Optics to act as a mirror or transparent medium depending on incident angle

[nikosb]

I am looking for an optical piece that can act as a mirror when the angle of incidence is close to normal (90 degrees) but acts as a see-through glass when the angle of incidence is less than 60 degrees?

Alternatively it can be a filter placed in front of a mirror that passes a lot of light at an angle of incidence close to normal but blocks light at an angle of incidence less than 60 degrees.

Is there such a thing?

 

[BPHH85]

Hi

I'm note 100% sure, but I don't think that there is something that can be a perfect mirror or perfectly transmitting depending of the AOI. In general this also depends on the wavelength of the incident light. For dielectric mirrors, they are coated with a coating designed for a special laser line or for a given bandwidth and for a defined AOI. Mostly for 0° or 45° AOI. But if you use such a mirror with for example R = 99,9% and designed for 0° AOI at a AOI of 45° and above, a certain amount of light won't be reflected and will be transmitted instead because the design condictions won't match anymore.

Maybe it might be good to know what you want to do?

 

[sophiecentaur]

If enough layers are used in an interference filter, it can do exactly what you specify - in principle and for a narrow wavelength range. For use over the whole visible spectrum, problems could arise for angles near the transition between pass and stop angles. You could expect to get strong colouration effects here because the the red end could begin at a different angle from the blue end.

There is an alternative approach which could use the Critical Angle for Total Internal Reflection. (TIR) That could work but the 'mirror' would need to be at the appropriate angle. TIR works 'off axis' so you would get total transmission for normal incidence and total reflection beyond the critical angle. That's around 40 degrees but depends on the actual substance being used so you would need to be 'inventive' in your choice of actual angles ( the mirror would need to be tilted or would need to be in a box of some depth.

Yet another approach could involve the mirror being in a 'tunnel' with an aperture which could limit the angle range of the incoming light. Not a 'flat mirror' but it could do the job. Play with a small mirror and a cardboard box to see what I mean.

 

[nikosb]

TIR and prisms were my first thought as well but I wanted to ask on this forum in case there is another technology that I had not thought of.

The application involves the entire visible spectrum not just the a specific wavelength.

 

[sophiecentaur]

TIR and prisms were my first thought as well but I wanted to ask on this forum in case there is another technology that I had not thought of.

The application involves the entire visible spectrum not just the a specific wavelength.

It would depend on the sort of image quality you're after. You could get away with a 'fresnel' technique with a lenticular mirror plus TIR if the viewer is going to be some distance away. Not easy to DIY that sort of manufacturing, I would imagine.
It's always worth kicking these things around. Sometimes (rarely, I must admit) a fresh view on a problem will hit pay dirt but it will more often bring up snags and save you significant time on line alleys.

As usual, it would be nice for PF if you keep us up to date with successes and failures. Third party, virtual. problems are good fun for some of us PF oldies.

 

[tech99]

I am looking for an optical piece that can act as a mirror when the angle of incidence is close to normal (90 degrees) but acts as a see-through glass when the angle of incidence is less than 60 degrees?

Presumably you take a sheet of glass and silver the central area.

 

[sophiecentaur]

Presumably you take a sheet of glass and silver the central area.

Haha. Or just a small mirror? I don't think that's what the OP had in mind but it's what an illusionist would do.

 

[LURCH]

TIR would seem to be the answer, but would this effect need to work on a stationary observer, while the optics move, or a moving observer while the lense holds still? Also, will this be viewed in person, or through a camera or aperture?

 

[tech99]

TIR would seem to be the answer, but would this effect need to work on a stationary observer, while the optics move, or a moving observer while the lense holds still? Also, will this be viewed in person, or through a camera or aperture?

If I am in a glass tank of water, as a goldfish, I can see out through a "porthole"; and each side of the hole the side of the tank behaves as a shiny mirror due to TIR. But the OP is asking for the opposite effect, which I cannot create without using meta materials to produce negative refractive index.

 

[sophiecentaur]

If I am in a glass tank of water, as a goldfish, I can see out through a "porthole"; and each side of the hole the side of the tank behaves as a shiny mirror due to TIR. But the OP is asking for the opposite effect, which I cannot create without using meta materials to produce negative refractive index.

Yes I did think of that and it need not be an issue if the 'mirror' needs to have no 'depth' to it or, as I suggested above, it has a lenticular / fresnel structure with stripes. A supplementary mirror strip would be needed to avoid the goldfish bowl effect, I think. I didn't;t draw it out, though and there could be a lateral inversion problem if the stripes are too wide.
We would need to know the exact requirement for a good solution.

 

[Klystron]

I am looking for an optical piece that can act as a mirror when the angle of incidence is close to normal (90 degrees) but acts as a see-through glass when the angle of incidence is less than 60 degrees?

Alternatively it can be a filter placed in front of a mirror that passes a lot of light at an angle of incidence close to normal but blocks light at an angle of incidence less than 60 degrees.

Is there such a thing?

OP requests a lense or filter as for photography. Could this be solved by electronics assuming the light is collected and converted? Incident angle could be determined from comparing inputs with a 2nd sensor then attenuation applied automatically.

[Edit: Add term "Assume" as first word. Thanks, Sophie.]

 

[sophiecentaur]

OP requests a lense or filter as for photography.

Can we be sure of that? We need the OP to confirm that or give a fuller explanation of the problem, I think.

 

[rrogers]

I'm thinking of the holograms on credit cards; where you see different pictures depending upon angle. Take two pictures
1) One a mirror
2) A black plate.
and place them in place of the "pictures".
I realize that this isn't quite what was asked but perhaps some variation? My knowledge of designing custom holograms is nil.