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Old Jun 19th 2016, 09:55 PM   #1
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distance based transparency

Hi,

Is there any optical sheet or film which is transparent to object only if the object is touching or extremely near to the sheet otherwise it acts as opaque sheet?

for example, If we look through a transparent glass window of an apartment, from a distance in day light, we canít see what or who is inside. All the light is reflected off the surface of the transparent glass giving us an illusion that itís reflecting and opaque. But the moment we start going near the window we start to see who or what is inside.
I was wondering whether there is any film or surface which will behave like opaque surface if the object is away from it. But the moment that object is brought extremely near to that surface or say it touches the surface, it will start becoming visible.
For example in the image below the object is say 0.5 mm away from the film the film behaves opaque. If the object is 0.25 mm near the surface we are able to see the object but not fully, but if the object surface is touching the film, we are able to see the object fully and the film behaves fully transparent.
Will a diffuser film behave this way? I have noticed that if we keep our hand touching the diffuser film then we are able to see our hand but moment we take our hand away it fades and we are only able to see the white diffusing surface.
I believe diffuser film is made of arrays of very small lenses which diffuse light. so can the focal length of those lenses be adjusted such that only objects which are very near to the film or touching the film are only visible, and the visibility reduces greatly with distance of the object from the film.
In short I am wondering whether there is any surface -which can behave VERY PRECISE opaque or transparent based on the distance of the object from the surface of the film.
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distance based transparency-opaque-transparent-distance.jpg  
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Old Jun 20th 2016, 05:25 AM   #2
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But please look at this video below. its of fiber optic plate or compressed fiber lens

here it seems that the object- which is the book or letters here, is visible only if the plate is touching it. other wise the lens looks opaque.





even the picture of the same fiber optic plate in hands behaves similarly- at-least while looking at its picture. here the finger print or the ridges of the finger print are visible as though the fiber optic plate is transparent. the rest of the finger is almost not visible.

http://hpi.de/fileadmin/user_upload/..._faceplate.jpg


Need help in understanding physics behind this fiber optic face plate and will it actually behave the way its visible in video and pic..
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Old Jun 20th 2016, 01:04 PM   #3
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I think that what is happening is that light from other angles is being deflected , by the bumps on the diffuser film, into the path of the light from the object.
If the distance between the object and the film is small, then there is no room for light from other angles to get in and disrupt the view.

Note that almost the same argument works for the reflections.
As you get closer to the glass, the ratio of transmitted light to reflected light improves so that you can see the view inside, rather than just the reflection.
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Old Jun 21st 2016, 04:12 AM   #4
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thanks you sir..
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