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# Distance of laser light traveled. How do you...

#### Bravestdog

##### New member
Distance of laser light traveled. How do you determine how far it will go?

Curious how you folks determine how far a laser light will travel so it can be seen?

Do you attach it to a tripod and start walking in the distance it is pointed and look back towards the laser...?

How does one determine how far the laser light will travel and still be seen with the naked eye?

Do you have to be careful when looking back towards the laser light?

Is there a distance where you are safe from eye damage for brief looks directly into the laser light?

Comments?

#### lazerlover

##### New member
Re: Distance of laser light traveled. How do you..

I'm also interested in this, I know it has to do with the ouput power of the laser, but I don't know the specifics. I do know that you need to be very careful when looking back towards the laser pointer to help prevent eye damage.

#### stevetexas

##### New member
Re: Distance of laser light traveled. How do you..

the light from a laser (or candle for that matter) will trave an almost infinite distance. How far away you can see it is another matter. If you are looking straight at it (don't do this) you could probably see it from over 1000 miles away or more (depends on the sensitivity of your eyes). the problem is that you are almost unable to look directly at anything from such distances. a slight movement in the laser or your eye will move it off the area of your eye that gathers light and will not be visible to you. The light will still be traveling towards you but you may not see it.

looking at a laser beam from the side is another matter entirely. a low powered laser beam can only be seen within a few degrees of the source. the higher the power, the greater the number of degrees away from the source you will be able to see it. I don't think this is a linear correlation though - it is probably an exponential curve...

#### Justin

##### New member
Re: Distance of laser light traveled. How do you..

Stevetexas: It's not really true that you will need to align your eye to the laser to see it from an extreme distance. All lasers spread as they travel, according to their divergence, so at any long range the beam will be quite wide and your eye will be able to detect it as long as the brightness is sufficient.

In fact, measuring the visible distance of a laser is one of the hardest things to do in this business. Laser retailers who list "range" as 50-60 miles are really misleading you, because while some light from the laser may technically reach a distance of 50 miles it is so spread out and diverged by that point that it is not detectable to the human eye. For example, a laser beam with 1.0mRad divergence will be about 100 feet wide at a range of 25 miles. A laser beam that is over 100 feet across is in no way visible or detectable by anything other than very sophisticated photodetector equipment.

Visible range is really determined by a combination of output power, divergence, wavelength (green is more visible than red, etc.), local air pollution, light pollution and your own eye's sensitivity. This is obviously not a simple equasion and there is no straight answer when it comes to range. I usually estimate range as either in the hundreds of meters (for 5mW greens) or in the thousands of meters (for high power handhelds). Anyone who claims to know the "exact" range of their laser is either mistaken or just not honest, because it's really not that simple.

#### stevetexas

##### New member
Re: Distance of laser light traveled. How do you..

I see where you're coming from Justin and agree.

My point was more that, brightness and divergence aside , the human eye is a very small target at 1000 miles ( a 100 foot wide beam doesn't even cover 1% of the sky at 1000 miles).

#### Justin

##### New member
Re: Distance of laser light traveled. How do you..

You're right, but a range of 1000 miles is basically meaningless when you're talking about a commercially available DPSS laser. Considering the curvature of the earth, there is no way to shine a laser for 1000 miles unless you or the target is in space.

#### RobTheVIP

##### New member
Re: Distance of laser light traveled. How do you..

I suppose it would be awsome to know the exact point where your laser is stopping, but is it really necessary when you have such a high output!?

#### Justin

##### New member
Re: Distance of laser light traveled. How do you..

The point is that there is no exact distance where your laser "stops", unless it hits something opaque. If you aim it into space, technically the photons will travel until they strike something or are absorbed by some form of matter. (Space dust, etc.) Theoretically a photon from your laser could reach Andromeda given enough time, regardless of the power or divergence of your laser. (2.52 Million years, to be exact.)

Aliens living in Andromeda would have a tough time detecting a single photon from planet Earth, mind you, but it's a neat thought. Oh, and it wouldn't be 532nm green anymore due to redshift, so you'd have to be looking for a more yellow-green photon at that point. I'm not enough of an astrophysicist to tell you the exact wavelength shift over 2.52 million lightyears, but if anyone can I'll be impressed!

#### Tallaxo

##### New member
Re: Distance of laser light traveled. How do you..

Yes Justin is correct. Its due to a phenomena known as doppler shift. you experience it with sound here on earth. Take a police car or ambulance with its siren on, when its coming towards you its pitch seems to increase until it goes past you, then its pitch seems to get lower. the pitch changes due to the frequency of the tone when its coming towards you (compresses the wave) to going away from you (stretching the wave).

Light has similar properties. a galaxy or star moving towards earths relative position, would appear to us to be more blue than it actually is (higher frequency , or compressed light) while a galaxy or star moving away from earths relative position would appear to be more red than it actually is (lower frequency or drawn out light).

Quite what the frequency of your 532nm light hitting Andromeda would be is totally dependant on how fast the galaxies are moving apart from each other. But as we know for certain that we are moving away from Andromeda, we can say for sure that the spectrum would be shifted further into the red by a few nm (redshift) making your laser appear more yellow than green.

Phew, that was a bit heavy :

Jase