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Comment by cthulhuology | Hacker News Reader

Reading the article it hints at the reality that it doesn't actually work like passive glasses as it implies. the blueshift effect happens because they're using a laser to excite the crystal and it later emits the shifted light, but you have to pump a lot of energy in for that output. Simple conservation of energy implies this won't work for normal night vision applications, instead they want to shine a laser spotlight and use the reflected infrared laser light for vision. cool material science for sure, but not a replacement for nightvision goggles which don't broadcast your position with a bight spotlight.

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> The crystals act as antennaes that receive a photon emitted in the infrared, combine them with a photon from a laser and then "upconvert" the combined photon to the visible spectrum.

It doesn't sound like you need to illuminate the scene with a laser spotlight, it sounds like they will input a small amount of laser light into the crystal film on the glasses. The externally sourced IR photons will combine with the internally generated laser photons and emit a visible frequency photon.

The amount of laser energy pumped into the crystal film will probably be proportional to the amount of energy you want out of your "screen", i.e. brightness

Is there any chance that the "combined photon" will have the same direction as the original one? Otherwise there will be no image, just light.

> Simple conservation of energy implies this won't work for normal night vision applications

Not at all. For a single photon, sure. But in aggregate many low-energy photons can charge a material to emit fewer high-energy photons. Ex. https://www.newport.com/f/near-infrared-nir-sensor-cards

Yea, in fact I think it's the second law (no entropy decrease) rather than first law (energy conservation) which is the restriction. Devices up-converting photons to higher frequencies require power input.

But you can turn three infrared photons into a single red photon, if the energy levels add up.

That would, for instance, allow you to make safety goggles for detecting hot spots. Could be useful for firefighters or factory workers.

The problem with a passive device would be that if you walked past something hot enough your goggles would white out (well, red out), and you might be effectively blind. At least with the passive device you can turn it off briefly so you can see where you're going.

> But you can turn three infrared photons into a single red photon, if the energy levels add up.

You can't do this with a passive device, because the single visible photon has less total entropy than the three infrared photons. Yes, it satisfies the first law of thermo, but it violates the second.

You absolutely can, there are existing products that do exactly this. See the link in my post.

Thanks for emphasizing this, and I agree I need to qualify my statement. I haven't completely understood the restrictions yet, but the effectiveness of this device has to be conditional on the fact that the IR laser is a laser, or otherwise is in some low-entropy coherent state. Otherwise, it could be used to transform part of a uniform IR bath into high-frequency photons, passively generating a temperature gradient from nothing.

The incoherent case is the relevant situation when you're talking about passive glasses for viewing a scene in ambient IR light.

Here's one place where this is mentioned:

http://physics.stackexchange.com/questions/98790/what-low-le...

Still looking for an authoritative treatment...

> but not a replacement for nightvision goggles which don't broadcast your position with a bight spotlight.

To be fair that's exactly where NVD technology started out https://en.wikipedia.org/wiki/Zielger%C3%A4t_1229

At least the crystals are excited.

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