digital holography

[verbes]

What I understand (not that much) is that the 3D image is the result of how the plate's surface has been altered by the 2 separate lightbeams's interference.
The detail? How detailed can hologram's be? What is the detail related to?
There's a weird gap in the thought process when I visualize myself putting an hologram under an microscope.

After understanding howmuch detail an hologram can store, I'm wondering if this can be equaled by means of digitally recording the received light.

[rzeheb]

Hi Verbes,
The information is recorded in the form of interference fringes within the photographic emulsion. The image is created by the diffraction of light passing through the fringes. Information is recorded in great detail and at very high resolution. If you were to look at the surface of the exposed and developed silver halide plate through a microscope (and it would have to be a very good and powerful microscope at that) you might see patterns of silver bromide grains. The spacing of the fringes would be on the order of 100 nanometers. Given that a single human white blood cell might be 20 micrometers in diameter the fringes are narrowly spaced indeed. I would recommend spending some time reviewing the Holowiki (link in the header, above), and then maybe invest in a book or two. Practical Holography by Saxby would be a good choice. Best of luck, Ron.

[Dinesh]

I'm wondering if this can be equaled by means of digitally recording the received light.

Not yet. A typical reflection hologram has roughly 3000 lines/mm or 3000*25.4 = 76,200 "dpi". The lines are also incredibly curved, so you can't simply draw 76,000 odd lines/inch.

Imagine the light is like a putty. This "putty" hits these very tiny set of lines, squeezes through and, in the process, alters it's shape. The "shape" of the light is representative of light that reflects off an illuminated object. Since the original illuminated object caused those lines in the first place, the "shape" of the light is an exact replica of the "shape" from the original object. When this "light shape" enters your eyes, your brain can tell no difference between this shape (of the putty) generated by squeezing through the fringes and the shape of light from the original object - ideally. The "wavefront" or "shape" of the light from an object has been 'reconstructed' by light oozing through these slits and coming out altered. In a real hologram, there is always noise and the lines are never faithfully recorded, so you can actually tell a hologram of a cat from a real cat.

"Digital holography" has many meanings, so there's a lot of controversy about what "digital holography" means. In one case, you record a series of images digitally from a standard digital camera or video recorder. Then, through optical means, you collapse these images (or video frames) into slits and record these slitted images holographically side-by-side onto a holographic plate. When reconstructed, the series of images all reconstruct simultaneously. However, since you've made slit images of the pictures, or frames, consecutive pictures, or consecutive individual frames, appear sequentially to your left eye and your right. You effectively see a stereo pair and hence you see 3D and you have a hologram of the original set of digital photos (or video). In another case, you compute the Fourier Transform of a CG object which gives you a series of lines. These lines - the FT of the CG object, are actually what you'd record if you made a hologram of a real object. So, by directly imaging the FT lines through a very, very good lens, you've made a hologram of an object that never really existed, except in software.

[Johnfp]

I must say I am really enjoying reading Dinesh's posts.

[rzeheb]

I wholeheartedly agree!

[walschuler]

I think you could add a variant to Dinesh's list: digitally create 2-d images, then output them through
the described conversion process to slit images, and record them as holograms. Again, no actual object
existed. Again, these would be multiplex stereograms. With modifications, you could make them
into Rainbow holograms.

[walschuler]

I would add that one of the most beautiful images I have seen of the sort I just mentioned
is a hologram of the design for Frank Gehry's museum of art in Bilbao, made from his 3-d
design database, I believe.