# Distance based opaque to transparent behaviour

• A
• deval vyas
In summary, a diffuser film made of small lenses can be adjusted to only allow objects that are very close to the film or touching the film to be seen.f

#### deval vyas

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 can’t see what or who is inside. All the light is reflected off the surface of the transparent glass giving us an illusion that it’s 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.

#### Attachments

• opaque-transparent-distance.jpg
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Any transparent plane set in a closed surface will do that ... provided bright light outside and an unlit interior.
The object becomes visible when close by because light from the observer side passes through the surface and scatters back through the surface.
We call these surfaces "windows".

Another approach: a metal sheet may effectively block an image forming in IR unless the object is touching the sheet. There are similar things with other forms of EM radiation penetrating materials.

@deval: I get the feeling that there is specific application associated with your post. When you use the word "opaque" are you actually meaning to not pass any light or are you meaning 'Obscure'?
You could make the surface of the window opaque by using a thin layer (or layers) of suitable dielectric material to suppress transmission (an interference filter technique - the opposite of 'blooming on lenses). If no light is emerging from the outer surface of the window then you could alter this with a coat of some other medium (water, even) which would destroy the cancellation of transmission by the original 'all stop' filter.
Alternatively your windows could be given a lenticular or pimpled outer surface which could produce total internal reflection of light trying to get out yet let light come in. Water would again stop the TIR and light would escape. This is the same principle as the Abbe reflectometer and the rain detectors that are used for automatic car windscreen wipers. However, this would be a useless system if it started raining. Everyone would see inside your apartment.
If you are looking for a 'one way mirror' effect then it is necessary for it to be a lot brighter outside and inside. Looking from outside with your head covered with a hood will let you see what's happening inside because the hood cuts out light coming from your side of the mirror.

Thanks a lot for your feedback Simon and Sophie. My one way mirror analogy was not correct I guess...

@Sophie: By opaque I meant that the surface should scatter light in all the directions so that when I look at that surface all I see is a white surface-Something like a sand blasted glass or translucent coated glass. I should not be able to see what is behind that surface unless I touch it from behind.

Say I have a transparent plastic or acrylic sheet which has two surfaces - A and B. What I am looking for is a manner in which I can make one surface-(A) of that sheet look opaque white (by light scattering) But when I touch or say press opposite surface (B) of that sheet, with any other another object say my hand then the light scattering should not happen at all and I should very clearly-in full fidelity- be able to see my hand from surface A . Again if I take my hand off that opposite surface (B) I should only be able to see white surface (A). this should keep on happening and its not a one time thing. Also, the object touching the opposite surface can't be any fluid or water--it has to be a solid.

I was thinking on lines of very small micro lens array or pimpled surface that could do that by TIR of light. Then if I brought any other object very near that lens it could cancel out the TIR by Frustrated total internal reflection there by allowing the light to pass again. But TIR is very angle dependant. Only light falling at certain angles will be reflected back and not all the light will be reflected back. So I will still get some transparency when viewed from certain angle. But this is my guess...not very sure...here I am looking at 180degrees angle of view.

@simon: can you please let me know if there are any surfaces which will block or scatter visible light unless an object is touching it from behind.

I guess I am looking for a way to make " touch activated opaque (or scattering) to transparent surface..."

I am attaching a picture and video of "fibre optic plate" which "seems to behave" this way..at least in below pic and video on you tube. however these are extremely costly things and I am looking for some thing I can buy from local market or a diy kind of thing...

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 when the plate is touching the letters. Once the lens is lifted, it behaves as though its opaque.

Even the picture of the same fiber optic plate in hands behaves similarly- at-least while looking at its picture.

Here too, only the finger print or the ridges of the finger print that are on physical contact with the plate are visible as though the fiber optic plate is transparent. The rest of the finger is almost not visible.