A4 Scanner light - Page 1
Forum section: electronics / general

author: StillTrying
author_rank: Super Contributor
timestamp: August 29, 2023, 09:59:45 pm
content: 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.

author: thm_w
author_rank: Super Contributor
timestamp: August 29, 2023, 10:17:37 pm
content: 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.htmlhttps://hamamatsu.magnet.fsu.edu/articles/scanningformats.html

author: tom66
author_rank: Super Contributor
timestamp: August 29, 2023, 10:28:54 pm
content: 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.

author: StillTrying
author_rank: Super Contributor
timestamp: August 29, 2023, 11:54:30 pm
content: 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.

author: bw2341
author_rank: Regular Contributor
timestamp: August 30, 2023, 02:59:01 am
content: 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-filmI 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.

author: StillTrying
author_rank: Super Contributor
timestamp: August 30, 2023, 07:47:08 am
content: Quote from: bw2341 on August 30, 2023, 02:59:01 amYes, 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.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 30, 2023, 08:19:33 am
content: 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.

author: bw2341
author_rank: Regular Contributor
timestamp: August 30, 2023, 11:35:12 am
content: Quote from: StillTrying on August 30, 2023, 07:47:08 amAre 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.

author: Psi
author_rank: Super Contributor
timestamp: August 30, 2023, 12:25:58 pm
content: 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?

author: StillTrying
author_rank: Super Contributor
timestamp: August 30, 2023, 05:34:08 pm
content: "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.