Tone-Transfer (OECF) Characteristics and Spatial Frequency Response Measurements for Digital Cameras and Scanners

Measurement of the spatial frequency response (SFR) of digital still cameras by slanted-edge analysis has been established for several years. The method, described in standard ISO 12233, has also been applied to other image acquisition subsystems such as document and print scanners. With the frequent application of the method and use of supporting software, questions often arise about the form of the input test image data. The tone-transfer characteristics of the system under test can influence the results, as can signal quantization and clipping. For this reason, the original standard called for a transformation of the input data prior to the slanted-edge analysis. The transformation is based on the measured opto-electronic conversion function (OECF) and can convert the image data to a reference-exposure signal space. This is often helpful when comparing different devices, if the intent is to do so in terms of the performance of optics, detector, and primary signal processing. We describe the use of the OECF and its inverse to derive the signal transformation in question. The influence of typical characteristics will be shown in several examples. It was found that, for test target data of modest contrast, the resulting SFR measurements were only moderately sensitive to the use of the inverse OECF transformation.