Selective filter based on angle of incidence?

  • #1
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Main Question or Discussion Point

I am a using a mirror to reflect an image at an angle of incidence of 10 degrees. The mirror is also at an angle of incidence of approximately 30 degrees from the viewer and it creates undesired reflections when looking at the mirror. Here is a diagram illustrating the situation.
Screen Shot 2020-02-05 at 12.45.16 PM.png


I am looking for an optical apparatus that can block visible light at large angles of incidence but at small angles of incidence let light pass through?

For example, I tried using a privacy screen like the ones used on smartphone or computer monitors. The privacy screen uses micro louvers to block light at large angles. I placed the privacy screen on top of the mirror. However the reflection was very diffuse and not usable.
 

Answers and Replies

  • #2
sophiecentaur
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Hi and welcome to PF.
It's not clear what you actually want. What does the red line represent in the diagram? Do you want to use the mirror for two purposes at the same time? If you want to block visible light at large angles, why not put the mirror at the bottom of a deep box? Alternatively, you could perhaps use bigger louvres. The micro louvres on a phone screen do not deal with reflection - just transmission of light.
If your problem is 'flare', due to non specular reflections on the mirror, perhaps you could use a 'better', mirror- very flat and with no dust or scratches on it. Conventional mirrors are silvered at the back of a glass sheet (to protect the silvering, especially when you are cleaning them). But there are always multiple reflections between front and back of the glass. (If you have a laser pointer you can see the effect very clearly on the mirror's surface).

High quality mirrors, such as the parabolic mirrors used in telescopes, use surface silvering and there is only one reflection at any point on the surface. Dust is still an issue, though and you can't afford to clean the mirror often or you can get scratches which will cause flare.

Give us some more details about the actual application and y ou may get some more useful answers.
 
  • #3
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Hi Sophie,

Thank you for the reply and for the suggestions. Your comments are good. Let me try to answer your questions. I made an updated illustration:

Screen Shot 2020-02-05 at 10.23.47 PM.png


The observer looks at a lens and sees a reflection of the real image. The blue line represents the ray diagram from the real image reflected off the mirror and reflected off the lens. The red line represents the ray diagram of lights sources that reflect off the mirror and reach the observer. The unwanted light source in my application can be the sun in which case it will be very unpleasant reflection.

A deep box would work but I am interested in a more compact solution due to space limitations.

In regards to the mirror, when I tried the privacy screen I was using a first surface mirror. It is a stainless steel mirror. A direct reflection of the real image off the mirror is clean but with privacy screen was attached to the mirror it was very blurry.

You suggested using bigger louvres. Why would that improve the 'flare'?
 
  • #4
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Don't small (sub-metre) telescopes often have internal baffles to do this ??

FWIW, when I saw the thread title, I thought it might be something you could solve using a diffraction grating. No, a very different problem...
 
  • #5
Tom.G
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Keep the big mirror out of the view of the observer.

Have the user look at the secondary (small) mirror thru a tube. Think of a small handheld telescope without any lenses.

Cover the box so only light from/to the object/small mirror can reach the big mirror.

Cheers,
Tom
 
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  • #6
sophiecentaur
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I have some problems with this diagram and with the use of words. Drawing correct ray diagrams can be quite demanding if you are not familiar with the conventions - arrows, showing the direction of the light can help.
The "lens" on this diagram would have to be a mirror (plane?) - lenses transmit light and don't reflect it. Also, your "real image" seems to me to be either an actual object or it's produced by some projector system that's not on the diagram. The projector would have to be somewhere top left of the label "Image". Are you actually talking about some form of head up display, perhaps on the console of a car? Conventionally, reflections from the Sun are avoided by the overhanging shelf (box idea).
Louvres: The louvres on a phone screen are not designed to reflect a wanted image. They are actually lenses (cylindrical) and would not work like the the box I suggested. My suggestion of louvres would be to replace the deep box with a number of shallow louvres all over the main mirror.

I wonder if your question is based on frustration at using ATM / Cash dispensers. Their designs seem to be ridiculous and without any acknowledgement of the possible presence of the Sun, behind the user. It's a much worse situation than in a car, where the user is in a more or less fixed position and the main source of interfering light tends to be from the windscreen, overhead. Car displays are pretty good on the whole - as long as you dust them off regularly.

@Tom.G suggests some sort of user-adjustable screen. That could be good but it wouldn't be vandal-proof. However, a motorised shutter inside the display could sense where the Sun is coming from and make some sort of adjustment. You can see my problem with not having the full story, here.
 
  • #7
sophiecentaur
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@nikosb Do you know about Pepper's Ghost? This is a way of putting an apparent image into a scene. That is a system that's ben used for a couple of hundred years right up till today. The link shows a stage illusion but it has all sorts of other applications. Semi-silvered mirrors are obtainable afaik.
 
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