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I am having a problem with reflection on an acrylic surface, in the IR part of the spectrum. This reflection is interfering with an algorithm that looks at objects, as it makes two show up when only one exists.

Some background:

An IR sensitive (and IR filtered) camera must view an IR backlit stage below at a steep angle, the walls of which are perpendicular to the IR light source, the cameras are at about 30 degrees tilted back with respect to the walls of the stage.

Because the walls of the stage must be translucent to visible light, they are made of acrylic (just about the same optical properties as glass).

A problem arises when an object comes close to the surface of the glass, as a reflection of that object appears from the acrylic. This confuses the software that uses the camera into thinking there are two objects. To get rid of this problem, a matte finish is applied to the inner surface of the wall to blur the reflection enough, and this works. However it doesn't work perfectly, and the inner surface of the wall would ideally be perfectly smooth.

The first question is: How can I eliminate the reflection due to the internal reflection, in the IR part of the spectrum, of my object?

Specifications:

  • A wall of acrylic is reflecting an object at a steep angle in the IR spectrum

  • The inner wall must be kept smooth (although could have some filter added, and then a smoothing film added over top, perhaps, if it would still elimiate the reflection)

  • The viewing angle is steep and cannot be changed

  • Visible light must be able to pass from the outside of the wall to the inside.

Proposed solution is now:

Using an IR filter applied to the outside wall to prevent the reflection from coming in and up to the camera as strongly, this leaves the inside smooth. Or applying a matte to the outside of the wall to blur the reflection, which also leaves the inside able to be perfectly smooth.

A second question is: will the proposed solution work?

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other tags are relevant, I just can't add them... –  St-Ste-Ste-Stephen Feb 24 '12 at 16:28
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Hi St-Ste-Ste-Stephen - while the background information is useful, you actual question is kind of buried. Could you clarify what exactly you are asking? –  David Z Feb 24 '12 at 20:51
    
Sure, no problem sorry! –  St-Ste-Ste-Stephen Feb 24 '12 at 20:51
    
The easiest way to accomplish your goal is as you said, add an IR filter to your camera. I would pick an "'interference" filter (eoc-inc.com/infrared_filters.htm). –  Antillar Maximus Jan 6 '13 at 15:27
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2 Answers 2

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If you have the budget for special purpose stuff, I would go for an anti-reflective coating. Since the angle of incidence is fixed and well defined and since you are filtering the IR illumination ideally what would happen is you use a very narrow pass IR filter and an anti-reflective coating that is specifically 'tuned' for the frequency and angle of illumination. You should be able to eliminate most of the reflection.

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How much slop is usually associated with these angles? Is it fairly dependent on the material, extremity of the angles, something else? Thanks. –  St-Ste-Ste-Stephen Feb 26 '12 at 21:14
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what about using a reflective lense on the camera or anti glare? or even adjusting the IR with some type of filter?

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