facilitates visual detection of astigmatism, vibration problems and errors in focus and alignment.
The USAF 1951 test chart below, designed to military specification MIL-STD-150A, is used very widely in determining the resolving power of optical systems and imaging materials. The large frequency range makes it applicable for the evaluation of almost any imaging system.
Figure 12: USAF 1951 test chart
The ultra-high resolution target consists of three groups of fifteen-bar, high-contrast test targets covering an image area 16mm x 16mm, centered on a 4" square, .060" thick glass plate. There are 11 targets in each of the three groups, with the smallest target repeated in the next smallest group of targets. The table below shows the spatial frequencies in cycles/mm for each of the three target groups. The density difference is greater than 2.0.
Figure 13: Ultra-high resolution test target
The Microcopy test chart is used for evaluating the output image quality of optical devices such as cameras, lenses, microfilm systems and enlargers. It also is useful for construction of test arrays. This chart is equivalent to the NBS 1010A Microcopy Test Chart and the ANSI/ISO Test Chart #2. The image includes 26, five-bar patterns from
1.0 to 18.0 cycles/mm, with each bar group labeled to indicate cycles/mm of the target. The bar length-to-width ratio is 24:1 and the cycles/mm progression from group to group is 1.1225 (sixth root of two).
Figure 14: Microcopy test chart
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