Canon CANOSCAN D1230 series Service Manual

Also see for CanoScan D1230UF: User guide Quick start guide User guide User guide User guide

CHAPTER 22 - 15COPYRIGHT © 2001 CANON INC. CANOSCAN D1230U/D2400U REV.0 MAR. 2001 PRINTED IN JAPAN (IMPRIME AU JAPON)3. CalibrationCalibration can be performed to normalize the pixels of a linear CCD so that each pixelproduces the same digital output code from the scanner when presented with the same imagelight intensity. This intensity ranges from black (no light) to white (maximum light intensity).The CCD’s analog output may have large pixel-to-pixel variations in their output voltage whenscanning the same white image (corresponding to errors on brighter signals). If these offsetsare subtracted from each pixel, and each pixel is given the optimum gain setting to correct fordifferent efficiencies, then these errors can be eliminated.Ideally the digital output code for any pixel would be zero for a black image, and some codenear the full scale for an image with maximum brightness. This code is called the target code.The analog IC eliminates these global and pixel-to-pixel offset and gain errors with itsCorrelated Double Sampling, Offset D/A converter, Variable Gain Amplifier (VGA), andProgrammable Gain Amplifier (PGA).Calibrating an analog IC-based system requires three steps:1) Offset CalibrationTakes a black image and normalizes the digital output code for each pixel at or near 0.2) Boot-Gain/Coarse-Gain Stage CalibrationFinds the optimum gain setting that places the output voltage from all pixels from x0.93 to x9adjustment range of these two stages.3) Shading Correction from ASICCalculates the gain required for normalizing the output from each pixel to the target code.4. Filter ProcessingWhen converting resolution and scaling, the image quality tends to be reduced. To preventthe image quality reduction, filter processing is performed according to the resolution. Filterprocessing includes interpolation processing and averaging.