Understanding transmission holograms and HOEs/gratings

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Nnnn
Posts: 39
Joined: Tue Nov 06, 2018 3:23 am

Understanding transmission holograms and HOEs/gratings

Post by Nnnn »

I'm trying to understand the difference between the setups for transmission holograms and diffraction gratings. I've yet to make a successful transmission hologram so far, using an extremely simply single beam setup. (See attachment). Attempting to play it back with a laser gave me nothing, but when viewed in white light I had a blurry rainbow colored series of images in the shape of the object. (I can get a picture of this later if needed.)

So, I have a few questions on the matter.

1) what is the difference in setup between diffraction gratings and transmission holograms?

2) Regarding playback of transmission holograms, I hear that you should be able to play them back with a laser even before drying. (Integraf pfg-03m processed with jd-4.) Is this entirely true or do they have to be dried to some extent? I wasn't able to find an image with the laser I used to make the hologram in the same place as where I took it, so I also tried to find the real image with a different 1mw laser pointer. If the frequency of the laser pointer is slightly different than the original, will that also make it impossible/harder to find the real image?

3) Regarding diffraction gratings: first of all, is "diffraction grating" synonymous with "HOE"? If not, what's the difference? Do diffraction gratings/HOEs have depth or are they always on "top" of the image? Are there any resourcres about how to shape them/place them on an image when I want to combine them with another hologram?

Apologies for yet another load of beginner questions.
Attachments
IMG_7841.PNG
IMG_7841.PNG (84.89 KiB) Viewed 6310 times
jrburns47
Posts: 165
Joined: Mon Sep 14, 2015 2:48 pm
Location: Oyster Bay, NY

Re: Understanding transmission holograms and HOEs/gratings

Post by jrburns47 »

Urge you to get a copy of Practical Holography, (any edition but 4th is best) by Graham Saxby (RIP) - 4th has Stanislovas Zacharovas as co-author. It will contain easy to understand answers to your questions. If you’re in NY, you can take an excellent and inexpensive basic course with Sam Moree. If in UK, I think Pearl John may give occasional introductory workshops. Good luck!
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Understanding transmission holograms and HOEs/gratings

Post by Din »

Nana wrote: Mon Mar 25, 2019 10:14 pm 1) what is the difference in setup between diffraction gratings and transmission holograms?
A diffraction grating is simply two beams, no object. You illuminate the plate with two beams and you have a grating. If the two beams are from the same side, you have a transmission grating, also called a 'surface grating'. If the beams are from opposite sides, you have a reflection grating, also called a 'volume grating'. So, they're both 'transmission holograms', but in one case you're scattering light off an object and the recording is the light from the object, and in the other case, the recording is just two beams.
Nana wrote: Mon Mar 25, 2019 10:14 pm Regarding playback of transmission holograms, I hear that you should be able to play them back with a laser even before drying. (Integraf pfg-03m processed with jd-4.) Is this entirely true
Yes, but at a different angle. The light refracts through the wet plate, so you have to change the replay angle.
Nana wrote: Mon Mar 25, 2019 10:14 pm
If the frequency of the laser pointer is slightly different than the original, will that also make it impossible/harder to find the real image?
No. For a transmission hologram any light source will show an image. If the source is reasonably coherent, like a laser, then any laser will show an image, if there is one.
Nana wrote: Mon Mar 25, 2019 10:14 pm Regarding diffraction gratings: first of all, is "diffraction grating" synonymous with "HOE"?
Not synonymous. A "HOE" is a Holographic Optical Element, any simulation of any optical element using holography is a HOE. A transmission grating is an optical element using holography, so a transmission grating is a HOE. But, it's one possible HOE, there are many others. For example, you can reecord a reflection grating with power, so that the hologram acts like a focusing mirror; this also is a HOE (in fact, these kinds of HOEs are used in AR/VR goggles).

One problem in your setup may be that the plate is 'seeing' very little object light. According to your drawing, most of the light reflected from the object is reflecting away from the plate. It should be reflecting to the plate, not away from it. I would suggest carding off the actual reference, ie put a card in front of the beam until you block the reference and only the object is illuminated. Then, look at the object through the plate. If it looks dim, then most of the object light is not hitting the plate. The ratio of object light to reference should be about 3:1 or 4:1, ie the reference should be about 4 times, or 3 times, as bright as the object. If the object is dim, the ratio is too low.
Ed Wesly
Posts: 513
Joined: Wed Jan 07, 2015 2:16 pm

Re: Understanding transmission holograms and HOEs/gratings

Post by Ed Wesly »

I urge you to check out: http://edweslystudio.com/Pedagogy/PedagogyFrameset.html

You're not by any chance in the Chicago area?
"We're the flowers in the dustbin" Sex Pistols
Nnnn
Posts: 39
Joined: Tue Nov 06, 2018 3:23 am

Re: Understanding transmission holograms and HOEs/gratings

Post by Nnnn »

Thank you all for your incredibly helpful replies! This is more than I could have possibly hoped for.
jrburns47 wrote: Mon Mar 25, 2019 11:13 pm Urge you to get a copy of Practical Holography...
I'm in the process of reading through it now, thanks. It's a wonderful read and I'm learning a lot already.
I'm just getting to the chapter on transmission masters, which indeed does cover a lot of what I asked... <<;;

I guess what I was surprised by was ending up with a grating in the shape of the object when I was trying to make a transmission hologram, which I hadn't seen mention of before.
Din wrote: Tue Mar 26, 2019 8:49 am ...
Wow, thank you for such a detailed reply! This covers just about everything I was lost on, and a bit more. I really appreciate the advice in particular about looking from the perspective of the plate to see how much light is being reflected from the object to the plate since it's not practical for me to get a light meter yet.

I do plan on getting my first front surface mirrors soon to improve my setup, though. I'm looking forward to being able to control the beam at least a little bit. ^^
Ed Wesly wrote: Tue Mar 26, 2019 10:36 am I urge you to check out: http://edweslystudio.com/Pedagogy/PedagogyFrameset.html

You're not by any chance in the Chicago area?
Excellent resource, thank you very much! I do visit your site with some frequency, but this is the first time I've seen this particular page.

And unfortunately I'm in neither NY nor Chicago but Tokyo. If there were some sort of class/learning opportunity here I would jump at the chance to take it, though. I do plan on attending the HODIC meetings at least!
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