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Old 05-22-2020, 04:47 PM
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What's the term for this property of light at the focal plane of a projector?


If you take a standard LCD projector and project a blank white image onto a screen in proper focus, you can observe some interesting effects by holding up objects close to the screen (the focal plane of the projector).

Shadows of objects are ultra sharp. You can clearly see changes in index of refraction in the air rising above a warm coffee cup. You can see how light is distorted by a transparent object, or how it's focused by things like eyeglass lenses, etc. If you hold up a film negative very near to the screen, the transmitted image that appears on the screen is nearly perfect - not blurred, etc.

What is the term for this? Planar? Collinear? Collimated? Merely "focused"?

Can I purchase a light source with this property, without the rest of the doodads in the projector? Can I purchase a light source that maintains this property over a longer distance, rather than just a thin focal plane?
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Old 05-22-2020, 04:54 PM
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When I say “what’s the term for this”, I mean, the term for the property of this configuration of light that allows these effects to be clearly seen.
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Old 05-22-2020, 05:27 PM
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Jen-Paul Marat performed these experiments in France before the REvolution. HE demonstrated the heat rising from Benjamin Franklin's bald head.

Yes, THAT Jean-Paul Marat. The one who was knifed in his bath by Charlotte Corday, got painted by David, and had a play written about it in the 1960s. He really did make a lot of optical discoveries, but is pretty much neglected today.

I don't know what he called the effect, or what it's called now. A more modern complex version of this involving a bit odf spatial filtering is called the Schlieren Effect

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

As that article points out, Robert Hooke was seeing these effects in 1665, but not with the modern system, which was developed in the 19th century.

I wrote an article about this, but unfortunately the online copy is locked behind a paywall.
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Old 05-22-2020, 08:46 PM
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Quote:
Originally Posted by Absolute View Post
What is the term for this? ... Collimated?
I think you've got it.
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Old 05-22-2020, 09:06 PM
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Very generally, it's low-étendue light.

Collimated isn't quite right, because the light isn't parallel here--instead, it's more like a pinhole camera, where all the rays can be traced to a single point. Collimated light is one example of low-étendue light, but not the only one.

In contrast, a large, diffuse radiating panel is high-étendue. All incoming light comes from a mix of emission from many points. So shadows and other projections are blurry.

Any low-étendue source will give sharp shadows, although there may be distortions depending on the nature of the light source.
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Old 05-25-2020, 06:22 PM
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Thanks, that was exactly the term I needed. I knew there must have been a more specific word for this.
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Old 05-25-2020, 10:38 PM
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Perhaps the word is - focused.
If you put a slide on the projector, the image on the screen is focused. The light at that point on the screen is coming primarily from that point on the projector glass plain. Imagine each point on the glass as a pinhole point of light. Light spreads from that point, in a cone toward the lens, where it is re-focused onto the corresponding point on the screen. Holding something close to the screen and it will mostly catch the light intended for that spot. mess with the linearity of the light beam -with heat-induced refraction - and some light maybe fails to arrive, thus making the point darker, and nearby points - where it is diverted - brighter. You see refraction patterns because they are turbulent, not uniform.

Last edited by md2000; 05-25-2020 at 10:38 PM.
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Old 05-26-2020, 04:14 PM
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Focused isn't really specific enough. Consider two projectors with the same focal plane distance, one with a large aperture and the other a small, almost pinhole aperture.

Hold an object at the focal plane--assuming the projectors are actually focused, you'll see a sharp image.

However, if you move the object a bit closer or further away, you'll find that the projection gets blurrier much more rapidly (with respect to distance) with the large-aperture projector.

Likewise, suppose you set the projectors to just emit white light and you hold a paper cutout at the focal plane, and then a screen some distance behind it. The small-aperture projector will give sharp shadows even with the screen some distance behind. With the large aperture, the shadows get blurry more quickly. Same effect with slides.

The difference is with étendue. The large aperture means that the incoming light for each point at the focal plane has a wide spread of angles. This spread blurs out shadows and other projections. The small aperture has much less spread and shadows are relatively sharp.

There are other ways to achieve the same thing. You could take a point source of light, say an arc lamp, and use a collimating lens to make the rays parallel. Done perfectly, you now have a light source that can cast sharp shadows at any distance. Or, just consider an unfrosted incandescent light bulb--it's also nearly a point source, and the resulting sharp shadows are one the reason why the classic bare, swinging bulb in a basement is so creepy.

All of this, in reverse, is also the reason why a large aperture on a camera gives a narrower depth of field--that is, everything outside your focal plane is out of focus. Use a smaller aperture and your whole scene can remain in focus.

Of course, this also requires more light--you're rejecting all light that doesn't come in from a very specific angle. Etendue is conserved, like entropy (and can be considered an analog of entropy in optical systems), so the only way to reduce it is to somehow separate the low-étendue parts from the high-étendue parts. You can do this, but you're losing the light that you throw away or ignore.
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