What is it?
"Overvoltage Crossover" is the terminology used by the IEC Standard for Scanner 61966-8. This refers to the effect of nearby colors that shift the scanned colors toward neighboring colors.
What problems does it cause?
This effect can cause large differences in the colors of scanned photos, making it sometimes impossible to scan a photo and get a colorimetrically accurate image file. These are not often seen in complex photos, but some are vulnerable. A scanned image of a hot air balloon in front of a bright sky brightens the lighter sky more than the balloon.
Accurate reproduction is highly desirable, among other things, for accurately duplicating and preserving historical images.
Here is a scanned image of a mid-high V850 Epson scanner. They are large, shifted L * values under each circle. The three circular spots on the left were printed the same way as the three on the right. The lighter ones were RGB (240, 240, 240), while the darker ones were RGB (118, 118, 118). The circles surrounded by white have L * s scanned about 6 higher than the same one to your right. This shift is from "trans-spanning large area".
This problem is caused by the reflection of light near parts of a document which propagate through the translucent, white and white rails on both sides of a horizontal aperture, through which the CCD scan is detected. It is as if the lux levels in the right white surround area would be increased by 20%. The increase in light had to increase L * from 85.7 to 92.7.
Question: Are there any reflective desktop scanners that have addressed this issue in their design, and / or is there software that corrects or searches for this error?