by Dinesh » Sun Jun 26, 2011 8:09 am
Martin wrote:Well, from what I read, you can have it both ways: there are edge lit reflection and edge lit transmission holograms. But they rely on different recording geometrics.
Actually, Martin, that's part of the problem. What exactly does "edge lit" mean. True edge lit holograms would require that the reference is an evanescent wave. However, such a reference is neither reflection nor transmission. People have used very (very!) steep angles in both geometries and called it "edge lit" but then one requires large prisms or blocks of glass. I reasoned that if it could be possible to generate an evanescent wave along the surface of the emulsion, then blocks of glass or large prisms would not be necessary. But, it's very difficult to produce an evanescent wave, you have to hit the back end of the emulsion at ju-u-st the total internal reflection angle. So, I reasoned that if the emulsion were thicker than would be found in commercial silver, I'd have a better chance.
Martin wrote:On the other hand, isn't it that processing DCG above 30µm or so becomes increasingly difficult too? So maybe the ultimate recording medium for this might be photopolymers (there are a couple of papers by Phillips where he described the use of Omindex films for edge lit applications).
I've thought of polymers for edge lit. I think there would be a great advantage, if the polymer were uniform throughout the thickness. This is what I meant when I said that dcg was a continuous medium; the index has to be (reasonably) continuous in order to produce evanescent waves. I think that the granularity of silver may prevent evanescent waves inside a silver emulsion.
favalora wrote:That's cool, Dinesh. I just Tweeted about it (for all of the, like, 9 people who follow me who aren't spambots). Fame! Fame for you!
You, too, can follow me, at @gfavalora .
Thanks, Favalora! As for tweeting, I'm afraid I have no idea how to do that! I'm afraid I'm the Last of the Luddites. My sister-in-law saw me whip an egg for an omelet with a fork and informed me that there was now a machine for whipping eggs. "Why ?", I asked. Actually, I just discovered texting. It's actually quite interesting, you don't have to indulge in unnecessary conversation. It took me about 3 minutes to text, "I'm at the pub" (the apostrophe threw me, I couldn't figure out how to generate an apostrophe). Now if I could generate equations on the keypad and text those.....
Jeffrey Weil wrote:It's a black and white transmission hologram developed by Steven Benton. It's related to a rainbow holograms, sometimes also known as a Benton type hologram.
Well, yes and no. It's one of the problems of holography that there's a surfeit of names and they all get in each others way. The so-called "achromat angle" comes because Benton used the association of the lens equation with Fresnel diffraction. This is a standard method of analysing Fresnel diffraction, but Benton applied it to the slits from an H1. Using this association Benton calculated that the slits are focused at different positions and angles relative to the H1, but they fall along a straight but slanted line. If the H1(s) is similarly slanted with three slits at rgb, then all the slits would superimpose and you'd get true colour.
However, the word "achromat" has also been applied to a hologram made with open aperture in transmission mode. In this mode, the H1 pseudoscopic image is focused onto the H2 plate (as normal), but the H1 is not slitted. If the image is focused at exactly the image plane, there is no dispersion because dispersion is a function of the image point's distance from the image plane. If the image is right at the plane, dispersion is zero. In practice, you can have an image that's about an inch in front and about an inch and a half behind and still get almost no dispersion. You can make it deeper using dispersion compensation, but you have to ask Kaveh abut that!
[quote="Martin"]Well, from what I read, you can have it both ways: there are edge lit reflection and edge lit transmission holograms. But they rely on different recording geometrics.[/quote]
Actually, Martin, that's part of the problem. What exactly does "edge lit" mean. True edge lit holograms would require that the reference is an evanescent wave. However, such a reference is neither reflection nor transmission. People have used very (very!) steep angles in both geometries and called it "edge lit" but then one requires large prisms or blocks of glass. I reasoned that if it could be possible to generate an evanescent wave along the surface of the emulsion, then blocks of glass or large prisms would not be necessary. But, it's very difficult to produce an evanescent wave, you have to hit the back end of the emulsion at ju-u-st the total internal reflection angle. So, I reasoned that if the emulsion were thicker than would be found in commercial silver, I'd have a better chance.
[quote="Martin"]On the other hand, isn't it that processing DCG above 30µm or so becomes increasingly difficult too? So maybe the ultimate recording medium for this might be photopolymers (there are a couple of papers by Phillips where he described the use of Omindex films for edge lit applications).[/quote]
I've thought of polymers for edge lit. I think there would be a great advantage, if the polymer were uniform throughout the thickness. This is what I meant when I said that dcg was a continuous medium; the index has to be (reasonably) continuous in order to produce evanescent waves. I think that the granularity of silver may prevent evanescent waves inside a silver emulsion.
[quote="favalora"]That's cool, Dinesh. I just Tweeted about it (for all of the, like, 9 people who follow me who aren't spambots). Fame! Fame for you!
You, too, can follow me, at @gfavalora .[/quote]
Thanks, Favalora! As for tweeting, I'm afraid I have no idea how to do that! I'm afraid I'm the Last of the Luddites. My sister-in-law saw me whip an egg for an omelet with a fork and informed me that there was now a machine for whipping eggs. "Why ?", I asked. Actually, I just discovered texting. It's actually quite interesting, you don't have to indulge in unnecessary conversation. It took me about 3 minutes to text, "I'm at the pub" (the apostrophe threw me, I couldn't figure out how to generate an apostrophe). Now if I could generate equations on the keypad and text those.....
[quote="Jeffrey Weil"]It's a black and white transmission hologram developed by Steven Benton. It's related to a rainbow holograms, sometimes also known as a Benton type hologram. [/quote]
Well, yes and no. It's one of the problems of holography that there's a surfeit of names and they all get in each others way. The so-called "achromat angle" comes because Benton used the association of the lens equation with Fresnel diffraction. This is a standard method of analysing Fresnel diffraction, but Benton applied it to the slits from an H1. Using this association Benton calculated that the slits are focused at different positions and angles relative to the H1, but they fall along a straight but slanted line. If the H1(s) is similarly slanted with three slits at rgb, then all the slits would superimpose and you'd get true colour.
However, the word "achromat" has also been applied to a hologram made with open aperture in transmission mode. In this mode, the H1 pseudoscopic image is focused onto the H2 plate (as normal), but the H1 is not slitted. If the image is focused at exactly the image plane, there is no dispersion because dispersion is a function of the image point's distance from the image plane. If the image is right at the plane, dispersion is zero. In practice, you can have an image that's about an inch in front and about an inch and a half behind and still get almost no dispersion. You can make it deeper using dispersion compensation, but you have to ask Kaveh abut that!