3. Calibration When scanning a document of even density, the image signal corresponding to each pixel may be uneven due to the following reasons: * Uneven light intensity of the scanning lamp * Variation in the sensitivity of the light phototransistors * A slight output from the light phototransistors with no light Calibration corrects these variations with the following three steps: Offset Calibration Coarse-gain Calibration Programmable Gain Amplifier (PGA). 1) Offset Calibration Normalize the output data from each pixel at the value of 0 or near 0 by using data aquired with no light. 2) Coarse-Gain Calibration Finds the optimum gain setting to have the output voltage from all pixels within in the adjustment range of Programmable Gain Amplifier. 3) Programmable Gain Amplifier (PGA) Calculates a gain coefficient to make output constant by correcting the output difference among pixels.
4. Filter Processing Changing the resolution and the scaling tends to reduce the image quality. To prevent this, the filter processing needs to be performed according to the resolution. The filter processing includes interpolation processing and averaging.
