contrast between fringe's

Dichromated Gelatin.
dannybee
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contrast between fringe's

Post by dannybee »

Dinesh ="This is an important parameter. So, as a photographer, judge how much the contrast between fringe and not-fringe must be to get a good photograph. The brightness of the hologram is directly affected by the contrast in the fringes."

This is good teaching dinesh, can you also explain how difrent medum types handle this difrently like silver, dcg , and polymer,like I know dcg need to be very close to 1 to 1 to get good bright results, and polymers have to be very even across the exposer to get good fringes that probly why duponte uses dcg master for its polymers (and can't handle high freqences).. where silver is a little more forgiving in the contrast curve and used as H1 masters:D ..can you explain more in detail?
Dinesh

contrast between fringe's

Post by Dinesh »

This can be a complicated question. Essentially, the photosensitive emulsion, be it dcg or polymer, receives an interference pattern consisting of variations in an electric field (the E field). These variations of the electric field give rise to variations of energy. When this interference pattern hits the photosensitive emulsion, there occur physical changes in the medium caused by the energy field of the interference pattern. The medium might, for instance get darker, or harder, or softer. However, the physical changes in the medium depend on the physical characteristics of the medium and so usually cannot track the changes in the energy field accurately. There is a "translation" between the value of the energy field and the value of the physical changes caused by the field. The corresponding physical change caused by some specific energy value is known as the "gamma". Specifically, if an energy of V(in) causes a physical change in the emulsion of V(out), then gamma is defined as,

gamma = d(log(V(out)))/d(log(V(in))).

Different photosensitive substances have different values of gamma.

In photography, the gamma value is determined by the variation of density in the film for variations of exposure. This gives rise to the Hurter-Drieffield (H-D) curve and you can determine the gamma of a specific film from this curve. Every film has a unique H-D curve, and the steeper the H-D curve, the higher the gamma and the darker the film for a given exposure. Below is a film with varying gamma

In holography, ideally, the interference pattern is a sin squared variation for two point sources. This type of variation rises very fast, slows down, then hovers at the maximum value, then falls, slowly at first then more rapidly. To get perfect reconstruction, the gamma of the emulsion must be able to follow these variations. In practice, no film can follow all these variations exactly, so the more subtle variations are lost. This causes a loss of efficiency and some distortion of the hologram. The other issue is the rate at which the energy field rises and falls. One complete rise and fall is one period of the energy field and the number of periods per mm is known as the "spatial frequency" and denoted as l/mm (some use lp/mm). So, if the emulsion can take 3000 l/mm, that means there are 3000 rises and falls of the energy field every mm or one rise and fall every 1/3000 cm (roughly 0.00001 inches). The ability of an emulsion to take a specific spatial frequency is known as the "frequency response" of the material.

When you record a hologram, one factor that affects the spatial frequency is the geometry of the recoring. The larger the angle between the beams, the larger the spatial frequency and the larger the frquency response necessary in the film. Typically, you generate, for a transmission hologram, about 1700 l/mm and, for a reflection, about 4000 l/mm. But, these are based on one point source directly in front of the film and one at 60 degrees at 530nm. If you're shooting a large object, for example, then the light reaching the film is coming in from a wider set of angles, since you need to capture light from the extreme edges of the object as well as the middle. This means that you need a larger frequency response. If you're right on the edge of the response necessary for a source of light directly in front of the film and the film is not capable of the larger response needed for the edges of the object, you'll lose the edges of the object. The exact amount of edge you lose will depend on the frequency response, and, if that is low, you may get almost nothing outside the centre. Some films have a more limited response such as the resists typically used by embossed mastering facilities (1800 series). This has a response function of 1500 l/mm. which means you cannot have large reference angles. I don't know what the response of the different polymers is (I'm sure their technical handouts will contain that). The Bayer Bayfol HX tech sheet quotes a high frequency cut-off 4525 l/mm.

Another factor is the variation in the physical parameter - the actinic reaction. In some cases, like unbleached silver, the actinic reaction makes dark lines at the exposed areas. In other (most) cases, the actinic reaction causes the exposed areas to become harder physically, ie it increases the density. This increase in density translates into an increase in the refractive index (through the Kramer-Kronig relation). The efficiency - brightness - of the hologram now depends on the difference between the actinised areas and the non-actinised areas. Kogelnik wrote a paper around 1969 analysing the various differences between the efficiencies and the actinic reactions. he concluded that if the actinic reaction consisted of only a darkening of the film, then the max efficiency the film is capable of is about 33%. If the actinic reaction consisted of a change in density, then the efficiency of the hologram depends on the change in index corresponding to the change in density (these holograms are called "pure phase holograms"). The higher the change in density, the greater the efficiency. This variation of density/index is known as "index modulation". The index modulation depends basically on two factors: the beam ratio and the spatial frequency of the source, ie the difference in path lengths between the two beams. You get the greatest efficiency only for zero path length, all other things being equal. If the path length is non-zero, then no matter what the coherence length of the laser, there is a fall-of in the modulation, which gets worse as the path length changes. A path length greater than the coherence length of the laser results in zero modulation. Also, the efficiency varies depending on whether it's a reflection or transmission hologram. In addition, it depends on the spatial frequency response of the material, on the geometry of the exposure and on the orientation of the Bragg planes within the emulsion, ie the twists and turns of the planes. For example, Rallison has quoted the efficiency of dcg holograms as,

eta = )(tanh^2)(pi*delta n)*d))/(lambda*cos(theta))

But this is not true for display holograms, it's only true for holograms with unslanted gratings.
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