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Comment by adastra22 | Hacker News Reader

Very few people in the comments seem to understand what holography is. A hologram is not a “3D display” that mimics a sci-fi movie or TV show. A hologram is photographic media / display technology that records and transmits phase as well as intensity of incident photons. By capturing all the properties of the electromagnetic field that strikes the film, and re-emitting the same EM wave when stimulated, the hologram ends up being a full “3D picture” because after all, the light that hits your eyes ends up being the same as if you were looking at the actual scene. But that’s a consequence, not the defining property.

3blue1brown has a great explainer video on what holograms are: https://youtu.be/EmKQsSDlaa4

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Well, yes, but actually, no...

"Hologram" is one of those words that has a wide variety of overlapping meanings. You can be a strict prescriptionist about it and insist that a real hologram involves laser light, interference patterns, 3D scene replication, phase preservation etc, but there will always be something that doesn't quite meet your definition that nevertheless is called a "hologram" by large numbers of people, even expert optical scientists:

A digital holographic stereogram prints an array of hogels (holographic pixels); the hogels themselves are holograms, but are mutually incoherent across the plate. When implemented to the highest level, the result is perceptually and behaviorally indistinguishable from an analog hologram.

A rainbow hologram (such as are found on credit cards) has single parallax only - it certainly does not capture all the properties of any particular electromagnetic field. No purity here, it's just a nifty tech for making things look 3D that happens to exploit wave interference. And everyone calls those holograms.

A scratch hologram is, to a first approximation, an extremely coarse version of a rainbow hologram, made by hand by engraving curved grooves in a reflective material. Now we're away from lasers and wave interference, and for this we sacrifice color - but geometrically we are still dealing with directional microreflections on a surface. Hologram? Depends who you ask.

Lenticular holograms use refraction rather than diffraction to achieve essentially the same imaging model as a rainbow hologram, but in a decidedly less finicky way, at the expense of some resolution. The same imaging data could be used to make a lenticular or a rainbow hologram. No lasers, no diffraction. Hologram? Depends who you ask...

My advice, as someone who has worked with holograms for a decade, is to give up. Saying "AKTUALLY Tupac/Abba isn't a hologram" gets old after the 1000th time. In the real world, "hologram" means "intangible optical simulacra", regardless of how it was achieved. If you want more precision than that, you have to specify the actual technology - Denisyuk, rainbow, lenticular, etc...

When someone asks "can you make a hologram of this", you don't lecture them on optical physics. You say "sure, you want the expensive kind or the plastic kind?"

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.

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.

Do these all actually only capture and display in stereo ?

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

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

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

That's like saying a CRT doesn't display a picture because it energizes pixels only one at a time.

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

I'm still a bit sad that light field cameras didn't take off.

2012, commercial availability (failed in 2018 and eaten by Google). 2017, used in scientific settings. 2021, sold commercially to consumers through kickstarter. 2026, multiple companies still sell light field cameras at modest quantities.

I wouldn’t say they “didn’t” take off. They’re gradually acquiring market share.

Was there anything besides the Lytro?

Its problem seems to be that it was a solution to a problem that was fast disappearing, at the cost of unacceptable tradeoffs.

The Lytro sacrifices a ton of resolution and produces crappy images, in exchange for being able to refocus. And they did that in an age where excellent autofocus was already available.

These days we have cameras with AI models that recognize birds and animals and focus flawlessly almost every single time. And they don't sacrifice any resolution to do it. And they are proper, comfortable cameras rather than a weird stick.

Their thing was more being able to change the focus after the fact

Problem is you can already take a fairly deep focus image (with a stopped down lens) and then fake shallow focus later if you want.....

There are still light field cameras available, but they're all call for pricing industrial cameras.

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.

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).

A properly constructed light field image can do this, so can a virtual reality headset or other "video game technology" system if it properly models refraction with ray tracing or another method.

A headset only ever renders one viewpoint at a time. A hologram records and re-emits the whole light field all the time., so it is simultaneously viewable from every perspective.

Well, two viewpoints, one for each eye. And it adjusts what it renders when you move around so the effect is similar to a hologram and I'd say, competitive with it. If you are just one person the other views are wasted, and more than one person can share the same VR scene and get their own two viewpoints.

Holograms are cool but it is a finicky technology and speckle makes them visually ugly.

"Another method" is doing a lot of heavy lifting there: computational holography is possible, has been for a while, but comparing it to raytracing is an apples-the-fruit to Apples-the-computer comparison.

Holography is the display, not the rendering process. It contains, inherently, how the light is focused, so your eyes can refocus on near and far objects within the scene even though it's a static image.

But more to the point, a hologram is less like a computer display and more like a photograph.

As a person trained in physics I’d say ray tracing is based on the ray theory of light whereas computational holography is based on the wave theory of light.

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.

> 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".

We can imagine a holographic display though. It would look like a window. Not a metaphor of a window, but an actual window. Like, a pane of glass you look through to see a real, physical scene on the other side, indistinguishable (to resolution limits) from the real thing.

One can hope we someday have this capability.

We have holographic displays, they have expensive components the picture sucks, and they're are on optical workbenches, but there is real hardware.

It's not exactly like looking through a window, they can do that, but they can render an image in front of the window as well as behind it.

You can walk in front of a window, but you can't walk to the side of the window and see the opposite side. A hologram is not like a window, a hologram is an object you can walk around. Not 360 degrees, but way more than 180.

I don't see the difference.

"Screen", sure, that implies 2d. "Display", is a much more general term covering screens, LED readout panels, a shadowbox of physical objects, and more. There are a lot of things that are displays that are 3d already.

Not all holograms are displays, except in the sense that any piece of photographic film is a display.

The hologram described above certainly is.

Again, it sounds like you haven't actually seen a real hologram. It is different, and very much so.

The irony is that actual holography is arguably more impressive than the sci-fi version once you understand what's happening

Speckle tends to make holograms look awful.

Holograms compete with many other display technologies such as stereograms, VR headsets, as well as volumetric and light field displays. This technology is described as "holographic"

https://lookingglassfactory.com/

but it looks better than holograms because it uses incoherent light field technology that doesn't have the speckle, plus the pipeline to make content for it doesn't require lasers, special equipment or special skills.

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