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I am a photography novice, and am trying to understand how polarizing filters and light reflections work.

I'm trying to understand how to reduce reflections in a glass corner. The context is trying to photograph a book in a type of display case, where the book is open at an acute angle, and each page is covered by a pane of glass to flatten it. This diagram (from this site) demonstrates approximately how the glass platen looks (but shows a larger angle; also, I'm photographing the book with just one camera instead of two):

Book scanner diagram from http://diybookscanner.org/

The problem is that each page shows a reflection from the other page. (Similarly, if just one page is covered by a pane of glass, that pane will show a reflection of the opposite page.) The reflection doesn't seem to improve with increased light.

I understand from questions such as this and this that circular polarizing filters can reduce a camera's view of light reflected on glass. Would a circular polarizing filter be appropriate to use in this type of context (where there are two reflective glass panes facing each other at an angle) for reducing reflections? If so, I'd be grateful to understand why. Are there alternative approaches that you would recommend that I learn about for reducing reflections in this context?

(For the purpose of my learning, imagine that the camera can be placed anywhere with a clear view of both pages of the book, whether straight down or at an angle. Similarly with light sources.)

  • Light reflections on glass are linearly polarized, not circular. See Wikipedia on Brewster-angle. – Grimaldi Feb 21 '17 at 20:31
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    @Grimaldi "Circular" polarizers are linear polarizers with an additional plate that repolarizes the light in a circular polarization. – Michael C Feb 21 '17 at 21:10
  • What happens if you put a paper sheet in the plane of symmetry? If the problem is caused by reflection only the polariser should solve the problem. – Crowley Feb 22 '17 at 13:18
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Light as it travels, consisted of particles called photons. These are one component of the electromagnetic light wave. These vibrate; the motion is often compared to the motion of water waves. The difference, water waves ungulate up and down only, light waves are not restricted, they vibrate in any and all planes. Visualize the polarizing filter as having parallel lines drawn closely together all over its surface. Light waves can pass only if the direction of vibration is aligned (parallel) to the ruled lines. Thus light after passing through the polarizing screen consist of light waves undulating only in one plane. The polarizing filter consists of tiny flakes of mineral imbedded within. The term polarized light came from early scientist trying to explain, they visualized that light had a north and south pole much like a magnet. They incorrectly named changes in the direction of vibration after refection or transmission as “polarization”.

When light radiates out from a lamp, its direction of undulation is omnidirectional (non-polarized). When light hits polished material it can be reflected. The composition of the material may absorb some or all of the light. Some materials, mainly non-conductors like glass, reflect light vibrating in just one plane i.e. this reflected light is polarized. Some transparent materials will also affect the direction of vibration during the passage.

A polarizing screen (filter) can mitigate reflections. We mount the polarizing screen and rotate it. We rotate to find a position that best position that rejects unwanted reflections. This is accomplished by rotating the filter as you observe the effect through the viewfinder or viewing screen. As to mitigating reflections from glass, the maximum effect is dependent on the angle camera-to-subject. We move the camera and the position of the lights to optimize this anti-refection ability. You may find it impossible to midrate reflections simultaneously from two glass surfaces set at different angles. Experimentation is your only option. You may need to place polarizing filters, available in large sheets, over the lamps. You will rotate all the filters and experiment for best effect.

About circular polarization: Most cameras will work just fine with a standard polarizing screen. In modern times, especially with digital cameras, the internal mechanisms can be defeated if a polarizing filter is mounted. We mount a circular polarizing filter. This is two filters sandwiched together. The first is an ordinary polarizing filter. This does the trick, minimizing reflections and improving saturation. The second is a “retarder”. This second filter de-polarizes the image forming rays minimizing harm to the auto focus and metering system.

  • The question was not "what is a polarizing filter?" I do not see where the specific question is answered here. – ths Feb 21 '17 at 22:23
  • Alan-Marcus, thank you for your thoughtful response! @ths, this does answer the "Why would a polarizer be useful?" aspect of my question, and also suggests that trial and error with polarizers on the light sources as well as camera lens might be the only way to know for sure in this case, which does answer the rest of the question. I'm waiting to see what other answers will come in in the next couple of days, but am considering marking this answer as accepted. – J L Feb 22 '17 at 14:44
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The light as currently positioned is at an angle to the glass and the reflection will be polarized. A polarizing filter on the lens should help.

For the most faithful reproduction orient the camera perpendicular to the page. It's not to reduce reflections, but if it is oriented at another angle you could skew the image.

Try a ring light on the camera. This makes spurious reflections very near, and hopefully unnoticeable, to the original image. It will also reduce light from the page not being photographed.

Is there another method to flatten the page without glass? A frame around the edges?

Use glass with an AR coating.

Put a black cloth (paper, etc.) over the page not being photographed. I presume there isn't any reason both pages need to be photographed simultaneously. This also looks like a time intensive project anyway. A split second to cover the other page shouldn't matter much.

Turn off extraneous sources of light, turn off room lights, etc.

Use a lens hood or build a hood that extends all the way to the page.

Some combination of these tips should work.

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I believe you can solve your problem by making sure that the camera is perpendicular to the glass. If the platen angle is 90 degrees or greater (some commercial units use 100 degrees) and the camera is out of the light, and the room is dark then you won't get a reflection of either the camera or the adjacent page. You may already know about this site (you seem to have taken your diagram from the page) but there are tidbits of useful information to be gleaned...

Polarizers won't work in this case because they only work if the light is bouncing off the glass at an angle (57 degrees- Brewster's angle, I believe, provides the maximum polarization).

  • The question states that the display case glass is at an acute angle, though. – junkyardsparkle Feb 21 '17 at 19:59
  • BobT, thank you for your answer! I very much like and am familiar with the DIY Book Scanner site you linked. @junkyardsparkle is correct: What makes this question different from previous questions (both here on SE and on diybookscanner.org) is that I'm specifically curious about instances where the panes of glass are at less than 90 degrees to each other (e.g., at 70 degrees). The closest the book scanner website has come to discussing this is here, I think, but it didn't really go into the reflection aspect. – J L Feb 21 '17 at 20:04
  • Ah... Have you tried a black cover sheet for the page not being photographed? Something nonreflective? – BobT Feb 21 '17 at 23:30
  • @BobT, that's certainly something I'm considering now; thank you. Ideally, I'd be able to photograph both pages at once, though. I think that my immediate next step will be to try replacing glass with acrylic that's been treated to be less reflective (the "anti-glare" type). – J L Feb 22 '17 at 16:03
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The light is in the wrong place. You need to have the light hit the glass at a 45 degree angle or less. (90 degrees is right angle to the glass)

To be successful you have to be able to turn the other light off. That is, you want the non-photographed page to be unlit.

The camera should be at 90 degrees to the page.

If the display above is inside a secondary case, you want to use a shield so that no light from your source is hitting the top glass in the field of view. This may not be possible if the outer case has minimal clearance from the display.

You can also reduce the reflections off the glass by polarizing the source with a cheap polaroid plastic filter. There should be an orientation that minimizes the reflections.

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