Polarized light.... no more reflections!

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Polarized light.... no more reflections!

Postby kasslloyd » 21 Dec 2010, 21:47

I have a plan to 100% eliminate reflections on the glass...

Best Option:

Use a sheet of linear polarizer positioned vertically in front of the lights illuminating the book, then a sheet of linear polarizer positioned horizontally in-front of the camera lens. What this will do is polarize all the light illuminating the book, and by putting the polarizer positioned the opposite way in-front of the camera all the light that is directly reflected by the glass will be eliminated, but all the light hitting the paper will be refracted and won't be polarized anymore and will pass through the filter in-front of the camera. One downside is each filter acts as a neutral-density filter reducing the amount of light that ends going to the camera, so brighter lights will be necessary.

See the picture how all light is blocked by positioning the films opposite:

Image

Second Option:

Only use a polarizer filter in-front of the camera, and then angle the lights to minimize the glare, I think 45 degrees from the camera. This I don't think is the best option, but should eliminate far more glare than no polarizer filters.

Notes:

Also you need to make sure there isn't anything else that can refract the light and reflect on the glass.. so black fabric all over the place, behind the lights, behind the camera, etc... Paint everything flat black. Also bright lights are hot and any plastic polarizing film near it will melt, so be careful there too...

Source to get polarizing film, might not the be the cheapest, but was the first I found that has good options:

http://www.polarization.com/polarshop/

How does it work?

Heres a little graphic to illustrate how polarizers work

Image
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Re: Polarized light.... no more reflections!

Postby russca » 23 Dec 2010, 00:33

kasslloyd wrote:Use a sheet of linear polarizer positioned vertically in front of the lights illuminating the book, then a sheet of linear polarizer positioned horizontally in-front of the camera lens. What this will do is polarize all the light illuminating the book, and by putting the polarizer positioned the opposite way in-front of the camera all the light that is directly reflected by the glass will be eliminated, but all the light hitting the paper will be refracted and won't be polarized anymore and will pass through the filter in-front of the camera. One downside is each filter acts as a neutral-density filter reducing the amount of light that ends going to the camera, so brighter lights will be necessary.
I wish you would put a drawing depicting how it would work in a bookscanner setup. I am having hard time visualizing vertical and horizontal lines against lights and camera. You mean two polarizers should be at 90 degrees to each other as far as lines go?
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Re: Polarized light.... no more reflections!

Postby Gerard » 23 Dec 2010, 04:51

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Re: Polarized light.... no more reflections!

Postby kasslloyd » 23 Dec 2010, 14:30

russca wrote:
kasslloyd wrote:Use a sheet of linear polarizer positioned vertically in front of the lights illuminating the book, then a sheet of linear polarizer positioned horizontally in-front of the camera lens. What this will do is polarize all the light illuminating the book, and by putting the polarizer positioned the opposite way in-front of the camera all the light that is directly reflected by the glass will be eliminated, but all the light hitting the paper will be refracted and won't be polarized anymore and will pass through the filter in-front of the camera. One downside is each filter acts as a neutral-density filter reducing the amount of light that ends going to the camera, so brighter lights will be necessary.
I wish you would put a drawing depicting how it would work in a bookscanner setup. I am having hard time visualizing vertical and horizontal lines against lights and camera. You mean two polarizers should be at 90 degrees to each other as far as lines go?


Linear polarizers, think of them as a fence like in the picture above except the spaces between that let light go through are microscopic, have lines that go vertically up and down one way. These let in light of only one type of vibration through. Light coming from the light source is a mixture of all types of ways the waves can be angled. A polarizer will filter out only one vibration pattern. Once you polarize the light for one vibration pattern, unless it's refracted by something (changing it's vibration pattern) then if you orient another sheet of linear polarizer 90 degrees from the first, i.e. filter for a different frequency, then all unrefracted polarized light will be blocked, so any that was from a direct reflection from a shiny surface.



This is an example of what I mean, LCD screens have a liner polarizer infront of them so the light coming from a LCD screen is polarized. The camera filter hes holding up as he rotates it once it hits 90 degrees of the polarizer in the laptop screen it blocks all light, then progressively lets more in as he rotates it back away from that position


Gerard wrote:maybe you will get an new problem
http://www.instructables.com/id/Capture ... bjects-Us/


Whats going on here is the same effect in the above video, except hes putting a transparent object that refracts the polarized light infront of it. Plastics will do weird things to polarized light, specifically cast plastic. I'm not sure how acrylic sheets react to polarized light if they refract or just transmit, but I would use this setup with glass personally, glass will give the best images over plastic.

The term is photoelasticity

Glass should not have this effect...
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