Test and Calibration Procedures for Image Scanners

Film scanners will be used for many years to come and it is well known that not even the best and most expensive scanners are not free of errors. Scanner testing and calibration procedures are necessary to achieve a high geometric and radiometric scanner performance. It is also a means to produce or make use of cheaper scanners that do not rely on expensive mechanical positioning and optical parts. The paper refers to flatbed scanners employing linear or area CCDs, thus to both photogrammetric and DeskTop Publishing (DTP) scanners. It first presents different slowly and frequently varying errors of geometric and radiometric nature and their sources. The focus of the paper is on general testing procedures for high precision scanning. Different test patterns for detection of various errors and requirements on their quality are presented. Appropriate conditions for performing the tests are formulated. Test procedures for detecting and correcting various errors using test patterns are presented. Finally, some requirements for scanner vendors are proposed. The work is closely related and was partially done within the OEEPE/ISPRS Working Group on the Analysis of Photo-Scanners. CCDs or multiple optically butted linear CCDs. Reference to these specific sensors in the text will be made using the ac ronyms A-CCD, L -CCD, ML-CCD re spec t ive ly. Photogrammetric scanners employ either area CCDs that scan the image in tiles or linear CCDs scanning in multiple swaths. Flatbed DTP scanners use one or multiple linear CCDs to scan the image in one swath. Here only the major errors will be mentioned. Other errors can occur depending on the design, construction, and parts of each individual scanner. Whether some errors are slowly or frequently varying depends on the quality and stability of the scanner, e.g. in photogrammetric scanners the positioning mechanism is accurate and stable, while in DTP scanners the positioning errors vary from scan to scan or even within one scan. For linear CCDs the following convention will be used. Horizontal direction is the direction of the linear CCD, vertical the direction of the scanning movement. A. Slowly varying errors 1. Distortions due to lens or other optical parts This refers mainly to geometric errors like symmetric radial and tangential distortion. Radiometric errors like vignetting, shading, and secondary reflections can also be introduced by the optics. 2. CCD blemishes (A-CCD) They usually occur only with large area CCDs. Blemishes are single pixels, lines/columns or areas whose grey values differ significantly (e.g. 16 grey values or more) than the average grey level of their neighbourhood due to fabrication faults. 3. CCD misalignment and overlap (ML-CCD) The multiple CCDs may have different direction or not be collinear. If their overlap is not correctly estimated by a sensor calibration, then overlaps or gaps will occur. 4. Subsampling errors (L-CCD, ML-CCD) When scanning with a resolution less than the original one, the pixels in horizontal direction are low-pass filtered and resampled, while in vertical direction larger pixels are created by increasing the scanning speed. This leads to different treatment of horizontal and vertical features and can lead to loss of information if the scanning speed is not increased by the correct amount and is not properly synchronised to the integration time. 5. Smearing (L-CCD, ML-CCD) Due to the high scanning speed horizontal features, especially lines, will appear thicker and with lower contrast than vertical ones. This effect corresponds to a low-pass filtering.