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

Using a polarizing filter to suppress glare and lighting reflections?

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hacecalor
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Joined: 28 Sep 2016, 08:16
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Using a polarizing filter to suppress glare and lighting reflections?

Post by hacecalor » 05 Nov 2018, 10:01

The biggest problem I had with my last scanner (which I scrapped) was the dreaded reflections and glares caused by the glass platen. Even worse, I was photographing books with glossy-coated pages, so I was getting reflections from the opposite page that were strong enough to throw off the OCR. This is a problem I want to address in my next scanner (should I ever get around to building it :D )

When I heard about Circular Polarizers (CPLs) https://en.wikipedia.org/wiki/Polarizin ... otography), I thought it sounded exactly like something you folks would be interested in. Even if you don't have a DSLR, I bet you could jerry-rig something to hold a cheap CPL in front of the camera lens... and they're not even that expensive (assuming you don't need professional photographer-grade CPLs), like $10 on Amazon for the smallest one.

Just to show you what I'm talking about, here's some screenshots I shamelessly stole from Amazon:
Screenshot_2018-11-05 Amazon com AmazonBasics Circular Polarizer Lens - 58 mm Camera Photo.png
Screenshot_2018-11-05 Amazon com AmazonBasics Circular Polarizer Lens - 58 mm Camera Photo.png (481.75 KiB) Viewed 3295 times
Screenshot_2018-11-05 Amazon com AmazonBasics Circular Polarizer Lens - 58 mm Camera Photo(1).png
Screenshot_2018-11-05 Amazon com AmazonBasics Circular Polarizer Lens - 58 mm Camera Photo(1).png (501.51 KiB) Viewed 3295 times
I think I may buy one of those $10 CPLs and do an "experiment" to see what happens in a typical bookscanning setup, but before I do, does anyone have any input? Has anyone tried this before?

dpc
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Re: Using a polarizing filter to suppress glare and lighting reflections?

Post by dpc » 05 Nov 2018, 14:34

The solution to reflections off the platen glass showing up in your scanned images is a well known scanner design issue involving the platen angle and the location of the camera(s) and scanner's light source.

To eliminate the reflection off the adjacent platen glass you need to increase the angle between the two platen planes (100 degrees is a good starting point). Daniel has a video that demonstrates this.

If you're seeing a reflection of the scanner's light source off the platen glass showing up in your images you need to move the light position so that it is farther away from the platen (typically raising it), or decrease the camera's viewing angle by moving the camera farther away from the platen surface, or possibly a combination of both of these.

If you're seeing other parts of your scanner show up as reflections off of platen glass, then you need to decrease the light reflecting off of them (paint the surfaces matte black, wrap with black velvet, etc.) and attempt to eliminate any other light sources in the room.

Finally, when chasing down reflection issues it's helpful to put a piece of black paper in your scanner under the platen glass as it will make it easier for the camera to see any reflected light off the platen.

hacecalor
Posts: 5
Joined: 28 Sep 2016, 08:16
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Country: United States

Re: Using a polarizing filter to suppress glare and lighting reflections?

Post by hacecalor » 05 Nov 2018, 15:51

Thanks for the suggestions. You're right, ideally the scanner would be designed to make glare a non-issue. When I start committing a design to paper or CAD, I'll take the cradle angle/light heights etc. into consideration.

I'm still wondering if using a polarizing lens would be a cheap and easy solution to fix an imperfect scanner. Obviously I'm designing my scanner to not have too much glare, but I'm not expecting it to be perfect by any means, so I'd be interested to see whether a CPL could alleviate those problems without needing additional hardware, for example having a huge light baffle.

I will take a look at that video of Daniel's. I'd be interested to know the math or trial-and-error that went into finding that perfect platen angle.

dpc
Posts: 296
Joined: 01 Apr 2011, 18:05
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Location: Issaquah, WA

Re: Using a polarizing filter to suppress glare and lighting reflections?

Post by dpc » 05 Nov 2018, 16:32

Dan's original post regarding platen angles here: viewtopic.php?f=17&t=1149

For some odd reason the links to the associated videos don't appear to be in that post any longer. Here they are:
Part I
Part II
Part III

I can provide the mathematical routines for determining the minimum camera and light distance that will ensure that the light won't be reflected back into the camera view based on a platen width, platen angle, and light width.

hacecalor
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Country: United States

Re: Using a polarizing filter to suppress glare and lighting reflections?

Post by hacecalor » 06 Nov 2018, 09:42

Thanks for the resources.

And yeah, looking at the design for the Archivist I can see the ideal angle is 100 degrees.

I wouldn't mind taking a look at those formulas. My primary goals for my scanner are portability and price (hence trying to cut corners with the polarizing filter), but the more I can get right in the construction, the better off I'll be.

I've ordered a cheap CPL on Amazon to do some testing. Now to get my hands on some glass...

dpc
Posts: 296
Joined: 01 Apr 2011, 18:05
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Location: Issaquah, WA

Re: Using a polarizing filter to suppress glare and lighting reflections?

Post by dpc » 08 Nov 2018, 16:13

I'm so pissed right now!

I typed up a response comprising multiple pages explaining the formulas and examples of how to use them to calculate light and camera distances based on page sizes and platen angles and when I clicked on 'Preview' to check the formatting this POS website took me to the login page and lost all of my input (probably because I was working on the post for hours this morning and it auto-logged me out).

Maybe I'll get around to posting the info later, but I need to go cool off and do something else for a while. Sorry.

hacecalor
Posts: 5
Joined: 28 Sep 2016, 08:16
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Country: United States

Re: Using a polarizing filter to suppress glare and lighting reflections?

Post by hacecalor » 09 Nov 2018, 13:14

Oh shoot, I'm sorry to hear that! The forum is behaving strangely for me, it threw a 500 error at least once and is taking forever to load.

I really do appreciate the effort you put in, even if I can't see it yet :)

The CPL arrived yesterday, and it may be premature but I think this thing is magic. I don't have any sort of scanner setup to show off how it reduces glare, but I'm optimistic about what it'll be able to do. The images I posted from Amazon are completely realistic. I think using it in combination with a decently-designed scanner setup will be enough to get some nice images, especially in imperfect conditions.

Writing up a huge response over the course of several hours was tremendously nice of you to do. I'm sorry the forum decided to screw you over. Take your time, it's going to be quite a while before I start the actual build.

cday
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Re: Using a polarizing filter to suppress glare and lighting reflections?

Post by cday » 09 Nov 2018, 18:00

dpc wrote:
08 Nov 2018, 16:13
I typed up a response comprising multiple pages explaining the formulas and examples of how to use them to calculate light and camera distances based on page sizes and platen angles and when I clicked on 'Preview' to check the formatting this POS website took me to the login page and lost all of my input (probably because I was working on the post for hours this morning and it auto-logged me out).
I know the feeling well, that has happened to me a small number of times on forums, whether due to a timeout or an inadvertent misclick. Now when composing a lengthy post I try to remember to save the text to a text file periodically: most, or even all, formatting is saved although any attachments uploaded will have to be uploaded again and placed in the post.

hacecalor wrote:
09 Nov 2018, 13:14
The CPL arrived yesterday, and it may be premature but I think this thing is magic. I don't have any sort of scanner setup to show off how it reduces glare, but I'm optimistic about what it'll be able to do. The images I posted from Amazon are completely realistic. I think using it in combination with a decently-designed scanner setup will be enough to get some nice images, especially in imperfect conditions.
I shall be interested to learn the result: I did suggest recently that a polarisation filter might be beneficial but nobody picked up on the idea. When it comes to enhancing scans, whether to improve the appearance of pages when viewed or to improve OCR accuracy, the evenness of illumination ultimately sets the limit to the result that can be achieved.

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