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Comment by eps | Hacker News Reader

You'd expect at least a passing reference to Fresnel lens in a piece that talks about flat lenses.

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

Here is a video explaining the difference between a Fresnel lens and a photon sieve: Photon sieves used to visualise optical wave front propagation

https://www.youtube.com/watch?v=TshYfYIxR9E

I agree, this lens looks like a Fresnel lens taken to microscopic extremes:

the new fisheye lens consists of a single flat, millimeter-thin piece of glass covered on one side with tiny structures that precisely scatter incoming light to produce panoramic images

>> this lens looks like a Fresnel lens taken to microscopic extremes:

Except I don't think it is. It's probably more like a diffraction grating or a hologram. You can actually make a flat lens by creating a hologram of a real lens and it will be flat.

That sounds like it would make it ridiculously easy to make VR headsets that are as thin as the prop glasses used in Westworld.

What are the downsides?

Wavelength specificity. You only get the original focal depth at the wavelength you used to make the hologram, and it changes proportional to the wavelength. When you consider that blue light is 2/3 the wavelength of red light, you can see that this is chromatic aberrations big brother.

Diffractive elements are very wavelength dependent so while something like this would only work for monochromatic light (like a laser), but not for multiwavelength light. For that you would need to do some colordiversity.

It’s not. The mechanism by which it acts is quite different. The lens is a metasurface lens, which has small structures that cause the phases of light to constructively or destructively interfere. The result is caused by the fact that, by carefully picking how light interferes, one can form an image on the other side of the lens. This differs from a Fresnel lens (or other classical lenses), which essentially form images by having rays of light which emerge from one spot, on one side of the lens, converge to another spot on the other side.

(In particular, wave theory is not needed to predict the behavior of classical lenses.)

> [...] which has small structures that cause the phases of light to constructively or destructively interfere

This is slightly unclear. I meant more specifically:

[...] which has small structures that change the phase of the light, which, in turn, causes it to constructively or destructively interfere [...]

Well, a fresnel lense (or any lense) also just changes the phase of which in turn causes it...

But i will read the paper later.

Yeah, I should have said "subwavelength" structures, since I really didn't specify what "small" meant, but thought it was getting too technical.

You can use the Fresnel lens in an old projection TV as a laser.

https://www.youtube.com/watch?v=OmlXR2nsis4

Laser is light amplification by stimulated emission of radiation. Fresnel lens doesn't amplify light it focuses it. It is not stimulated, it is just passive component.

This is true, but I don't think many people think of laser as anything other than "dangerous light." That's how I used it. Colloquial meanings can be frustrating when they clash with the technical meaning, but trying to fight them is typically futile.

https://www.merriam-webster.com/dictionary/laser

>> "2 : something resembling a laser beam in accuracy, speed, or intensity "

I gave up on fighting "CPU means the entire computer" a long time ago.

> This is true, but I don't think many people think of laser as anything other than "dangerous light."

"Many people" have taken first year physics, or at least read a wikipedia article, and understand that the defining feature of a laser is its coherence.

But, sure. It's 2020. We live in a post-factual society, words have no meaning and it's impossible to know anything. Why resist?

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