Difference between revisions of "Experimenter's Corner"
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of for the amateur
== Home-made Holographic Stereograms ==
== Home-made Holographic Stereograms ==
Revision as of 21:30, 14 May 2013
[From Wikipedia:] Since the beginning of holography, experimenters have explored its uses. Starting in 1971, Lloyd Cross started the San Francisco School of Holography and started to teach amateurs the methods of making holograms with inexpensive equipment. This method relied on the use of a large table of deep sand to hold the optics rigid and damp vibrations that would destroy the image.
Many of these holographers would go on to produce art holograms. In 1983, Fred Unterseher published the Holography Handbook, a remarkably easy-to-read description of making holograms at home. This brought in a new wave of holographers and gave simple methods to use the then-available AGFA silver halide recording materials.
In 2000, Frank DeFreitas published the Shoebox Holography Book and introduced the use of inexpensive laser pointers to countless hobbyists. This was a very important development for amateurs, as the cost for a 5 mW laser dropped from $1200 to $5 as semiconductor laser diodes reached mass market. Now, there are hundreds to thousands of amateur holographers worldwide.
In 2006, a large number of surplus Holography Quality Green Lasers (Coherent C315) became available and put Dichromated Gelatin (DCG) within the reach of the amateur holographer. The holography community was surprised at the amazing sensitivity of DCG to green light. It had been assumed that the sensitivity would be non-existent. Jeff Blyth responded with the G307 formulation of DCG to increase the speed and sensitivity to these new lasers.
Many film suppliers have come and gone from the silver-halide market. While more film manufactures have filled in the voids, many amateurs are now making their own film. The favorite formulations are Dichromated Gelatin, Methylene Blue Sensitised Dichromated Gelatin and Diffusion Method Silver Halide preparations. Jeff Blyth has published very accurate methods for making film in a small lab or garage.
A small group of amateurs are even constructing their own pulsed lasers to make holograms of moving objects.
Holography kits with self-developing film plates have now entered the consumer market. The kits make holographs and have been found to be fairly error tolerant,and enable holograms to be made without any other specialized equipment.
Home-made Holographic Stereograms
A stereogram is an image that is able to convey a sense of depth to the viewer. Usually, two photographs of the same subject taken from two slightly different positions are used. A special viewer is used to present one photograph to the left eye and the other to the right to create the stereoscopic effect. Holography makes it possible to record a sequence of two or more photographs on a single hologram; the hologram requires no special viewer.
The method presented here is by Holography Forum member, lobaz, in the thread http://holoforum.org/forum/viewtopic.php?f=11&t=718
Taking the pictures
Point your camera to the subject, take picture 1. Move camera to the left/right, shoot another. Do not rotate the camera! It is much easier go get good results from a set of pictures with optical axes parallel to each other. The reason is simple: if you slightly rotate the camera so that it points to the subject, the background changes a lot. In stereograms, you need to keep disparity in some reasonable range, and keeping optical axes parallel helps to achieve it. Some people would argue that you are loosing resolution using this setup because the subject has to cover just a small part of the image. That's true. But you don't have to worry with many aspects of the crossed axes setup, this parallel setup just works. And with contemporary high resolution cameras there is room to sacrifice some pixels.
How much to move the camera? It depends. If you want to display the subject in 1:1 scale, then about 60 mm (inter-ocular distance) is OK. I also assume the camera is in the same distance from the subject as the viewer, i.e. not too far and not too close. If you want to make e.g. macro photographs of a LEGO model of the house and want it to look like a real house, then the camera movement should be just several millimeters (i.e. the inter-ocular distance of a LEGO man). And so on.
Editing the pictures
Let us assume for simplicity you took just left and right pictures. If you display them simultaneously with e.g. anaglyph method, then points with zero disparity will lie in the plane of the display. Normally, with parallel axes setup, those would be points in the background. Everything else will be in front of the display plane. That's not very good. When you look at a stereoscopic point near an edge of the picture, there is a psychological conflict. Your eye axes try to cross somewhere in front of the display plane, but due to the proximity of the edge your eyes want also to focus and cross there. This problem is very serious if the stereoscopic point is in front of the display, not very serious if it is behind the display plane, and nonexistent if there is no content near the edges.
So, what to do?
Pick some point you would like to lie in the display plane. If you are shooting a portrait, then it is the best to pick the eye. Then most of the head will be behind the display plane and the nose will slightly go in front of it. So, load the pictures into different layers of a picture editor (e.g. PAINT.NET or Photoshop), make the layers semitransparent and move them so that the eyes are at the same location of the screen. Crop the picture so that every layer is fully covered by the particular image, make the layers opaque and print them. It is also important to choose the right size of the picture. As the printed pictures get bigger, then disparity also gets bigger and 3D effect is enhanced. If you want to control 3D effect precisely, then you have to take this into account when choosing inter-camera distance when shooting!
I also recommend to do check your photographs with anaglyph. While in the editor, take the red channel from the left picture, the green and the blue channels from the right picture, and make a composite image. Put your anaglyph glasses on (red filter - left eye, cyan filter - right eye), display the picture in intended print size and check the 3D effect.
I will describe the procedure for rainbow holograms but other hologram types are similar.
Cut a strip of a holographic plate, put it into a holder. Make a screen that covers most of the strip except for the left part. Stick the "left" photograph to the picture holder and make a hologram. Then change the screen to the one that does not cover right part of the strip. Replace the photograph. Take care - left and right photographs have to be perfectly aligned! Do next holographic exposure. And so on, if you had just two photographs, you are done. If you had eight photographs, you have to imagine your H1 strip is split to eight parts, prepare eight screens that uncover just one part, and one by one carefully change the screens and the photographs. Very tedious!
That's easy! Just make H2 the same way you would do common H2. For best results put your H2 plate to the same distance as was H1-photoholder distance.
Ferric Ammonium Oxalate
Ferric ammonium oxalate (FAO) and possibly ferric ammonium citrate (FAC) have possibilities as a substitute for ammonium or potassium dichromate in gelatin. Experimentation is chronicled in two Holoforum threads: http://holoforum.org/forum/viewtopic.php?f=7&t=440 and http://holoforum.org/forum/viewtopic.php?f=7&t=497 .
Test plates were fabricated using:
The coating thickness was estimated to be 7 to 9 microns. Plates were exposed three days after coating. Geometry was a two-beam transmission grating at zero degrees and 24 degrees (~900 l/mm) at 457 nm from a solid state Melles Griot laser. The pictured plate was a 60 mJ exposure (mJ/cm^2?).
Processing was as follows:
The second picture is a single-beam reflection hologram take with a 25 minute exposure.