Having also spent a long time in actual holography, I would agree with your assessment generally and your catalog of different "holographic" techniques.

I might quibble with lenticular displays, since they have a name that predates holography.

In the ideal world, a larger umbrella term would have emerged. "Autostereoscopic" describes any display that provides different views without glasses, but that typically suggests discrete views.

"Plenogram" would have been reasonable following Adelson and Bergen's plenoptic function, but that didn't and won't happen.

"Hologram" now basically means a sufficiently advanced three-dimensional display in popular culture. And that even leaves of other real holograms, such as the ones used for non-destructive testing.

What about the kind where you just slap an " H " for Hologram on someone's forehead and then tell everyone they're a real life hologram ?

Red Dwarf may have gotten this one right for comedic purposes :-D

https://en.wikipedia.org/wiki/Arnold_Rimmer

There’s such a big difference between the meaning that it took me a while to fighter this story out “ gravity is holographic” https://news.ycombinator.com/item?id=49845998

>You can be a strict prescriptionist about it

*prescriptivist

Fascinating, but I do wonder if you're not being the pedantic one here. All of these depend on reproducing the phase of a wavefront. Even a traditional hologram fails this purity test - you don't get a full intensity + phase map without taking multiple images with different phase of reference beam.

Mutually incoherent hogels ... the mind boggles.

Yes, it's a tech that can be used to display in 3d, but not sure that's the best categorization choice overall.

You can also make things like holographic lenses [1].

And, "display" doesn't seem right. If you recorded a still image, it's a (literal) record of the light field of a scene that lets you view the scene in 3d, like a magnifying glass functioning from all perspectives [2]. You do get the "display" of a 3d scene out of it, when used that way.

Fun fact, you can make your own at home with gelatin, some photo chemicals [3], a laser, and a ludicrously stable table. DIY hologram is going to be one of the first things I do when I retire.

[1] Used in HUDs, like in fighter jets. https://en.wikipedia.org/wiki/Holographic_optical_element

[2] https://www.youtube.com/watch?v=iMdvBmePm9U and https://www.youtube.com/shorts/DnjATyPuFFk

[3] https://holowiki.org/wiki/Dichromated_Gelatin

It's a light field rather than merely 3d, so you can record not only a 3D image of a microscope but also if you look down the hologram's eyepiece you get to see the thing it is focussed on: https://www.youtube.com/watch?v=K-ahG_Q2xRc

(I own a similar print, different microscope).

> I don't understand the distinction. You said it's not a 3d display, then described a 3d display technology.

It's possible that you haven't actually seen a real hologram. It's quite impressive. The ones I have seen are 3-dimensional objects in the air in front of you, and you can walk around it, typically not the full 360 degrees, but it's just.. there. It's nothing at all like what we normally call a "display".

A CRT doesn't display a picture, it produces a dot of photon emission which varies in color, intensity, and position over time across a 2D surface. The picture happens in your mind.

You can do things with holography that are impossible to do with any other 3D display. I. e. comparing two objects for sub-wavelength differences.

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Good catch, link updated!

It's "just" an engineering problem at that point though. It works. First it'll be at Disneyland for tens of millions of dollars and then it'll be in Vegas for millions of dollars and then eventually we'll be able to get it at BestBuy for $1,999 in 40 years.

And a battery powered laser

https://youtu.be/P5VEu77qe4Q

Oh, I'd love to have a spare $6000 sitting around that I could buy^H^H^H reserve a holographic printer.

The traditional emulsion had peak sensitivity at 633nm. I don't know much about it otherwise but it was much easier to handle as glass plates rather than film. Russia led the science of this for decades.

Silver halide based holographic plates are like conventional black and white photographic material, only extremely high resolution and very low sensitivity.

That's required because off-axis display holograms have spatial frequencies ranging from about 1200 line pairs per mm for transmission holograms to maybe 5000 for reflection holograms.

Because the grain size is so small, these plates tend to be pretty insensitive to light. That's why they have limited use outside of holography these days.

Other types of emulsions such as dichromated gelatin also exist but are also hard to find. DCG is even less sensitive to light.

In either case, the wavelength the material is sensitive to matters when exposing with a single frequency laser.

Then there are issues about the substrate if the emulsion is on film related to polarization preservation.

The price of entry to kick off digital photography was a fraction of a megapixel, the industry really kicked into high gear around VGA resolution which is about 1/3 of a megapixel and it was VERY plausible for the computers of the time to manipulate it, store it, transmit it, display it, edit it, etc.

A very "entry level" not good but maybe enough to kick off an industry digital transmission hologram would be like "hundreds of gigapixels" or at least dozens and thats not realistic for computers of this time. The super cool reflection holograms are much worse, like "many dozens of terapixels" to even begin to play with.

I guess its good news there are still frontiers of computation out there to reach. A lot of people believed progress would cease when computers could do high enough resolution video, and they're mostly correct, progress has halted for the general public. But if we ever get smart phones with low petabyte fast on device storage and display, those could realistically have "star trek" like transmission holograms.

That beings up some interesting issues WRT RF engineering. Shannons law and all that. Even with unimaginably complex compression algos will something like a hand held hologram video conference (similar to sci fi tropes) ever be possible? Maybe 60 PB/s would not be unreasonable. Consider the LHC only generates about 1 PB/s when its running and that takes a rather large distributed system and a lot of electricity. I don't think video conference holograms will be realistic... Strange thought experiment the only way to do a "holographic video conference" with sane data transfer rates and power levels might be to send the other side a LLM prompt and ask for a locally AI generated video hologram. "Here's 1000 jpeg pictures of me as raw data, render a hologram of me saying 'scotty beam me up'"

The display hardware tech would be pretty wild. I think it could be done but it would require a minimum of 1980s nanotechnology hype. Like DLP but using individual atoms not "large" silicon mirrors.

In summary the MVP for digital photography required early 1990s hardware, but the MVP for digital holograms requires a LOT more than 2020s hardware. Maybe in the 2040s, 2100s, who knows.

Probably true, but I reckon there's surely at least some money to be made for a commercial enterprise if anyone wanted to go big, and nobody has. Get the price down via mass production and automation. Get some holograms into the shops. Sell them in an online store (you can buy holograms online already, but only a few from small operators at high prices, or second-hand). Let people send you their 3D models or 3D scenes and you'll send them a hologram of it. You can even do short animations (there's nothing stopping every new perspective being a different frame!). Have your AI convert people's photos to 3D and then output a hologram of them.

You do need to have a decent point light to shine on it, but you can sell that too. You can even sell a frame that has a clip-on light that's just right[1], although a perfect lighting setup is not required.

[1] Like Dutch Holographic Laboratory does: https://holoprint.com/pages/components.php#Telescope

It wasn't that prohibitive. When I did it, I had a helium neon laser recovered from an old supermarket scanner, but it can be done with one of those $10 keychain lasers. The chemicals weren't too expensive, but the holographic plates could get pricey if you wanted a lot of them.

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One of the most magical parts of the hologram for me is that if you cut it (the film media) in half, you have two copies of the hologram. The reduction in size is a loss of resolution - but the scene stays the same.

Though that also is part of the reason that a they're not more common. Each hologram is the film rather than a "print".

If someone wants to make 100 holograms to be able to sell them as more than a one off art piece you're making 100 units of the film.

The Science Museum in London used to have a life-sized holoportrait of Denis Gabor. It looked like a milky plate until you pushed the button that turned on the laser, and... there he was. Stunning.

There was a short-lived flurry of art establishment interest in high quality glass holograms, but the process is just too expensive and fragile to create non-niche art.

I always expected that at some point we'd have dynamic holographic displays, but the processing and resolution requirements are insane.

Holographic film has a resolution in the region of 100,000 to 250,000 ppi, which is quite a bit more than 300dpi for print or a few thousand PPI for plain old film. If you could build a 27" display with that resolution you'd need maybe 1TB per frame at a minimum.

You could probably apply quite a bit of compression to get that down on input, but that's still the order of a workable frame store memory.

Multiply by three for colour.

You are describing transmission holograms, the best visual fidelity but it had to be illuminated by laser light. Reflection holograms had a small field of depth before getting blurry but we're visible in white light. Dichromate Gelating (DCG) holograms were also white-light visible, had a smaller depth field than reflection holograms, but we're very sharp and bright. A lot of novelties were made using the DCG process, like novelty glasses that had alien eyeballs in the lenses.

They're also useful for analytical purposes.

It's possible to do this with digital methods, too, but the lower spatial resolution of most digital LCDs and most digital camera sensors (compared to film) limits it somewhat. But I think it'd be more practical than the analog film based analytical holograms.

There was one in London in Covent Garden market around the same era (perhaps a small chain, or perhaps just a similar name).

One image that I remember being particularly impressed by was a model of a radio telescope dish. You could look around the dish and see all the support gantry. The contrast between the smooth dish face and the structure on the other side made the 3D nature very dramatic.

On the counter they had a nice mirror illusion¹ of a floating "help" button - not a plate hologram but a really nice effect and you can still get these. They tend to be cheap plastic and dust and scratches affect them over time, but it's still a really nice effect if you've not seen it before.

¹ https://sandradodd.com/airpig.html

The Camera Obscura in Edinburgh has a selection of holographic images or used to.

I want them to find a smarter way that doesn’t make my experience suck.

There’s enough sites who don’t have this annoying feature.

I have no idea. I'm just curious if the fact that nobody uses "holografic sound" in the marketing means that we don't have equivalent tech for sound waves.