Hi Guys,When the eye sees an object, there has to be light

[Pierre007080]

Hi Guys,
When the eye sees an object, there has to be light reflecting from or emitting from the object itself. How does this happen with a hologram? Does the interference pattern actually scatter the light so we can see it?

 

[jambaugh]

Look at how your eye works in more detail. Your eye focus on a point on that object by focusing the rays emitted by that point. Imagine those rays passing through a window.

If you close one eye, and paint on that window the color and brightness of the object you see in that direction you form a conventional picture of the scene before you. If you look with the other eye though the image will not have depth because your other eye must turn to point at that part of the window where the first eye sees an image of a point on the object.

Now if instead you make the window a hologram, it reflects coherent light in the same direction as the light which was moving from the original object. Let me elaborate.
Let's let the object be a single point. The light from that point spreads out in all directions and so hits every part of our window. Let's take some drinking straws and glue them together across the face of the window at angles which parallel the motion of the light coming from that point.
something like: (looking down at...)
... A ... [eye]
\ \ \ ... / / / [window of straws]____*____ [image of the point]Now if you look through the window of straws at say a white background you'll only see light coming from the same direction as was coming from that point. You'll see the image of that point behind the window.

You can do this more efficiently using a photographic emulsion and lasers. The interference pattern of the laser light bouncing off the point mixed with the light hitting the window directly to form just the right pattern we need for a diffraction grating which will have the same effect (on the laser light) as the straws did above. They direct the light only in the direction it was going when emitted from the point.

Now do this for every point on a large object and you eyes will see each point with the depth it had originally even though all the light is being bounced off of our holographic "window".

... A ... [eye]
XXXXXXXXX [holographic plate]*%$*#^% [complex image]

Note that since each small piece of the holographic plate must diffract light for every point on the complex object there is some loss of fidelity in the image. Holograms tend to give grainy (noisy) images. But making the holographic emulsion thicker will improve fidelity.

On as side note. Conventional emulsion holograms are typically made using red lasers but they come out looking green because the emulsion shrink during development. This shortens the wavelength of light which is diffracted in the desired way.

On another side note. If instead of an emulsion you use a photoresist on a metal plate and then etch it you get a reflective hologram which is encoded in the surface roughness of that plate. You can then press the hologram onto plastic (e.g. credit cards) or even http://www.nytimes.com/1997/04/02/garden/where-no-candy-has-gone-before-light-as-the-secret-ingredient.html" for mass production.

 

[Andy Resnick]

Hi Guys,
When the eye sees an object, there has to be light reflecting from or emitting from the object itself. How does this happen with a hologram? Does the interference pattern actually scatter the light so we can see it?

Don't forget that your eye is an imaging system- light scattered or emitted by an object is not simply collected by your eye but focused onto your retina (among other things).

A hologram is a diffractive optical element (either through reflection or transmission), so in some ways it's like any other scattering object. One essential difference is that you are not imaging the actual hologram (the diffractive element) but the far-field diffraction pattern made by the hologram.

 

[Pierre007080]

A hologram is a diffractive optical element (either through reflection or transmission), so in some ways it's like any other scattering object. One essential difference is that you are not imaging the actual hologram (the diffractive element) but the far-field diffraction pattern made by the hologram.

Hi Andy, Thanks for the reply. I think I follow what you are saying, but you use the words "element" and "other scattering object". This narrows my question. How can an interference of EM waves diffract or scatter waves from the point of interference? The light we see must be scattered spherically from that point? It is this aspect of the generation of the new "wave front" from a non-existent "element" or point that I can't get my head around.

 

[Pierre007080]

Hi Guys,
I think I have got it now ... lost the plot for a moment.
Thanks.