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Optimum Print Width
Excerpt from Metrics for High-Quality Specular Surfaces
Measuring the quality of optical images is required in photography and for visual devices as well as for the manufacturing control of many image-forming systems. Formerly, this process depended on the visual assessment of images produced by a variety of test patterns, such as point sources or resolution test charts. These subjective results were too variable for present needs.
The current, most widely used objective methods for measuring image quality include recording the distribution of intensity in the image of a point source, line, or edge or (more recently) measuring the contrast and position of periodic test patterns having a sinusoidal intensity distribution to determine the Optical Transfer Function1 (OTF). Although supported by international standards, these methods do depend on the use of expensive, high-precision test benches. In addition, when used for diagnostic purposes, many separate measurements are required across the image field, for various apertures and for settings through the focus.
However, a much simpler metric is needed for rapidly comparing the performance of different cameras, for optimising controls, and for assessing changes in performance with use. The Optimum Print Width2 (OPW) metric quantifies the information content of an image in relation to the performance of the average human eye. In the case of the digital camera, where the image is sampled by pixels, the OPW is the width of the image when viewed as a print at arm’s length (500mm), where the effective size of the pixels is matched to the size of the cones in the eye. The OPW can be considered as a measure of the image's information content produced by the optical system.
OPW measurement involves displaying a star test pattern, (a), shown magnified in (b). The diameter (D) of the unresolved central disk in (b), called the camera footprint2 (CF), is measured and divided into the largest side dimension of the image. The OPW is calculated by multiplying this number by the diameter of the visual footprint2 (VF) of the average eye (namely 2.3mm when using a star pattern with 36 bars as shown below).
- L. Baker (Editor), Selected Papers on Optical Transfer Function: Measurement (MS60), SPIE Press, Bellingham, WA (1992).
- L. Baker, “Digital cameras: Optical footprints in a performance metric,” Proc. SPIE 7102, 71020K (2008).