Interesting colors

kaveh · Post by **kaveh** » Tue May 18, 2004 10:58 am

Again, let's make things simple to begin with and forget white light. Let's work just with a laser of one color.

To get a clean holographic image, as we all expect, you need a specular (i.e. non-diffuse) reference beam. In other words your light must emanate from a single point. (In the case of a laser this is a point at infinity.) In reconstruction, you also use a point source, and you get a sharp image.

The way to look at the present problem is to regard one or the other of the squares as a set of reference beams, and the other as the object. So you have an infinite number of (weak) reference beams, making lots of recordings of your object. When you reconstruct with a point source laser beam, all component holograms are reconstructed, each slightly offset from the others. The result is a brightish square, with a squarish halo around it. So a very noisy reconstruction.

In the case of reflection, I think a similar result is seen.

When you use white light to see the image, you will see the same kind of image, but dispersed, as before. So you will get a spectrum of light, but as the image is more blurred and spread, the spectrum will not be as saturated in color.

I think this is what you will get. If you get time, very interesting experiment to try.

JohnFP · Post by **JohnFP** » Wed May 19, 2004 10:14 am

So if I keep the mirror on the back and put mirros on the sides I should not only see the rainbow on the bottom and on the back wall but also on the sides. The side walls should be the same as what is on the back wall, that is, going from red at the bottom to blue at the top. But as opposed to going all the way up to the top like the back wall, the side rainbows should just go up a short way, basically just the distance up as the reflected white card is wide at the viewing angle. But above that rainbow should be another rainbow which is the reflection of the far mirror which has the reflection of the white card in it and so on.

This may turn out to be a very, very interesting hologram. I may try it with the bottle and without. Guess I will be stopping by the State surplus house and picking up a old microfiche machine. They have nice large first surface mirrors in them.

JohnFP · Post by **JohnFP** » Wed May 19, 2004 10:38 am

Do you think I will see this with mirrors on the sides?

kaveh · Post by **kaveh** » Wed May 19, 2004 10:41 am

OK. It's a bit difficult to visualize, but here is how to work out what you'll see.

Let's assume you have enough coherence length to record everything in the scene, including reflections. Remember each reflection is adding to the path of the object beam.

Illuminate the scene and look at it from the direction of the recording film. Everything you see will be dispersed into the spectral colors, as you have seen. The angular dispersion will be the same for all points, so the further away the image point from the recording plate, the more dispersion you'll see. Here is a diagram to illustrate it:

The bottle is in the plane of the recording plate so it will be completely sharp.

So if the bits of light you see on the side are nearer to the plate, they will look less dispersed.

JohnFP · Post by **JohnFP** » Wed May 19, 2004 11:01 am

Dang, I forgot the reflections of the side mirrors in the back mirror.

Colin Kaminski · Post by **Colin Kaminski** » Wed May 19, 2004 11:13 am

"Now, Greek salad...!"

I have a few pages of greek salad from PCG you kindly wrote for me!

JohnFP · Post by **JohnFP** » Wed May 19, 2004 11:24 am

Crude but I am just trying to visualize.

kaveh · Post by **kaveh** » Wed May 19, 2004 1:07 pm

Are the reflections on the side multiple reflections from the mirrors? I am not sure what is happening with the mirrors. If you can take a photo of the setup some time, it will help.

Two comments:

If these are multiple reflections, then the colors on the side will be effectively further and further away from the hologram plate, so the dispersion will be more. You have the red and blue getting closer as you go up.

Also, you have the dispersion angled on the side. All dispersion will be in the direction of the diffraction, so as you look at the hologram, red will always be below the blue, not angled, as you have shown.

kaveh · Post by **kaveh** » Wed May 19, 2004 1:09 pm

Again, main thing is that all spectra are up and down, and they therefore blend and merge into one another. Also note that the amount of dispersion will be proportional to the apparent distance of the object from the plate, taking all multiple reflections into account.

JohnFP · Post by **JohnFP** » Thu May 20, 2004 6:37 am

Yes, in my crude drawing the reflections on the sides and now the back for that matter are reflections of the card in multiple mirrors. I do not have the side mirrors yet but hopefully by the end of this weekend I will and should have the holo done.

Kaveh, I think you hit the nail on the head. I do believe that when I incorporate the side mirrors in the set-up the effect will be quite dazzling. Each and every card that is reflected in the mirror and the multiple reflections of the card will be a rainbow with red at the bottom and blue at the top. I am very excited about producing this hologram.

BTW, the back mirror just happens to be very close to 45 degrees. I didn't plan it that way it just worked out that way. I am going to keep DCG concentrations and processing the same because I really want to run with this effect. I am almost afraid to move to 5 x 7 as I will be in experimentation mode again. It would be nice if I could get two sessions in. One on more 4 x 5's with side mirrors then another session with 5 x 7's.