Yes, I was thinking single beam Denisyuk. Those might be really silly questions, but your answers are on point. Thanks much.
Now with the same geometry, imagine I illuminate an object and it casts a shadow. How could I create a hologram so that the shadow follows the direction of the reconstructing light... shining the light bottom left, the shadow extends up right and shining the light bottom right, the shadow extends up left. Would this just be a matter of taking multiple exposures, changing the incident beam direction each time?
vocabulary
Re: vocabulary
Not sure what you mean here. Are you bottom referencing from one side or the other? Also, what same geometry?Brian wrote: Now with the same geometry, imagine I illuminate an object and it casts a shadow. How could I create a hologram so that the shadow follows the direction of the reconstructing light... shining the light bottom left, the shadow extends up right and shining the light bottom right, the shadow extends up left. Would this just be a matter of taking multiple exposures, changing the incident beam direction each time?
Generally, in a single beam Denisyuk geometry, the shadows take care of themselves. This is because the reference beam and the recon beam are in the same direction, and so whatever shadow the reference beam throws, so will also the recon beam. Of course, you will need to place something in the object space that takes shadows. It used to be popular back in the 80's to place a white screen behind the object, so as to show the shadow of the object on the white screen (usually a glass pane spray coated white). This added to the depth of the image field, always assuming that the coherence of the laser was long enough. So, for example, you place your porcelain cat behind the recording plate, you then place a spray painted white plate behind the cat. For stability you'd probably want to have the entire package - recording plate, cat and white background plate on the same base, so that your package looked like an "L", with the recording plate on the vertical part and the cat/background plate on the horizontal. I guess that would be more of an "U", with the cat between the two verticals of recording plate and background plate. Now, when you illuminate the cat, it throws a shadow on the background plate and you have both cat and shadow.
If you want the shadow to be on the ground as it were, you'd have to place a "background plate" at right angles to the plane of the recording plate and object. So, in the "L" geometry, the horizontal part would be a white glass plate. However, remember the coherence constraints. For an object of height h in a beam inclined at theta to plate normal, the shadow would be h/tan(theta). If the plate-to-object distance is d, then you'd need a coherence length of greater than d + h/tan(theta). If this were not so, the shadow would taper off at the top edge.
Re: vocabulary
Hello Brian , is this what you had in mind, I call it a shadow-gram as the onject is not seen , only black space they occupied .....
http://youtu.be/NBPIftmlqto
http://youtu.be/NBPIftmlqto
Re: vocabulary
Ah yes, my apology for the vague description, must use my vocabulary! 
Dinesh, let's go with the Denisyuk single beam and your geometry description with holoplate making one side of the U and white boards for the other side and short floor space with the porcelain cat sitting on the floor. Then we'll set the reference beam coming from the top right direction so that it is angled down and left to create a shadow on the left side of the cat. Then the white light to reconstruct the image would also have to come from the top right direction and the shadow in the image would be seen on the left.
Now overall what I would like to see is, when the white light reconstructing the image is panned from top right to top left, the shadow moves behind the cat from left to right. I'm thinking maybe this could be done by making multiple exposures where the reference beam is redirected for each exposure. So for the final exposure, for example, the reference beam comes from the top left and is directed down and right to create a shadow on the right side of the cat.
Holomaker thanks for the response. Not what I had in mind, but that is a very cool, fun display. Is this a two beam hologram where the object beam is lighting the object from behind?
Dinesh, let's go with the Denisyuk single beam and your geometry description with holoplate making one side of the U and white boards for the other side and short floor space with the porcelain cat sitting on the floor. Then we'll set the reference beam coming from the top right direction so that it is angled down and left to create a shadow on the left side of the cat. Then the white light to reconstruct the image would also have to come from the top right direction and the shadow in the image would be seen on the left.
Now overall what I would like to see is, when the white light reconstructing the image is panned from top right to top left, the shadow moves behind the cat from left to right. I'm thinking maybe this could be done by making multiple exposures where the reference beam is redirected for each exposure. So for the final exposure, for example, the reference beam comes from the top left and is directed down and right to create a shadow on the right side of the cat.
Holomaker thanks for the response. Not what I had in mind, but that is a very cool, fun display. Is this a two beam hologram where the object beam is lighting the object from behind?
Re: vocabulary
Well, this is interesting!
You can't use multiple exposures as you suggest because you'll get massive cross talk for the multiple shots you need to make this effect happen. If you record a hologram with a given reference, and then reconstruct with a recon that's close to the original ref, you'll get an image that's not too aberrated. We mentioned this before. You can rotate or displace the hologram over a range of small angles and distances and the image may change colour and position, but will still look OK as a display hologram, ie it'll still look like a cat, albeit maybe with a more elongated nose. However, let's record with two references that are fairly close to each other, say at 60 deg and 65 deg, Let's call the first shot (60 deg) A and the other shot B. When you now try to reconstruct with A, you'll get A and a slightly distorted B at the same time. This is because had you just shot A and positioned it - reconstructed it - at angle B, you'll still see a cat, albeit slightly distorted. Similarly vice versa. So, if you shoot both A and B, either recon will show both images. This is known as 'cross talk'. To avoid cross talk, the refs must be fairly well angularly separated, which will not get the smooth effect you're looking for.
After discussions with our Resident Holographics Expert Advisor (RHEA, not to be confused with a goddess of the same name), namely wife Joy, we may have a solution. Deliberate use of aberration. If you shoot a Denisyuk with a very divergent beam, then reconstruct it with a collimated or much less divergent beam, the image "rolls", much as if it were on the surface of a ball. Kaveh (my old mentor) called it the "football effect". However, this football effect is worse as a function of the depth of the image. So, if you now shoot with a highly divergent beam, keep the shadow as far back as your coherence will allow, and recon with a collimated (or even convergent) beam, you may get the effect you're looking for.
[Disclaimer ] I may have got this backwards. You may need to shoot with a collimated (or converging) beam and recon with a divergent beam. Neurons are a bit sluggish on Monday morning!
[Suggestion]This works even better if you reconstruct the pseudo, but the pseudo image is upside down, so you'll have to shoot the original upside down. It may be instructive to take a look at the pseudo anyway.
Single Beam Denisyuk. Image unmoved, but light toggled from left to centre to right. We weren't going for the effect so the recon is not too far from the ref. But, you can still see a slight rotatoion of the shadow.
You can't use multiple exposures as you suggest because you'll get massive cross talk for the multiple shots you need to make this effect happen. If you record a hologram with a given reference, and then reconstruct with a recon that's close to the original ref, you'll get an image that's not too aberrated. We mentioned this before. You can rotate or displace the hologram over a range of small angles and distances and the image may change colour and position, but will still look OK as a display hologram, ie it'll still look like a cat, albeit maybe with a more elongated nose. However, let's record with two references that are fairly close to each other, say at 60 deg and 65 deg, Let's call the first shot (60 deg) A and the other shot B. When you now try to reconstruct with A, you'll get A and a slightly distorted B at the same time. This is because had you just shot A and positioned it - reconstructed it - at angle B, you'll still see a cat, albeit slightly distorted. Similarly vice versa. So, if you shoot both A and B, either recon will show both images. This is known as 'cross talk'. To avoid cross talk, the refs must be fairly well angularly separated, which will not get the smooth effect you're looking for.
After discussions with our Resident Holographics Expert Advisor (RHEA, not to be confused with a goddess of the same name), namely wife Joy, we may have a solution. Deliberate use of aberration. If you shoot a Denisyuk with a very divergent beam, then reconstruct it with a collimated or much less divergent beam, the image "rolls", much as if it were on the surface of a ball. Kaveh (my old mentor) called it the "football effect". However, this football effect is worse as a function of the depth of the image. So, if you now shoot with a highly divergent beam, keep the shadow as far back as your coherence will allow, and recon with a collimated (or even convergent) beam, you may get the effect you're looking for.
[Disclaimer ] I may have got this backwards. You may need to shoot with a collimated (or converging) beam and recon with a divergent beam. Neurons are a bit sluggish on Monday morning!
[Suggestion]This works even better if you reconstruct the pseudo, but the pseudo image is upside down, so you'll have to shoot the original upside down. It may be instructive to take a look at the pseudo anyway.
Single Beam Denisyuk. Image unmoved, but light toggled from left to centre to right. We weren't going for the effect so the recon is not too far from the ref. But, you can still see a slight rotatoion of the shadow.
- devil shift.jpg (48.87 KiB) Viewed 3493 times
Re: vocabulary
Yes, this is what I was afraid of. Either all the shadows blur together from too many reference beams or you'll have the shadow jump in a few discrete steps from too few references. When I can get to that point, I will definitely use your suggestions.Din wrote: To avoid cross talk, the refs must be fairly well angularly separated, which will not get the smooth effect you're looking for.
I have many more questions. But I also have a lot of work to do before June when I can play around in the lab. So I'm going to try limiting myself to one question per week. Here is this week's question...
How does everyone take such nice photos (and movies) of their holograms? In particular my photos show a white spotlight that is some kind of diffuse reflection of the reconstruction beam. It is not so bad viewed in person, but my point and shoot camera sees that reflection as much more intense than the light making up the holo image.
Also I'm wondering where this diffuse reflection is coming from. I keep thinking the glass surface of the plate should exhibit specular reflection. But I know I made a big improvement putting the plate on a glossy black surface instead of matte black surface, so I don't think the diffuse reflection is coming from that.
Re: vocabulary
It is a problem. Capturing a photograph of a holographic image has never been easy. All too often, you see photos of holograms that look flat and fuzzy, and is accompanied by the disclaimer: "This does not do justice to the hologram" (No shinola!) Different people have different techniques. Perhaps others can chime in, Dave Battin (holomaker) is pretty good at this. Putting the holo on a black surface is definitely a good idea, blocking any stray light from any spurious reflection also helps. You may not see all the diffractions from light hitting the holograms having reflected off a metallic object nearby, but the camera may. Another point is the brightness and cleanliness of the hologram. If the hologram is noisy, this manifests itself as a halo around the image. The camera then captures a fuzzy image due to the noise surrounding the holographic image, especially if the hologram is weak. Another factor is the angle of the ref/recon.Brian wrote:How does everyone take such nice photos (and movies) of their holograms? In particular my photos show a white spotlight that is some kind of diffuse reflection of the reconstruction beam. It is not so bad viewed in person, but my point and shoot camera sees that reflection as much more intense than the light making up the holo image.
Difficult to say without actually seeing the reflection. It could be scattering from the emulsion or scattering from dirt on the glass surface. Diffraction effects from very low frequency grating structures may be a possibility too. The books always tell us that when you record a Denisyuk hologram, you get a set of Bragg planes. While this is mostly true, it's not completely true. The moment you dunk the hologram into a liquid, you distort the surface. This distortion of the surface has low Fourier components, which act as a low spatial frequency grating. The grating frequency is too low to cause dispersion, but, without significant dispersion, the diffraction field could look like Lambertian scaterring.Brian wrote:Also I'm wondering where this diffuse reflection is coming from. I keep thinking the glass surface of the plate should exhibit specular reflection. But I know I made a big improvement putting the plate on a glossy black surface instead of matte black surface, so I don't think the diffuse reflection is coming from that.
Re: vocabulary
Yes I'm hoping others will add.
I've tried to photo in complete surrounding dark. I don't think that the hologram is particularly noisy or weak. I used pretty much the same angle of ref,recon beam for all my tests, so that is something to vary. Top surface is clean, good specular reflector (I've seen clear reflections of camera in some photo attempts).
I've tried to photo in complete surrounding dark. I don't think that the hologram is particularly noisy or weak. I used pretty much the same angle of ref,recon beam for all my tests, so that is something to vary. Top surface is clean, good specular reflector (I've seen clear reflections of camera in some photo attempts).
Yep I suspect it is some kind of scattering. If it isn't the top surface and it isn't the bottom surface, the only thing left is the volume in between.Din wrote: It could be scattering from the emulsion [or] the diffraction field could look like Lambertian scaterring.
Re: vocabulary
Tested a new laser making a couple Denisyuk holograms. The plates were tilted vertical, so used half wave plate to rotate the beam polarizations vertical. No woodgraining, yay. Blackened the plate edges with a marker. None of that wavy intensity change across the picture, yay. If only I had not slipped and made a mark on the plate with the indelible ink marker. Next time I'll use tape or a dry erase marker.
So what did I see... well a shiny coin hologram showed good depth, you could read the date at the bottom of the coin that was about an inch away from the plate. With a 532 nm beam, the coin looked green face on, but tilting this way and that you could see it fade to blue and violet. I did a really bad job matching the beam divergence to the LED divergence... feel silly about that, but didn't have much time. The coin is quite elongated when viewed with the LED at an angle about 60 degrees from plate's normal. If you hold the LED so that its central axis is parallel to the plate, the coin is round again. I count this error as a good learning experience.
By accident I put the plate into the recon beam upside down and saw a cool pattern of thin dark horizontal lines. Just mentioning because it was fun, I knew immediately it was upside down.
Practiced photographing holograms with a SLR camera instead of a point and shoot; the results are better, but I've just as much to learn about photography as I do holography. Maybe I'll get to try again end of next week. Will ask a question before then.
So what did I see... well a shiny coin hologram showed good depth, you could read the date at the bottom of the coin that was about an inch away from the plate. With a 532 nm beam, the coin looked green face on, but tilting this way and that you could see it fade to blue and violet. I did a really bad job matching the beam divergence to the LED divergence... feel silly about that, but didn't have much time. The coin is quite elongated when viewed with the LED at an angle about 60 degrees from plate's normal. If you hold the LED so that its central axis is parallel to the plate, the coin is round again. I count this error as a good learning experience.
By accident I put the plate into the recon beam upside down and saw a cool pattern of thin dark horizontal lines. Just mentioning because it was fun, I knew immediately it was upside down.
Practiced photographing holograms with a SLR camera instead of a point and shoot; the results are better, but I've just as much to learn about photography as I do holography. Maybe I'll get to try again end of next week. Will ask a question before then.
Re: vocabulary
I think the only two holograms I ever saw before recently was a couple of transmission holograms illuminated with HeNe laser light. One showed a toy car and the other had a magnifying glass in front of a phone dial... I can't remember if the numbers it was magnifying were on a rotary dial or touch tone... but it gives you some idea how long ago that was.
Anyways, I was thinking about the magnifying glass and using it to increase the size of small things in a reflection hologram. What unintended consequences may result from trying to use a magnifying glass in this way? Any peculiar HOE effects, like with a mirror? And if the magnifying glass has a piece of dust on it, will the dust show up as concentric rings in the hologram image?
Anyways, I was thinking about the magnifying glass and using it to increase the size of small things in a reflection hologram. What unintended consequences may result from trying to use a magnifying glass in this way? Any peculiar HOE effects, like with a mirror? And if the magnifying glass has a piece of dust on it, will the dust show up as concentric rings in the hologram image?