General > General Technical Chat
A4 Scanner light
StillTrying:
--- Quote from: bw2341 on August 30, 2023, 02:59:01 am ---Does the light flash rapidly between red, green and blue as it scans? If so, you have a CIS (contact image sensor) scanner.
By carefully varying the density of the marks, with very few near the LED and many more at the far end, you can create a very evenly emitting strip of light. This is the uneven spacing that you can see in your photo.
--- End quote ---
Yes, if I scan the camera up and down during the 1/3 sec. exposure the white dots separate into RGB, plus I can see the white is made up of RGB times by eye.
If you etch evenly spaced marks along the light guide strip, the whole strip will emit light, but the light will not be even. The end of the strip in contact with the LED will be much brighter than the far end.
Yes, I suppose the LEDs / 1mm plastic bumps of light dots could actually be a light guide.
I think it calibrates the CCD for even light level across the full A4 width by looking at a white or grey strip before it enters the clear A4 glass area.
The unevenness / missing dots at the right of the A4 are only visible as a darker vertical line when I have to adjust the contrast of a photo scan a lot, it's not visible on document or magazine settings.
RoGeorge:
Stronger light at the far end might sense when the sensor is a long CCD bar. In a CCD only the last photo cell is connected out. The reading is made by pouring the charge (shifting) from one cell to another until all cells in a row are read.
The shifting of charge from one cell to the next has some losses at each step. Thus, for equal light, the charge in the far-end cell will arrive attenuated to the reading amplifier, in comparison with the nearest cell to the reading amplifier.
To compensate for reading losses of the far end cells of the CCD, might make sense putting more light into the far end.
bw2341:
--- Quote from: StillTrying on August 30, 2023, 07:47:08 am ---The unevenness / missing dots at the right of the A4 are only visible as a darker vertical line when I have to adjust the contrast of a photo scan a lot, it's not visible on document or magazine settings.
--- End quote ---
Are the missing dots less bright than the others or are they completely missing?
If they are less bright, dust has probably fallen right on the feature. Blowing the dust off the scanner bar should make the light more even.
If they are completely missing, it is probably a manufacturing defect. At 1mm feature sizes, it might be possible to mould them directly into the plastic. Some missing features would be expected. As in your scanner, there seems to be enough calibration range to make up for some defects.
Psi:
I assume you tried looking at it from different angles to rule out that it only 'looks' that way from the angle you're viewing it at?
StillTrying:
"Stronger light at the far end might sense when the sensor is a long CCD bar."
Yes, I wondered about that CCD thing, but don't remember it being a big problem up to about 2k cells.
I'm quite sure it's a light tube ATM, with less dots at the bright end and many more dots at the far end. Now I've had a proper look in daylight the dots are all small rectangles roughly 0.4mm x 0.6mm.
I think the missing dots/rectangles at the bright end are probably correct, they just haven't got the radomness quite 100% correct, it might depend on the exact way a photograph surface reflects.
"Are the missing dots less bright than the others or are they completely missing?"
The dots appear to be slightly raised rectangles, I thought the missing ones might be just painted black but even in daylight with a magnifing glass they appear to be completely missing.
"I assume you tried looking at it from different angles to rule out that it only 'looks' that way from the angle you're viewing it at?"
Yes, I have to view it on the side where the very thin CCD line is and 25-50 degrees off vertical to see three bright rows, not much light goes vertically up.
It's switched off in these 2 images, it's just bright daylight showing the bright dots and the missing left hand ones.
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