research for a talk... beam ratio March 22 2004 at 5:16 PM
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lets say refrence beam is 100mj/2 or a 100 units.... at what point on the object beam at at the plate will you not be able to record fringes 100/25:1/4 or 1/8 or 1/16.... does any one know
im trying to determan the toneal range the can be recorded on a object, it seem holography is a lot like slide film ...the range was one stop over and 2 stops under...... i know we are dealing with interfrence and recording the interfrence...each point on the object creates its own wave front, at what point will the light be to low to interfer..
it is at that number .. whe can come up with a tonel( the number of grays that can be recorded)chart... it is with this information we can learn to work the light & object
I think what you're looking for is the dynamic range. At the top, the interference pattern can record the brightest object reflections up until the point where the object light becomes too strong for the reference beam. As the object light decreases, the interference pattern gets weaker and weaker. Physically, it will continue to exist down to the point where there's just a single photon of object light. Practically, it will be limited by other factors, such as noise in the final hologram. So the question is how clean and noise free can you practically make a hologram?
Oh, and I claim that something about diode lasers limits the noise floor to reduce dynamic range, compared to HeNes. No theories or facts, just empirical observations...
This depends a lot on the medium as well as the dynamic range. In the old, pre-bleach days they went primarily on the H&D curve. They basically treated the problem like an electronic amplification problem wheree the H&D curve was the "amplifier response". they then arbitrarily (well, not too arbitrarily) decided to choose a particular point about halfway on the straight line portion and went up to roughly the top and bottom of the straight line portion, which turned out to be the famous 4:1. This of course relied on the slope of the curve, the gamma, which in those days were determined to be about 2 for ideal holographic film material by Nick Phillips et al. Today with finer materials the gamma is probably higher. I'm sure Sergei can give us the gamma for PFGs.With bleached materials and pure phase materials like DCG and resist, the ratio, k is tied into the index differences, MTFs etc. and is no longer dependant on gamma since the H&D curve is not valid for phase materials.
"With bleached materials and pure phase materials like DCG and resist, the ratio, k is tied into the index differences, MTFs etc. and is no longer dependent on gamma since the H&D curve is not valid for phase materials."
Is that one of the reasons DCG is so bright, because the index difference is 1 to 1.5?
"Is that one of the reasons DCG is so bright, because the index difference is 1 to 1.5?"
Yes. This also is the cause of the high brightness in SHSG.
DCG has the largest modulation range I can think of, making it almost perfect for holography. There two possible mechanism - a "crack" theory and a "bubble" theory - and to the best of my belief, the exact mechanism has still not been settled. One is that the rapid dehydration causes microbubbles to form in the interference lines causing a large index difference between air and gelatin and the other, due to Shankoff who wrote one of the earliest papers on DCG as a holographic material is that cracks appears due to rapid drying stressing the emulsion. Hans seems to favour the "bubble" theory since he talks of "voids" forming in the emulsion. Personally, I like the "crack" theory because DCG is so clean and I think bubbles would create noise.