General > General Technical Chat
A4 Scanner light
StillTrying:
Any ideas why the RGB LEDs on the moving strip bit of an A4 scanner are not evenly spaced?
They're spaced at one side and get closer and closer together towards the other side. :)
thm_w:
Kind of hard to see from the photo.
Maybe they are using a light guide or mirror that is on one side of the scanner, so light required varies across the scanned area?
https://global.canon/en/technology/support10.html
https://hamamatsu.magnet.fsu.edu/articles/scanningformats.html
tom66:
Could be computational optics. They use an inexpensive lens which distorts the light but then position the source LEDs to create even light levels through this distorted lens. In a distantly related world, see computational lithography.
StillTrying:
I'd had a read of the canon link, and now the other, I used to work with linear CCDs years ago. :)
I'm sure it's a full length CCD where I've drawn the red line.
On the left of the fuzzy pics where the LEDs are less dense one seems to be out which is visible on a scan, just, or definitely when I turn the contrast up.
I still don't know why the RGB LEDs are 5 times more dense at one end. :)
RightHandSide is ~10% of A4 width plain white but with gamma and saturation turned up.
bw2341:
Does the light flash rapidly between red, green and blue as it scans? If so, you have a CIS (contact image sensor) scanner.
The CIS element is sensing the reflected light from your document. It is not flashing light at your camera. You're looking at the LED light source. A bright RGB LED is cycling between red, green and blue so that the CIS element can capture each of the colour channels separately for every scanline. The goal of the LED is to evenly light the whole width of the page.
https://www.plexiglas.de/en/applications/thin-thinner-light-guide-film
I couldn't find a good image of how it works, but I suspect that the scanner is using an edge-lit LED and a light guide film. A PMMA plastic film or sheet is laser etched with fine details so that the total internal reflected light from the LED is let out perpendicular to the sheet.
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.
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.
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