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Uneven light

Posted: 14 Jan 2013, 06:34
by spomwii
Hi. I have a problem with uneven light on my scanner. The problem is when I have my light source is on the top I get a brighter light on the top of the page that is closest to the light source. Se sample her:
Image
https://www.dropbox.com/s/7z2q0yblamcog ... 0light.JPG

This is not a problem when scanning text books but I am also scanning comic books with glossy paper and then the difference in brightness gets very visible. Does anyone know how I can avoid this problem?

Image
https://www.dropbox.com/s/bzqhhwourdau316/3.jpg

Re: Uneven light

Posted: 14 Jan 2013, 19:32
by dpc
Light intensity falls off at 1/d^2, where 'd' is the distance. So moving the light source farther away from the page will decrease the intensity difference between the leading edge of the page and the gutter. This can help, but likely won't eliminate it. Other things to try would be to diffuse the light source to try to get more even lighting. You could also attempt to filter the amount of light hitting the "hot" edge of the page with some screen or filter cells covering the outside edge of the light.

I've experimented in compensating for uneven lighting by shooting a blank white piece of paper across the entire platen surface before scanning pages and then using the resulting image of that blank page to adjust the pixel values of the scanned pages in a post-processing step. In order for this to work, the platen/light/camera positional relationship must remain constant during the entire scan of the book (which they are for the Hackerspace scanner). The pages being scanned will move around a bit from scan-to-scan under the platen and that's OK because the blank page was capturing light intensity at a point on the *platen* that will be the same no matter what you're shooting underneath it. This isn't a cure-all for uneven lighting problems, but just another tool of many available to tweak things a bit.

Re: Uneven light

Posted: 15 Jan 2013, 02:36
by kalyan
You can try with an image editing software for post processing. Also explore the "Custom White balance" option available in the camera.

Re: Uneven light

Posted: 15 Jan 2013, 19:16
by spomwii
dpc wrote:Light intensity falls off at 1/d^2, where 'd' is the distance. So moving the light source farther away from the page will decrease the intensity difference between the leading edge of the page and the gutter. This can help, but likely won't eliminate it. Other things to try would be to diffuse the light source to try to get more even lighting. You could also attempt to filter the amount of light hitting the "hot" edge of the page with some screen or filter cells covering the outside edge of the light.

I've experimented in compensating for uneven lighting by shooting a blank white piece of paper across the entire platen surface before scanning pages and then using the resulting image of that blank page to adjust the pixel values of the scanned pages in a post-processing step. In order for this to work, the platen/light/camera positional relationship must remain constant during the entire scan of the book (which they are for the Hackerspace scanner). The pages being scanned will move around a bit from scan-to-scan under the platen and that's OK because the blank page was capturing light intensity at a point on the *platen* that will be the same no matter what you're shooting underneath it. This isn't a cure-all for uneven lighting problems, but just another tool of many available to tweak things a bit.
Sorry. Forgot to mention that I have the hackerspace scanner. Thank you very much for your inputs. I was thinking about trying to put the LED higher up and maybe add another LED. I will try this next weekend.

I am not sure that I understood what you did with the white page you scanned. How do you adjust the pixel value afterwords? I totally understand if it is to complicated to explain here.

Re: Uneven light

Posted: 15 Jan 2013, 20:38
by daniel_reetz
spomwii, I'm very interested in fixing this lighting problem for hackerspace scanner owners. Do you also have the standard 10W LED that is included with the kit?

Re: Uneven light

Posted: 15 Jan 2013, 21:27
by dpc
spomwii wrote: <snip>

I am not sure that I understood what you did with the white page you scanned. How do you adjust the pixel value afterwords? I totally understand if it is to complicated to explain here.
Think of the image of the white page as a light map. I look at the image of the white page and determine the color value of what is to be assumed to be a perfectly lit white pixel (this will typically be near the middle of the page and toward the outer margin, closest to the light). The difference between this "white" pixel's color value and the color value of a pixel elsewhere on this blank page (that may not be lit as well) will the the amount that I brighten that same pixel location on each of the scanned pages. It's actually a bit more involved than that, but that's the basic premise.

Re: Uneven light

Posted: 16 Jan 2013, 17:50
by spomwii
daniel_reetz wrote:spomwii, I'm very interested in fixing this lighting problem for hackerspace scanner owners. Do you also have the standard 10W LED that is included with the kit?
Yes I have the standard 10w LED. I bought the kit From Mark. But after a lot of testing on comic books I had to buy a new more powerful LED to get more light for my cheap A2200 cameras.
I found a 20w 6500k LED that gave a much better result so now I just need to solve the uneven light problem.

Re: Uneven light

Posted: 16 Jan 2013, 23:23
by daniel_reetz
Is it a similar design to the one that came with the kit? Can you please post a picture of this better setup?

Re: Uneven light

Posted: 17 Jan 2013, 14:29
by spomwii
No the new LED is larger so I have just removed the top and balancing the LED between the top LED Holders. I have to make something more permanent but I am just testing now to find the best setup. I think this setup works better than the original for me because of two things. 1. more light helps my cheap cameras a lot. 2. By placing the LED like this the reflections from the LED moves a little bit further to the side.
I bought this LED in Norway so I guess there is no point in inserting the link to the webstore here.
Large LED
Large LED
led2.jpg (117.81 KiB) Viewed 14173 times
Large LED 2
Large LED 2
led3.jpg (68.33 KiB) Viewed 14173 times

Re: Uneven light

Posted: 17 Jan 2013, 20:02
by daniel_reetz
Handsome build you have there. Thank you for posting your updated setup. Interesting that only a few centimeters of movement (plus a much brighter light) seems to have solved the issue.