Right exposure

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Updated: 2005-03-28 by HoloM (the god)

Right exposure

Post by Tony » Mon Jun 07, 2004 8:48 am

I've made Denisyuks as well as H2 reflection copies of transmission masters using the same subject, using Agfa 8E75 (which is similar to PFG-01). The H2's were much brighter than the Denisyuks, but I also notice another interesting difference: the H2's have a much narrower viewing angle than the Denisyuks. My simple single beam Denisyuk is viewable over about 150 degrees. This was due in part to the setup used to make the H2, where a fairly narrow illumination beam was used on the H1.

I think this all boils down to antenna theory, where (neglecting losses for a moment) the gain (brightness) is proportional to the directivity (viewing angle). The reflection hologram is acting like an antenna, directing the incoming energy in different directions. If it has to direct the fixed amount of incoming energy over larger angles, then the energy density at any single angle is lower.

Controlling brightness ratios when making the H2 also helps, by maximizing the use of the available range of density changes (gamma) of the film in a way that can't be achieved in a simple Denisyuk, without going to more complex setups that utilize side-beams for additional object illumination.

Michael Harrison

Right exposure

Post by Michael Harrison » Mon Jun 07, 2004 10:15 am

Yes, you are right. I have only modest setup (laser + one mirror). I was doing H2 from transmission master. I had H2 emulsion side away from H1. Is it right?

In general, yes. Most commonly the H2 is flipped for replay so that the emulsion is away from the viewer and protected. If you want to protect the emulsion in another way, there's no reason you couldn't make the H2 with the emulsion facing the H1. There's no real magic involved with the direction the emulsion faces.

Are PFG-03M better for copies, than PFG-01? I've read, that thay are better for reflections holograms.

In theory they're better because PFG-03 is a finer grain emulsion. In practice, I've never had good results with PFG-03 but I seem to be in the minority in this regard. I find PFG-01 perfectly acceptable for both transmission and reflection holograms.

If someone says it can't be done but they haven't tried it, don't believe them.

Are you going to the next PCG Gathering?


Right exposure

Post by Tommy » Mon Jun 07, 2004 2:18 pm

A while ago, I tried overbleaching, to see how significant of a variable it was.

So, I made two single beam refelection holograms, developed for the same time, then bleached one for clear + 30 seconds (total time of about a minute and a half), and the other for about 5 minutes.

The two holograms were both about the same brightness. Neither were bright though, so perhaps the bleaching variable was swamped by something else I was doing.

This was either PFG-01 in JD-2 chemistry, or Agfa 10E75 in D19 and potassium dichromate and sulfuric acid bleach. I don't have my notes here...


Right exposure

Post by Dinesh » Mon Jun 07, 2004 3:49 pm

There are many variables to brightness. There are also many variables to reconstruction. I mention this because comparing your hologram to a picture of a hologram does not give you a real comparison unless both holograms were made in roughly the same way and lit roughly the same way. It's fairly easy to make a dim hologram appear very bright by illuminating it with a very bright halogen in a completely darkened room. With no other visual clues to judge against, the eye is completely swamped by the holographic image. And then of course there's Photoshop, but we're all too honest not to admit that we've digitally enhanced the picture, right? Right?
The first thing to consider is that you're taking a photograph of a set of very, very tiny lines. For a typical single-beam Denisyuk, these lines are 0.3 microns apart (a human hair, by comparison, is about 50 microns wide). These lines are also not stark dark lines seperated by stark white lines, the darkness varies sinusoidally. This means that the lines gradually become darker and then reach a peak darkness, hold that peak for a very short distance (about approx 0.03 microns) and then lighten to white gradually. having taken such a picture, you need to develop the picture to conform as closely as possible to this ideal, recorded pattern. Too much development and the gradual rise and fall of the lines is lost (this is similar to an overdriven amplifier), too little and there is not enough contrast between the dark peak and the lighter surrounding. Now you bleach. Where there was a dark, the bleach converts that area of the emulsion so that it is physically harder than the surrounding medium. Now it's hardness that gradually varies, as the darkness did on development. Too much bleach and the same results happen, ie a flattening of the ideal profile. Due to the extremely small sizes of the detail in your photograph (remeber 1/100th of the width of your hair), the grains of Silver halide in the emulsion needs to be even smnaller, otherwise it could not distinguish between darkness levels across one line. The entire line would be of the same darkness. However, there is a little problem. The smaller the size of the Silver Halide grains, the longer the exposure. As a comparison, your typical tourist film, say ASA 100, has a grain size of about 10 microns and this enables you to take pictures at exposures of 1/10 secs or even 1/100th seconds. As the need arises for faster and faster exposures, so must the grain size get larger and larger and so, therefore, must the resolution (the smallest detail capable of being recorded) get larger amd larger. Typically, holographic film has a grain size of about 0.03 microns. So why not make the grain really, really small? Because there is another factor, light hits the grain and bounces around inside the emulsion, finally hitting a grain it was never meant to hit. This is known as scattering and causes the film to develop in places where there was no image. This is dependant on grain and increase rapidly as the grain gets smaller and smaller. An ideal grain size has to compromise between resolution and scattering.

The factors that determine the brightness of a hologram are very dependant on the contrast in the fringes and their number and the cleanliness of the hologram is very dependant on the shape of the fringes - their profile. There is a thing known as the 'gamma' of the film that translates between the actual light entetring the camera and the ideal darkness or contrast that results in the film. Since for holography, the film has to go awfully dark very quickly (fram complete light to complete dark in 0.03 microns) thje gamma nneds to be very high. As a comparison, the gamma of "tourist" film is about 0.5 while the gamma for holographic film is about 2 or more.

In terms of actual exposure, the contrast of the fringes is fairly dependant of the ratio of the the two beams. This ratio dependance varies from film to film, for example VRP is very ratio-sensitive but the old Agfa was not too sensitive. No idea about PFG, never tried it. However, the ideal ratio is 1:1. With a single beam Denisyuk, you can never achieve this because the light hits the film at a particular power, goes through the film and loses some power, hits the object and some of it comes back onto the plate thus losing even more power. In the end, the actual ratio for a single-beam Denisyuk is probably more like 1/2:1 or 3/4:1 In addition, if the brightest part of the reflected light does not correspond to the brightest part of the reference beam, the ratios are even worse. It may help to throw some additional light onto the object, if you have the coherence length, but remember that this additional light must hit the object and reflect back onto the plate at the britghtest part of the reference - if the reflected light hits a corner of the plate where there's barely any reference, it's wasted, and, even worse, if it doesn't hit the plate at all, you've completely lost light that could be going to the reference.

When exposing, you've got a balancing act between making the highes possible contrast and maintaining the sinusoidal profile. Too much development and you "flatten" the fringes, too little and you lose contrast and hence brightness. Generally, the film needs to go to about 90% to 99% in about 2 to three minutes. If it goes too fast, you've overexposed, if it stubbornly refuses to go dark enough in over 5 minutes, you've underexposed. If the middle of the hologram is dark but the outer edges of the images are bright, you've overexposed. Sometimes, it's possible to see a white image if you hold the developed (but not bleached) hologram to a light from the back. This transmission, white image is an indication that you've got a good bright image coming up. Manufacturers give exposures in millijoules/sq cm (mJ/cm^2). This is a unit of energy while the laser characteristic is given in terms of power, or energy per second. So, let's say your laser is about 10 mW/cm^2 (ie it gives 10 mJ of energy per second over an area of 1 sq cm).Now take this laser around various optics and let's say by the time the laser hits the plate (both beams remember) you have 5mW total. If this laser is spread over 4cm by 4cm, you have an area of 16 sq cm. Each square cm is now receiving 1/16th of 5 or 0.3125 mW/sq cm or 31.25 microwatts.. The manufacturer says your film has a sensitivity of 50 microjoules/sq cm (1 mj?sq cm). If the laser is delivering 0.3125 microjoules of energy per second on each sq cm and you need 50 microjoules to get a good image, you need to expose for 160 seconds. You can see (http://www.slavich.ru/english/holo/holo.html ) that Slavich says the sensitivity of PFG-01 is 100 microjoules.sq cm

Bleaching converts the darkness in the fringes to hardness. It can also cause noise. Too much bleaching and you create noise. As a general rule, bleach till dark and then wait for about 30 seconds. Some people wait a full minute or even longer, this depends a lot on the chemistry and is a matter of experience. Eventually, you'll figure out a chemistry you like and swear it's the World's Best Chemistry!

H2's are simply secondary holograms made from laser transmission H1's. They can be any secondary hologram from reflection to rainbow. What distinguishes this is the direction of the H2 reference and whether or not you've slit the H1. With an H1-H2 system, you can control ratios much better by simply increasing or decresing the beam that reconstructs the H1. Remeber however, that the more power you put in to reconstruct the H1, the less power you have available for the H2 reference. The less power you have for the H2 reference, the longer will be your exposure and the more critical will be your stability rerquirements. This is also true for large H1's. The larger the H1, the more power it requires and so the less available is the power for H2. This might have a bearing on the sand table discussion elsewhere.

The angle of view of the image is dependant on the cone formed by the (expanding) object beam. In a Denisyuk, there is no boundary to the expanding object beam anbd so the angle of view is very large. Inan H1-H2 system, the cone of light of the object beam is determined by the H1. You have to image a point image and take imaginary lines from that point to the edges of the H1 and this is the angle of view. A simpler way to look at it is that when you see an H2, imagine there is an imaginary window in front of the H2. The onle view you get is inside this window. In fact, if you look carefully at a real H2, right on the edges, you'll actually see the edges of the H1 used for that H2 at the extreme edge of the image. Clearly, the larger the H1, the greater the angle of view. However, bear in mind the earlier point: the larger the H1, the more light you need to illuminate it and so the less light for the H2 and the greater (in some cases much greater!) the stability requirements.

Sorry to be so long. Hope I've not bored you too much!

John C K

Right exposure

Post by John C K » Mon Jun 07, 2004 8:09 pm

Dinesh, it’s not boring at all, nor too long. It is information like this that has me checking this forum at least once a day.


Right exposure

Post by Anonymous » Mon Jun 07, 2004 9:26 pm

"Hope I've not bored you too much!"

I am sorry Dinesh and I really hope this does not hurt your feelings, but you really suck at being boring :-P

Mark Cavin

Right exposure

Post by Mark Cavin » Mon Jun 07, 2004 9:27 pm

it was.