by Dinesh » Thu Jan 17, 2013 3:26 pm
Joe Farina wrote:By the way Dinesh, if I remember correctly, you worked at Physical Optics Corporation? If so, I would like to ask a question. Today I saw a paper called "Constructive use of high order harmonics in holographic Lippmann mirrors" by Chris Rich and George J. Vendura, Jr. (both of Physical Optics Corporation of Torrance, CA). This was in SPIE 1212, Practical Holography IV (1990). They used DCG as the recording material.
I wanted to ask, if you are familiar with this paper, was there any thing useful which could be applied to reflection display holograms in DCG? I'm not able to understand it. If you don't have (or remember) this paper, I will be happy to scan and send it to you.
Joe, I'm not familiar with the specific paper, but I am familiar with the concept. I'm still in touch with Chris (he's now at Wavefront, just down the road from me), but I have no idea who George J. Vendura, Jr is (sounds like a character in a mystery play!). However, I believe there is an interesting back story and not a little irony in this paper.
I've talked this before in regards to that efficiency curve you showed for eta in dcg. You remember that efficiency curve you showed from a paper from the University of Arizona group? Remember that it showed a rise, then a fall, then a rise again? Well, the reason for the fall and then the rise is due to harmonics from the modulation profile. Ideal recording would produce a perfect sinusoidal modulation profile, but no material can track the eact values of a sinusoid, so the real profile of a real emulsion will have harmonics - Fourier components due to the deviation of the modulation profile from a strict sinusoid. You can also create harmonics by deliberately developing the emulsion to create some specific modulation profile which will generate specific harmonics.
The irony and back story to this is that sometime in 1987, we (POC) was asked to create a reflective filter at around UV wavelengths in the 200nm range simultaneously with a reflective filter in the vis range. At that time, we had no UV lasers, so we could not do the UV part of this. However, at that time, it occurred to me that there were two ways of doing this: pseudo-colour and harmonic manipulation. In the pseudo-colour technique, you shrink the emulsion to generate a blue-shifted image, but no one had achieved pseudo-colour in dcg. So, I gave it a try, developed a few methods and succesfully created a UV grating by shooting at 488 and shrinking it. Then I shot a plate at 488 and then shrunk in to create two gratings: one at 488nm and one at about 350nm. It then occurred to me that pseudo-colour would never get me below 350nm odd, so perhaps I could get lower using a harmonic technique. I then worked out some techniques to develop the plate to a given modulaton profile to generate a filter at 244 (half of 488). Well, cut a long story short, the methods I came up worked and I got a grating at both 488 and 244. Now this was all done in between working on real POC projects (both Chris and I would often chat about unconventional methods to create unconventional holograms and then try these out when we were in "downtime"). Anyway, I showed what I'd done to Joanna and she got mad at me and told me not to waste my time on all this and throw all the plates out. About 6 months later, she asked me if I still had those plates at 244 and I mentioned that she told me to throw them all out. By 1990, I'd left POC so it's interesting that they then wrote a paper about exploiting the use of harmonics!
Is it useful for display? I don't know offhand. I suppose if you had a need for display UV holograms it might be. Then again, you could exploit the different efficiencies to creat an image where a difference in efficiencies might be beneficial, for example a bright foreground with a (controlled) dimmer background. If you overmodulated the lighting for the background, you'd end up with a more square-wave modulation profile for the "background" fringes. I must admit, off the top of my head I can't think of any particular way of exploiting harmonics.
Joe Farina wrote:Birds would be less susceptible to the Whittaker-Shannon theorem since they resolve more visual events per second, compared to humans*
*At least I assume so, since this is the first time I've heard of the Whittaker-Shannon theorem.
The Whittaker-Shannon theorem says that if you have a band limited function, so that given a function g(x), then it's Fourier transform G(f) = 0 for all f > f(c), you can scan that function by a series of sharp lines - Dirac delta functions. This will give you a set of numbers at specific positions on the curve, which defines a new function. If you take the FT of the new function (which is basically just a set of numbers), you get a series of sinc functions surrounding the points at which you scanned. So long as you the scan frequency is twice the highest frequency in the original function, ie f(c), and the function is band limited, you can recover the original function exactly. If the function is not band limited, you get what's called "aliasing". What I found interesting is that the ear is capable of Fourier transforming - a good ear can make out the harmonics in a tune - but the eye cannot Fourier transform - your brain cannot decompose a given colour into primaries. But, if the colour signal that enters your retina is band limited, you should be able to scan the colour signal and isolate it's harmonics. Why can't the eye do that? Perhaps because the retina integrates, but the cochlea is composed of tiny hairs that move under the influence of sound, but those tiny hairs have lengths that are harmonics.
[quote="Joe Farina"]By the way Dinesh, if I remember correctly, you worked at Physical Optics Corporation? If so, I would like to ask a question. Today I saw a paper called "Constructive use of high order harmonics in holographic Lippmann mirrors" by Chris Rich and George J. Vendura, Jr. (both of Physical Optics Corporation of Torrance, CA). This was in SPIE 1212, Practical Holography IV (1990). They used DCG as the recording material.
I wanted to ask, if you are familiar with this paper, was there any thing useful which could be applied to reflection display holograms in DCG? I'm not able to understand it. If you don't have (or remember) this paper, I will be happy to scan and send it to you.[/quote]
Joe, I'm not familiar with the specific paper, but I am familiar with the concept. I'm still in touch with Chris (he's now at Wavefront, just down the road from me), but I have no idea who George J. Vendura, Jr is (sounds like a character in a mystery play!). However, I believe there is an interesting back story and not a little irony in this paper.
I've talked this before in regards to that efficiency curve you showed for eta in dcg. You remember that efficiency curve you showed from a paper from the University of Arizona group? Remember that it showed a rise, then a fall, then a rise again? Well, the reason for the fall and then the rise is due to harmonics from the modulation profile. Ideal recording would produce a perfect sinusoidal modulation profile, but no material can track the eact values of a sinusoid, so the real profile of a real emulsion will have harmonics - Fourier components due to the deviation of the modulation profile from a strict sinusoid. You can also create harmonics by deliberately developing the emulsion to create some specific modulation profile which will generate specific harmonics.
The irony and back story to this is that sometime in 1987, we (POC) was asked to create a reflective filter at around UV wavelengths in the 200nm range simultaneously with a reflective filter in the vis range. At that time, we had no UV lasers, so we could not do the UV part of this. However, at that time, it occurred to me that there were two ways of doing this: pseudo-colour and harmonic manipulation. In the pseudo-colour technique, you shrink the emulsion to generate a blue-shifted image, but no one had achieved pseudo-colour in dcg. So, I gave it a try, developed a few methods and succesfully created a UV grating by shooting at 488 and shrinking it. Then I shot a plate at 488 and then shrunk in to create two gratings: one at 488nm and one at about 350nm. It then occurred to me that pseudo-colour would never get me below 350nm odd, so perhaps I could get lower using a harmonic technique. I then worked out some techniques to develop the plate to a given modulaton profile to generate a filter at 244 (half of 488). Well, cut a long story short, the methods I came up worked and I got a grating at both 488 and 244. Now this was all done in between working on real POC projects (both Chris and I would often chat about unconventional methods to create unconventional holograms and then try these out when we were in "downtime"). Anyway, I showed what I'd done to Joanna and she got mad at me and told me not to waste my time on all this and throw all the plates out. About 6 months later, she asked me if I still had those plates at 244 and I mentioned that she told me to throw them all out. By 1990, I'd left POC so it's interesting that they then wrote a paper about exploiting the use of harmonics!
Is it useful for display? I don't know offhand. I suppose if you had a need for display UV holograms it might be. Then again, you could exploit the different efficiencies to creat an image where a difference in efficiencies might be beneficial, for example a bright foreground with a (controlled) dimmer background. If you overmodulated the lighting for the background, you'd end up with a more square-wave modulation profile for the "background" fringes. I must admit, off the top of my head I can't think of any particular way of exploiting harmonics.
[quote="Joe Farina"]Birds would be less susceptible to the Whittaker-Shannon theorem since they resolve more visual events per second, compared to humans*
*At least I assume so, since this is the first time I've heard of the Whittaker-Shannon theorem.[/quote]
The Whittaker-Shannon theorem says that if you have a band limited function, so that given a function g(x), then it's Fourier transform G(f) = 0 for all f > f(c), you can scan that function by a series of sharp lines - Dirac delta functions. This will give you a set of numbers at specific positions on the curve, which defines a new function. If you take the FT of the new function (which is basically just a set of numbers), you get a series of sinc functions surrounding the points at which you scanned. So long as you the scan frequency is twice the highest frequency in the original function, ie f(c), and the function is band limited, you can recover the original function exactly. If the function is not band limited, you get what's called "aliasing". What I found interesting is that the ear is capable of Fourier transforming - a good ear can make out the harmonics in a tune - but the eye cannot Fourier transform - your brain cannot decompose a given colour into primaries. But, if the colour signal that enters your retina is band limited, you should be able to scan the colour signal and isolate it's harmonics. Why can't the eye do that? Perhaps because the retina integrates, but the cochlea is composed of tiny hairs that move under the influence of sound, but those tiny hairs have lengths that are harmonics.