Daniel Reetz, the founder of the DIY Book Scanner community, has recently started making videos of prototyping and shop tips. If you are tinkering with a book scanner (or any other project) in your home shop, these tips will come in handy. https://www.youtube.com/channel/UCn0gq8 ... g_8K1nfInQ

Uneven light

All about lighting. LED, CFL, Halogen, Other? Questions and info about lighting go here.
spomwii
Posts: 69
Joined: 17 Jun 2011, 07:57

Uneven light

Post by spomwii » 14 Jan 2013, 06:34

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

dpc
Posts: 268
Joined: 01 Apr 2011, 18:05
Number of books owned: 0
Location: Issaquah, WA

Re: Uneven light

Post by dpc » 14 Jan 2013, 19:32

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.

kalyan
Posts: 17
Joined: 11 Sep 2012, 02:51
Number of books owned: 0
Country: India

Re: Uneven light

Post by kalyan » 15 Jan 2013, 02:36

You can try with an image editing software for post processing. Also explore the "Custom White balance" option available in the camera.
Attachments
Sample- color.JPG

spomwii
Posts: 69
Joined: 17 Jun 2011, 07:57

Re: Uneven light

Post by spomwii » 15 Jan 2013, 19:16

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.

User avatar
daniel_reetz
Posts: 2776
Joined: 03 Jun 2009, 13:56
E-book readers owned: Used to have a PRS-500
Number of books owned: 600
Country: United States
Contact:

Re: Uneven light

Post by daniel_reetz » 15 Jan 2013, 20:38

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?

dpc
Posts: 268
Joined: 01 Apr 2011, 18:05
Number of books owned: 0
Location: Issaquah, WA

Re: Uneven light

Post by dpc » 15 Jan 2013, 21:27

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.

spomwii
Posts: 69
Joined: 17 Jun 2011, 07:57

Re: Uneven light

Post by spomwii » 16 Jan 2013, 17:50

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.

User avatar
daniel_reetz
Posts: 2776
Joined: 03 Jun 2009, 13:56
E-book readers owned: Used to have a PRS-500
Number of books owned: 600
Country: United States
Contact:

Re: Uneven light

Post by daniel_reetz » 16 Jan 2013, 23:23

Is it a similar design to the one that came with the kit? Can you please post a picture of this better setup?

spomwii
Posts: 69
Joined: 17 Jun 2011, 07:57

Re: Uneven light

Post by spomwii » 17 Jan 2013, 14:29

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.
led2.jpg
Large LED
led2.jpg (117.81 KiB) Viewed 5888 times
led3.jpg
Large LED 2
led3.jpg (68.33 KiB) Viewed 5888 times

User avatar
daniel_reetz
Posts: 2776
Joined: 03 Jun 2009, 13:56
E-book readers owned: Used to have a PRS-500
Number of books owned: 600
Country: United States
Contact:

Re: Uneven light

Post by daniel_reetz » 17 Jan 2013, 20:02

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.

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest