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When I've been to Astronomy-nights for the public at the local observatory, they always used green lasers to point things out.

Why is there such a preference for green lasers?

What's wrong with red lasers? (Which seem cheaper and more readily available.)

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closed as off-topic by Walter, Donald.McLean Aug 25 at 16:23

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I would guess it is because the human eye sees green better than red in low light conditions. Though the pointer usage of a few local astronomers isn't exactly proof that they all prefer green. –  zibadawa timmy Aug 12 at 1:34
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@zibadawatimmy - that's the shortest and best answer. Green lasers are easily visible at powers that are not dangerous. A red or blue laser would require much more power to be as visible as green, and the more power you put into the beam the more easily things could go wrong. That's all there is to it. –  Florin Andrei Aug 24 at 4:02
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this question is not about astronomy –  Walter Aug 24 at 15:02
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From the FAQ: "The purpose of this site is to provide expert level answers to questions on: Setting up, using and maintaining your astronomy related equipment" –  abelenky Aug 25 at 23:11
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@Walter I'm sorry, but saying this is off topic is like saying a question asking how a Bayer lens works is off topic. A green laser pointer is a piece of astronomical kit and questions about it should be allowed. –  Beta Decay Aug 27 at 8:29

3 Answers 3

up vote 10 down vote accepted

Mostly general purpose laser pointers are used for pointing things at smaller distances eg. Diagram or Equations in Powerpoint presentations, so the power of such laser pointers is quite limited/restricted to 5mw ( Class 3A or IIIa) or 10mW in some regions. Because of the low power and small aperture of laser pointers if you point them through empty space, you can only see them impact spot where it hits a surface.

So to point stars/planets as in astronomy club during observation sessions these low power laser pointers are not useful as there is no surface where the beam can hit & you can see the spot.

Some higher-powered laser pointers project a visible beam via scattering from dust particles or water droplets along the beam path. Higher-power ( Class 3B or IIIb lasers: greater than 5mW) and higher-frequency (Green or Blue) lasers may produce a beam visible even in clean air because of "Rayleigh Scattering" from air molecules, especially when viewed in moderately-to-dimly lit conditions. The strong wavelength dependence of the scattering (~λ^ −4 ) means that shorter (green & blue) wavelengths are scattered more strongly than longer (red) wavelengths.

[ Rayleigh scattering is the mainly the elastic scattering (in lay term collision) of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. Rayleigh scattering results from the electric polarizability of the particles (locally seperating positive & negstive charges by small distance apart inside the atom. i.e. temporarily separating centre of positive charge and centre of negative charge). The oscillating electric field of a light wave acts on the charges within a particle, causing them to move at the same frequency (hence the green colour). The particle therefore becomes a small radiating dipole (or small sources of light) whose radiation we see as scattered light (the beam passing from pointer to star pointed). ]

The intensity of such scattering increases when these beams are viewed from angles near the beam axis. (Thats the reason, if you standing close to the person pointing stars in astronomy club, you can see the beam bright n clearly whereas the ppl standing away can barely see it. So always stand near the presenter But avoid contact with your eye. :-) )

The apparent brightness of a spot from a laser beam depends on the optical power of the laser, the reflectivity of the surface, and the chromatic response of the human eye. For the same optical power, Green Laser light will seem brighter than other colors because the human eye is most sensitive at low light levels in the green region of the spectrum (wavelength 520–570 nm). The most sensitive pigment, rhodopsin, has a peak response at 500 nm. Sensitivity decreases for redder or bluer wavelengths.

Further, Green laser pointers are of moderate power (sufficient for Rayleigh Scattering in clean Air), they are compact and relatively cheap than blue one (even though human eye is less sensitive to blue colour, blue laser are little bit more expensive than green one.)

Avoid their contact with your eyes and dont point them at aircrafts. People have been given up to five years in jail for aiming a green laser at an aircraft.

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You gave a very long and partially wrong answer to a really simple problem. Scattering that makes laser beams visible is not via the Raleigh mechanism, it's simple scattering on dust motes and water droplets, so it's independent on wavelength. The reason why green lasers are preferred is simply because they require less power to produce the same visual impression. A red or blue laser would need to be more powerful to be as visible as green, and more power can be unsafe. Green is very visible at safe powers, that's all there is to it. –  Florin Andrei Aug 24 at 4:06

Green laser pointers are great for astronomers because at the most common power output (5-15mw for consumer grade 500-532nm laser) the beam is visible in most "dark sky" settings. Cost for a simple green laser with a visible beam ~$15

Red lasers on the other hand have an average power output of about 3mw, most commonly denoted with the consumer in mind as, < 5mw. For a red (632-660nm) to have a visible (to the unaided eye) beam, the laser would need to have a significantly higher output, on the order of 50-150mw. Cost for a "simple" red laser with a visible beam ~$200

The problem with that is that any laser with > 15mw output is considered class IV and there are heavy restrictions on these in most countries. There are a few places that get around the class IV restrictions and sell them as the lesser class IIIb, however to my knowledge there are no red lasers floating around that fit this category. Cost for "high powered class IIIb" red laser ~$70

With that being said, there is a preference for green lasers for both cost and safety reasons.

I personally prefer my red laser, however making everyone put on dark protective glasses at night is both over complicating a simple task of pointing out stellar objects, and at the same time hilarious, watching people try to figure out what the laser is pointing at, as the beam is all that they will see with the eye-ware on. Without safety goggles looking indirectly at the beam from the 12Mw laser can cause serious damage... again, that is why astronomers use a simple green laser.

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It's also perhaps worth noting that blue would generally be even more visible than green. You don't see blue laser pointers generally because they're not anywhere near that $15 price point at the present time. –  Shinrai Aug 22 at 5:02
    
@Shinrai That is not really true, as the peak sensitivity for human eyes is roughly around 550nm which is green. This means that as the wavelength decreases to a shorter wavelength, more blue, there is an logarithmic falloff in sensitivity. Think of a roller coaster, the incline is like light approaching from the red, as it reaches the peak in green, there is a steep drop off toward violet, which is far faster than the incline on the other side. Take a look as a black body diagram for a better idea of what I am referring to. –  BanzaiAstronomy Aug 23 at 15:24
    
That's correct, but I was referring to a laser shining up into the clear sky - blue is going to scatter more than green, so the actual visibility of the beam itself will be higher (you can't see a laser that isn't scattering, after all). At least, my subjective impression having actually seen them is that the scattering is a stronger effect in this situation than the change in your visual sensitivity to different wavelengths. EDIT: Also I'm not sure if the visual sensitivity of your eyes really looks anything like a blackbody...theres overlap between the cones that's mostly in the green. –  Shinrai Aug 23 at 22:07
    
@Shinrai - no, that's not how it works at all. Scattering that makes the laser beam visible is mostly on dust particles and water droplets, so it's not dependent on wavelength. The only thing that matters is visibility per watt, and that's maximal in green. Take three laser diodes of equal power - red, green and blue - fire them at the same time, and you'll see what happened. Green is preferred because it's easily visible at powers that are not potentially dangerous, that's all. Red and blue require much more power to create the same subjective perception. –  Florin Andrei Aug 24 at 4:00
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@FlorinAndei If you have such an important opinion that you have to point it out directly to the other answers, why don't you just add an answer? –  Beta Decay Aug 24 at 8:06

Have you ever tried shining a red laser up into the sky? If you have, you see nothing. Now, if you try shining a green laser up into the sky, you see the beam streaking off into infinity.

This is because green lasers are much stronger than red lasers. Red lasers are used so often in everyday life because they aren't very dangerous.

The colour red is near the infrared spectrum, which is a lower energy than the visible spectrum. This means that, when a photon of red laser light hits a particle of air or dust, it is just absorbed.

The colour green, however, is much further along the spectrum, meaning that it is a higher energy than the colour red. When a photon of green laser light hits a particle of air, it is absorbed. Then, because it is so high energy, it is re-emitted. Or, in lay mans terms, the beam reflects off the air particles.

This scattering is known as Rayleigh Scattering.

This reflection is used so astronomers can trace the beam up to where is being pointed. This is useful for finding stars, galaxies and a whole manner of other celestial objects.

P.S.

If you wanted the most powerful visible laser, you would have to go for a near ultraviolet laser. Of course, to generate such a beam, you would need power that is illegal in most countries.

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Green lasers are not "stronger" than other lasers. They are just more visible at the same power, because the eye is more sensitive to green light. That's all there is to it - that's why they are preferred. –  Florin Andrei Aug 24 at 4:08
    
@FlorinAndrei Although 'stronger' was the wrong word to use, my point is to do with Rayleigh scattering: light at smaller wavelengths (and therefore higher energies, which was what I was talking about) scatters more than lower energies. –  Beta Decay Aug 24 at 8:02
    
Raleigh scattering is irrelevant to the topic at hand. –  Florin Andrei Aug 26 at 6:53

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