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abeinszweidrei t1_j7fajh1 wrote

Reply to comment by ellipsis31 in Why are green and red laser pointers so cheap and available, but yellow ones not so much? by SurprisedPotato

This, but I think there's even more to it: When checking a plot of wavelengths vs color, (eg https://www.gentec-eo.com/media/3583/commercial_laser_lines.png ) one can see that the region of wavelengths perceived as yellow or cyan is rather narrow compared to red, green, or blue. So if you want a yellow laser, you need a laser running pretty exactly at the desired wavelength, so when frequency doubling around 1140-1180 nm. It is doable, but red or green is just much easier as its a wider range (and suitable lasers have been lon established, e.g. Nd:YAG at 1064nm to be frequency doubled to 532nm, which is a great green).

And I think another point comes into play: yellow is just really not convenient as a laser pointer, when most slides have a bright background. Red and especially green can just be seen mich easier also with low power.

So I think in the end it's a combination of harder/more expensive to engineer and just little demand for it.

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zerpa t1_j7fe0eo wrote

And why can't you just mix red and green, just like we do on a monitor/TV? The eye does not require a specific wavelength to perceive yellow.

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psycotica0 t1_j7fm3es wrote

True, but the thing that makes lasers lasers and not just tiny flashlights is that they are a single coherent beam of uniform light. This is what allows them to behave reliably for engineering purposes and stay together over long distances, etc.

If I were to build a yellow laser by having a green laser and a red laser, it would be hard to get them to converge on exactly the same point. Or put another way, getting them to converge at a particular distance would be easy, but as soon as you moved slightly closer or further the dots would probably misalign and you'd end up with a red and green dot near each other. Even if the two beams were mirrored into the same trajectory, it's possible they'd refract while traveling due to their different wavelengths and end up as two dots at the end anyway.

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Obsidian_monkey t1_j7g37vl wrote

That lack of mixing is actually a feature for fiber optic communications. There are optics that use four lasers with different wavelengths to send four data streams simultaneously down the same fiber line.

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Octavus t1_j7hb075 wrote

One doesn't need lasers for that, as long as the medium is linear the mixing of light is just superposition. It is no different than transmitting multiple RF signals across multiple different channels at once.

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dramignophyte t1_j7iogxg wrote

I think the staying together property is the important aspect. Light diminishes by 4 times every time you double the distance. Except not really because its just that it disperses at that rate by virtue of spheres. Light itself on an individual scale is as far as I know, infinite. So if you convince them to stay in parallel formation, you can transfer them in a vacuum infinitely.

Lasers don't perfectly align the photons but they do a pretty darn good job. Like take a flashlight and shine it at a wall and step back and the light in the wall keeps getting bigger and bigger. Now I have not verified this next part and I am vaguely remembering what someone else said on reddit so the size probably is a bit different than what I'll say but for the distances its kind of moot. But the lazer on space probes only goes from the base lazer size to about the size of a car going from pluto to earth. So they are shining a little light at earth and it only splits a very small amount. They also mentioned there are designs for perfectly straight lasers but to go from like 99.998% to 99.999% is obnoxiously difficult considering pretty much nothing requires that level and the biggest obnoxious part is it requires a larger and larger lens, eventually reaching infinite size in order to make the perfectly straight lazer. Again, any specifics, take them as a general idea and not a point you want to bring up in casual conversation without adding "I heard it was something along these lines" because this is all "something along these lines" after the point I mentioned that I was parroting off someone from reddit. The points before that I know with much greater confidence.

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Isotope_Soap t1_j7h9ixz wrote

Oddly I’ve experienced this to some degree. I have a scoped air rifle with an illuminated reticle that is capable of red or green crosshair/dot/circle etc. It also has a red laser mounted slightly below the scope. Both are sighted in for 30 yards and when I select the green reticle and turn on the red laser, they do appear almost yellowish when on target at 30 yards. Any closer or father from 30 yards and they diverge, with the red laser dot being above or below the scope reticle. I used to find this frustrating until I realised the amount of divergence actually became a crude range finder of sorts.

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etcpt t1_j7grczf wrote

>Even if the two beams were mirrored into the same trajectory, it's possible they'd refract while traveling due to their different wavelengths and end up as two dots at the end anyway.

Do you mean that they would refract differently passing through an interface, or that the two beams would interfere with each other? It seems like the former should be able to be controlled on the device side as long as you are careful with the optics (though shining the laser through an interface would split the beams, but nothing we can do about that). You could probably cheat your way around inter-beam interference by using a pair of pulse-width modulated lasers set out of phase so that the beams don't overlap and relying on persistence of vision for the laser to be perceived as yellow.

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SmitZTheMitz t1_j7gdvy0 wrote

You certainly can. But it requires some fine tuning and a beam splitter. I have done this In my photonics lab. Really it just comes down to cost.

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mfukar t1_j7g1qjw wrote

Because neither red nor green, in terms of frequency/wavelength, are yellow. The colour wheel is about visual perception.

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zerpa t1_j7hnytj wrote

I don't understand your point. The brain can't tell the difference between yellow wavelength and the mix of red+green that give the same excitation of red/green photoreceptors.

I get that it's not a coherent light/laser anymore, but it should still be possible to create a yellow pointer, if you can join the two beams and align them precisely.

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dramignophyte t1_j7ip4he wrote

I was typing a wrong response but I can tell you why it doesn't work: lasers run parallel and it's kind of an important aspect of them. In order to mix something, they need to converge. So you need to adjust the focus point of the two lasers to account for distance. I was thinking maybe fiber optics but that wouldn't change the fact you can't change the position of light in that way or it will diffuse or not converge to mix the colors.

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_GD5_ t1_j7glkeu wrote

Lasers are not great light sources for displays. You have to do something to get rid of the coherence, otherwise you get a lot of speckles that either distract you or make you sick.

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abeinszweidrei t1_j7fjtda wrote

Good question, I don't know. Sounds like it could work. A quick search only gave me some single wavelength yellow ones though. My guess is that they are just more expensive. You'll need two lasers, and also optics compensated for the chromatic shift and suitably coated. That's straightforward, but probably just makes it too expensive for normal laser pointers. But yeah, i don't see a fundamental reason not to make it like this

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Lukaroast t1_j7hubjg wrote

Lasers have a lot of use outside slideshows though. It’s very possible that yellow could be useful outside of that narrow isecase

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-Raskyl t1_j7fswxd wrote

Yellow also just doesn't travel as far. Which is why from far away, tress on mountains appear blue, not green.

Or at least that's what I've always heard.

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EverlastingM t1_j7fyhq0 wrote

Consumer lasers aren't going to be powerful or well calibrated enough for this to be an issue. The main phenomenon is Rayleigh scattering, the same thing that causes blue sky/red sunset, so red would travel farthest, and a hypothetical yellow would travel farther than green. There are some other less common factors like air pollution that could change how this plays out.

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_GD5_ t1_j7glxpn wrote

No, it’s because the atmosphere is scattering blue light between you and the mountain. Yellow light is unscattered.

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rcxdude t1_j7goyf1 wrote

Which is adding blue light, not removing yellow light (though the scattering also removes some blue light coming from the mountains: the scattered blue light from the much brighter sun more than makes up for it)

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