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There are several issues. Let me try and take them in order:
[quote=c23 post_id=73935 time=1792936870 user_id=12007]
With such a setup, I was able to see my first holograms almost at 1st attempt, using a simple white flashlight for reconstruction.
[/quote]

A flash light will not give you a lot of depth. The reason is that every point on the flashpoint generates its own image (especially if there's a diffusion glass on the flashlight) , all of which superimpose to give a blur beyond a certain distance. Right up against the plate, the blur radius (the "amount of blur") will be small, but as the image is further from the plate, the blur radius increase; you say that at 5 cm it was "a bit blurry", while at 20 cm it was "difficult to see". What you want ideally, is what's called a "point source", ie a source with (practically) no dimension at all, literally a point; with a point source, you won't get a myriad of images - blur - because the source has only one point . The nearest to an ideal point source for reconstruction is an led (be careful to match the led colour with the image colour as closely as possible). However, the sun is a pretty good source.

[quote=c23 post_id=73935 time=1792936870 user_id=12007]
Would you say that my current 2-beam setup produced successful holograms, because they are in fact simple Denisyuk holograms
[/quote]
The important thing in a Denisyuk (single or two-beam) is that most of the scattered light must hit the back of the plate. A lot of people use objects, such as cubes and spheres, that throw most of the light off the back of the plate. In a single beam Denisyuk, the way to tell how much light hits the back of the plate is to place a white card with a small hole in it against the front of the plate. The 'reference beam', will hit the card, and a small amount of light will go through the hole and the hit the object. How much light hits the back of the plate can be seen by how much light hits the back of the card. In a two beam Denisyuk, you simply place a card on the back of the plate - the light hitting the back of the plate is the same as the light hitting the card.

[quote=c23 post_id=73935 time=1792936870 user_id=12007]
it is still unclear for me what are the perfect angles at which the lasers have to illuminate the film. I tried these "alternative setup 1" or "2" diagrams few times, but it was a always a failure. I understood from the theory, that both beams have to be positioned with 45deg angle to the plate (making 90deg between them). But I don't understand if the right beam is supposed to go through the glass to illuminate the objects and then reflect to the back of the film, or if it has only to illuminate the objects, so that reflected light hit the film from back as well (the "alternative setups" 1 or 2).
[/quote]

Firstly, it is not true that theoretically both beams have to be placed at 45 deg to the plate. That may be true in a particular holographic optical setup - like a 45 deg mirror - but not true for display. Secondly, for a reflection hologram, you should have the ref (or the single beam) to be at about 55 - 60 deg to the plate. The reason is that you reconstruct with a reflection geometry - the light source reflects off the final plate. If the angle is too low, say 45 deg, the the reconstruction source will reflect into your eyes and you'll be partially blinded by the recon source.

[quote=c23 post_id=73935 time=1792936870 user_id=12007]
I can't say about laser coherence length, but I guess it is not less than 30cm (I have no optical measurement electronic tools. This is what I can obtain with my Michelson setup, then the fringes are too small. I can't say if they produce good contrast at that size).

Thanks!
C23
[/quote]

To get the coherence length from a Michelson, place both mirrors equally distant from the splitter. Look at the interference pattern, in particular, the density difference (called "the moduation"). Now pull one mirror - either one - back slowly, so the two mirrors are not equidistant from the splitter; the modulation will fall - the density difference decreases. At some point, the interference pattern will all but disappear, making the dark parts of the pattern a weak grey. The difference in the distance between the two mirrors at that point is the coherence length. So, if one mirrror is 10 cm from the splitter, and the other one is 25 cm when the modulation falls to almost zero, the coherence length is 15 cm.

<edit>
This business of having the two beams at 45 deg - 90 deg between them - may have to do with polarisation holography. This is a technique for determining polarisation, or de-polarisation - of the object. At this point, you should not worry about it. Most display holographers don't consider the polarisation - or depolarisation - of the object.

Welcome to the forum c23 :) You seem to have made excellent progress in holography in a short period of time, congratulations.

That was quite a post! (A lot of information to take in.) I'm just going to respond to a couple of things I saw at first glance. First, I don't know what the coherence length of your PL530 laser is, maybe you mentioned this somewhere. Second, the diagram with "my current setup" has two reference beams. There should always be one reference beam, as you have in "alt. setup 1" and "alt. setup 2."

You are doing split beam reflection holograms. This qualifies as advanced holography, as opposed to single-beam Denisyuk holography. While affording excellent control of object lighting, it also requires the maximum amount of stability compared to any holographic setup. While MDF (medium density fiberboard) has many uses, it is a very poor choice for a holography table. I'm quite surprised that you're getting any kind of decent holographic result at all. I would strongly suggest that you find another table material. Good choices would be a large concrete paving slab, a large ceramic tile (these have become more available in recent years in larger sizes), or better yet a piece of granite or stone countertop with a polished flat surface. People nowadays have a lot of these countertops in their kitchens, and a slab of adequate size should be obtainable without too much trouble. You can then use gravity mounts for your optics. Or you could buy a new or used optical breadboard and use commercial mounts.

Good luck!

Hello holographers from this forum!

Like many people who discovered ultra-niche hobbies, after watching few YT videos like the one from [url=https://www.youtube.com/watch?v=EmKQsSDlaa4]3B1B[/url], and after discovering that ChatGPT can do any optical maths homework :lol:, I decided to try holography with almost zero background in optics, thanks to all the infos from this forum.

My setup consists of a 70mW PL530 green laser (the eBay one advertised for holography...), a plano-convex and plano-concave lens to expand the beam to approx 5mm (arbitrary chosen because of [url=https://www.holographyforum.org/forum/viewtopic.php?p=73307#p73307]this[/url]), a 50/50 non-pol beamsplitter, a pair of FL10mm concave mirrors from Edmund for final beam expansion, Bayfol HX200, the cheapest silicon rolling pin I could buy, cheap 2mm glass plates, a small steel bench vice to lock the glass vertically and cover a bit the side of plate (because adhesive tape and plastic wasn't recommended in this forum), and for curing 3 modules in parallel of 9x Luminus SST-10 365nm LEDs from a german website (1 module is more than enough in fact). The table is a medium-density fiberboard 120*80cm, on 4 Sorbothane 30D 4x4x2cm pads from eBay, where I drilled and put 6mm metal inserts to lock the mounting bases/posts, like a real optical table.

After learning how to do good-enough laser collimations, a DIY Michelson interferometer helped me to see if the fringes are stable. I tested various laser exposure times (30sec, 1min, 2min), various curing times (1min to 15min). For curing, process seems complete in a bit less than 1 min or so (@800mA), so this is now my reference for my UV lamp. With such a setup, I was able to see my first holograms almost at 1st attempt, using a simple white flashlight for reconstruction. I was impressed myself I could achieve that.

[attachment=1]my setups.png[/attachment]

Now the questions :

- My current setup is the diagram at the top. I can generally see [i]Item A[/i] rather sharp. [i]Item B[/i], which is located 5cm from Item A is already a bit blurry but still ok. While [i]Item C[/i] located 20cm from Item A is difficult to see (maybe too dark because I might be able to read some big text on it, so I guess it is not blurry).
Before I started with my green laser, I started with this [url=https://hololaser.kwaoo.me/laser/modeanalysis.html#ML101J27]DIY red diode[/url], and did the classical beginner-style setup like [url=https://youtu.be/kSWvIBRQS00?t=119]this[/url]. It worked at first attempt. Would you say that my current 2-beam setup produced successful holograms, because they are in fact simple Denisyuk holograms like in the YT video, that worked accidentally with 2 beams? And that my setup contradicts the diagram I saw in a PDF file like below?
.
[attachment=2]Chapter 2 Holographic methods page5.png[/attachment]
.
- it is still unclear for me what are the perfect angles at which the lasers have to illuminate the film. I tried these "alternative setup 1" or "2" diagrams few times, but it was a always a failure. I understood from the theory, that both beams have to be positioned with 45deg angle to the plate (making 90deg between them). But I don't understand if the right beam is supposed to go through the glass to illuminate the objects and then reflect to the back of the film, or if it has only to illuminate the objects, so that reflected light hit the film from back as well (the "alternative setups" 1 or 2).
However, my tests were made before I bolted recently a steel frame below the fiberboard to add rigidity (and 10kg weight), so not fully sure about root cause. But usually, even if there were "minor" vibrations on my table, you could distinguish a blurred shape, while in this case, there was absolutely nothing. Any hints what to rotate and why, would be very appreciated.

- My concave mirrors are approx 15cm height above the ground (table), so the projected shapes are like 2 half-ellipses. How is this height relevant to the white light reconstruction angle? Should the objects be rather placed at the same height as mirrors, to optimize beam interference? Is it a must to have the reference beam be oriented 45deg to the plate and 90deg to object beam, or is it tolerable to play with +/-20deg anywhere without lowering image quality too much?

- how to guess the future angle at which I will be able to illuminate the film with my white flashlight? Ideally, if film is vertical and at my eye level, I want the famous 45deg from above my head. This angle from above seems not exactly the angle at which the reference beam shot the film. How to calculate that more or less?

- I haven't tried yet to play with spatial filters. I ordered parts to build one (10µm), but adjusting it is "time-consuming", I gave up for the moment. Would you say that there is a real WOW effect [url=https://www.holographyforum.org/forum/viewtopic.php?p=73356#p73356]like others experienced[/url], and that is currently my limiting parameter to get either better contrast or better view of depth?

I attached a draw.io diagram, in case it is easier to move some parts visually, than to explain in plain text. I can't say about laser coherence length, but I guess it is not less than 30cm (I have no optical measurement electronic tools. This is what I can obtain with my Michelson setup, then the fringes are too small. I can't say if they produce good contrast at that size).

Thanks!
C23