vocabulary

[Brian]

I've made some simple holograms, but I must await some parts and books before playing around some more this summer and starting some student project(s) next fall. So far I have just been going along by a feel of what should work, but now that I'm here, I shouldn't spend my time reinventing the wheel. But as I read old posts there is a lot of vocabulary.

Some vocabulary I figure out. For example, I know my simple holograms are called Denisyuk holograms.

Some vocabulary, when I figure it out makes me go "aha". Another example. I wanted to use a blue laser to make a hologram. It has an absolutely terrible spatial profile (a water leak upon it really messed up its output window... bad old science building). So instead of using a curved mirror to diverge the beam, I figured that sending the beam through a 50 micron pinhole would give a nice Fraunhofer pattern that expands as it moved further from the pinhole. It did, which was great, but there was too little power in the beam. So I used a short focal length lens before the pinhole to squeeze more power through. And this provided enough power to make a hologram. But then I'm reading here and realize I've constructed a spatial filter... "aha" ... nice job me, reinventing the wheel. And look, it would work much better if I replace the lens with an even shorter focal length, i.e. objective lens.

So I wanted to start a thread where I could ask about unfamiliar vocabulary words I come across.

[Brian]

First up.

Looking at simple holograms I've made under normal lighting, some have distinct swirls and they may even swirl around a central point. Since it does looks like the pattern seen around a knot in wood, this must be woodgraining. What causes this and how do you stop it?

[Ed Wesly]

It's the interference pattern formed by reflections from the front and back surfaces of the substrate. Aligning the polarization vectors to minimize the reflections helps, and they are virtually eliminated by shooting at the Brewster Angle.

Your simple single beam reflection hologram is a Denisyuk hologram; there are many types of single beam holograms, see the Seven Single Beam Project papers: http://edweslystudio.com/Pedagogy/7SBP/ ... e7SBP.html

[Brian]

Okay, tilt the plate's normal around 57 degrees from beam and polarize beam parallel to the plane of incidence. Very good.

Next up, big optics. Does this mean big mirrors? I don't need lenses larger than 1", or do I? For 2.5" square plates, how big are the plane and curved mirrors generally required? Where do you get big first-surface spherical mirrors?

[Din]

For 2.5 in plates, you don't need large optics. So long as the reference beam completely, and relatively uniformly, covers the plate, and so long the light reflected from the object can be seen all over the plate, the object will record. In this regard, I wouldn't use shiny objects like coins, the reflection is specular and won't uniformly cover the plate.

As for collimation, you probably don't need it at this level. Basically, the idea is that the closer the recon beam is to the object beam, the fewer are the aberrations. However, for an object in display holography, especially for a 2.5" plate size, the small distortions and aberrations coming from small changes in beam divergences will not be noticed. Say your spatial filter is a foot away from the plate, and you expanded it so that the inner third of the beam covers the plate (to take Gaussian into account). This means that the beam divergence is on the order of about 20 deg. Now, strictly speaking, you should reconstruct it with a beam also diverging at 20 odd degrees. Let's say you look at it with a halogen bulb with a divergence of 40 degrees, you probably won't notice anything. The nose of the porcelain cat may extend a bit more than it should but no one notices. The far greater problem is the distance behind the plate that the object is. The depth of the image is given by the dimensions of the white light source (strictly, it's given by the spatial coherence of the source). With a standard light bulb, you'll probably get a depth of an inch or two. If you use a halogen (smaller filament) you'll increase that to maybe 4 in, if you use a led (still smaller source) you'll increase it to maybe 6 in. However, with a fluorescent, you'll probably get an inch or less.

The real point of collimation is when you get to two beam H1/H2 setups. In this case, you make a master hologram (the "H1") that's laser viewable with the object a considerable distance behind the plate - say 8 to 10in. You then "flip the plate to reconstruct the real, pseudoscopic image. You reconstruct the pseudoscopic image, place a second plate into the plane of the (real) image and shoot a second hologram (the "H2"). But, if you're reconstructing the pseudoscopic, you need to reconstruct it with the conjugate of the original ref. However, the nice thing is that the conjugate of a collimated beam is also collimated, so you don't need to worry about matching the H1 ref with the H1 pseudo recon.

If you want to collimate (remember if you do, the reconstruction should also be white light collimated) for a small plate of 2.5", you're probably better off using a lens than a mirror. Simply place a positive lens at f in front of the spatial filter. Large lenses of the order of about 2" are cheaper than front surface colimators.

Where do you get big first-surface spherical mirrors?

Edmonds is not too pricey. (http://www.edmundoptics.com/ ) There's also Thor ( http://www.thorlabs.us/navigation.cfm you get a T shirt and a box of chips and stuff with the order!). I understand some hobbyists get stuff off EBay. Being a Luddite, I wouldn't know about that

[Brian]

This is really useful, every single bit. Well, except for Edmond and Thorlabs, those I know very well... I love lab snacks. Conversely, ebay is very difficult to use. Relieved I don't have to position big mirrors all over the place. I have a couple of 2 inch lens mounts.

I hadn't considered the spatial coherence and depth of view available from white light sources. I've only been using LEDs for viewing, so never even considered fluorescents.

Also hadn't thought about aberration. Just thinking out loud, if you don't use a collimated beam in the H1/H2 setup, then you are going to compound the aberration. Kind of a bummer though, having to use a lens with the LED when you want to reconstruct with white light. Or maybe an LED maglite flashlight, but the greater part of that output is still going to be uncollimated.

Still more vocabulary questions, but first I'm going to pause and think about this for a while. Especially I want to ponder viewing with fluorescent strip lighting.

[lobaz]

Talking about aberrations, viewing geometry considerations, H1/H2 setups...
I think that an excellent introduction to these topics (as well as to the holography itself) is given in the book "Holographic Imaging" by Steve Benton and Michael Bove. (This one is my most favourite book on holography - consider to have a look.)

One more addition to "woodgrain" - it is also wise to mask the plate edges, if you do not use a plateholder. Otherwise, light comes in through the edge, bounces there and back and creates stripes or other unwanted pattern. I use a tape, even a black marker helps.

You can also read the end of the Chapter 8 of Saxby's Practical Holography: there is a quick "Troubleshooting" section with images and explanation what went wrong.

[Brian]

Okay, add another book to the list.

Thanks Petr for the extra tip about light coming in from the edge. I think I've seen what you are talking about, light and dark bands parallel to the side tilted closest to the beam when the LED is held at a really shallow angle to the surface.

I have been using washi tape to hold plates on a sheet of black acrylic for viewing them with an LED. The black washi tape doesn't leave a residue when you peel it off again. So I will use that to mask the plate edges.

[Brian]

Just thinking, suppose you want to make a reflection hologram of something that reflects your laser color that is embedded in something that is black to the color. For example, some miniature stop sign exposed to a blue laser. The laser lights up the letters, while the surround looks black. You want to see the letters in the hologram, but you also want to see that the sign has octagon shape.

I'm thinking mounting the sign on a matte black background would be bad, and mounting it on a matte white background is likely best, for seeing the octagon "shadow." But what happens if you mount it on a mirror? Will you see the octagon edge?

Now let's switch to something color reflective embedded in a transparent medium. Like a daisy in a glass paperweight. Again you want to see daisy petal edges, but also you want some visual cue of the paperweight borders. What is best background on which to mount the paperweight ... if its shape is half hemisphere? ... if its shape is cubic, with cube face parallel to the plate?

[Din]

I'm thinking mounting the sign on a matte black background would be bad, and mounting it on a matte white background is likely best, for seeing the octagon "shadow." But what happens if you mount it on a mirror? Will you see the octagon edge?

Depends on the lighting.First off, if there were no sign at all, just a mirror, you'd get a HOE. The hologram would simulate a mirror and/or a lens, depending on the holographic medium. For silver halide, you'd probably get a mirror

I assume we're still talking single beam Denisyuk? In which case, still assuming no sign - just mirror, the incoming rays would reflect back onto the plate at the same angle as the reference angle, except reversed. That is, if the ref angle were theta to the plate normal, the light would hit the reverse side at -theta. You'd create parallel Bragg planes all of which would be parallel to the faces of the plate - the reason they're parallel to the faces of the plate is that the Bragg plane angle is the bisector of the two beams - and the spacing between the planes would be given by lambda = 2*d*sin(theta), where lambda is the recording wavelength, theta is the reference angle and d is the plane separation. If you now reconstructed in white light (note I haven't developed yet), the hologram would reflect light at a wavelength of lambda' = 2*d at normal incidence. You've essentially made a mirror, a HOE where the Element in Holographic Optical Element is a mirror. In other words, the hologram would act as a narrow band mirror, whose reflection wavelength is dependent on the reconstruction angle. Once you develop this hologram, the Bragg plane spacing would vary and the inter-Bragg plane distance population would follow some kind of Gaussian. This is no longer a narrow band mirror, in terms of angle and wavelength of recon. You'd get a swath of colours at all angles.

If you now put an object on the mirror that scatters, the scattered light would record, as well as the HOE. You'd get a swath of colours surrounding and superimposing onto the image field. Whether you'd see the black edge depends on the relative efficiencies of the two fields, the HOE and the scattering object. If the HOE is much more efficient than the scattered image field, then all you'd see is colour, with perhaps a dim edge. If your image field is much more efficient that your HOE, you may see a dim outline of the black edge, but this would be swampled by the light from the part that actually scattered, the centre of the sign. So, yes, it's possible to bring out the edge, but it'd still be surrounded by colour. However, note that if you reconstruct with a led (not a white led), then the colour of the led would determine whether you see the edge. If the colour of the led is not the diffracted wavelength of the HOE at some angle, the mirror would make no difference, except maybe to generate noise which would "fuzz" the edge a little.

Now let's switch to something color reflective embedded in a transparent medium. Like a daisy in a glass paperweight. Again you want to see daisy petal edges, but also you want some visual cue of the paperweight borders. What is best background on which to mount the paperweight ... if its shape is half hemisphere? ... if its shape is cubic, with cube face parallel to the plate?

If you're front-lighting it, ie a Denisyuk geometry, the light hitting the glass part of your paperweight would simply pass through the glass and not record. If the edge of the paperweight had rough edges, or the edges were inclined at some angle such that light from the edge would scatter back onto the plate, you may see a dim outline of the edge. Would you see the edge of the black part? It depends on how clean your paperweight were. A perfectly clean paperweight would send no light back to the plate, all the light would simply pass through. However a real paperweight does scatter and so, some of the light would scatter back onto the plate. This may allow you to see the edge of your sign. Remember here again, the light from the paperweight forms a HOE, so again, it's a balancing act between the efficiency of the HOE and the efficiency of the image field.

Of course, you could simply put white tape along the dark edge.