HP 8550 Service Manual - Page 210

Shading correction The output of the CCD is not necessarily uniform even when the light it receives is from an original with perfectly uniform density for the following reasons: l The sensitivity of each CCD pixel is different. l The level of lens transmission differs between the middle and ends. l The intensity of the scanning lamp differs between the middle of the lamp and its ends. l The scanning lamp deteriorates over time. Shading correction is performed to make up for these discrepancies. Shading correction overview The CCD measures the intensity of light reflected by a standard white plate (very uniform density) (callout 1). Values generated from this process are used to correct data gathered from each scanned original. Correction values are taken from the standard white plate each time START is pressed. If the standard white plate is ever replaced, the value printed under its bar code (callout 2) must be entered in service mode. The shading correction circuit compares the measurement data gathered by the CCD from the standard white plate and the target value from the numbers recorded on the standard white plate. It then uses this comparison to calculate a "shading correction value" that is stored in memory. 2 21 Copyboard glass Figure 98. Scanning lamp and lens 208 Chapter 5 - Theory of operation EN