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# long distance visibility of lasers?

#### ixfd64

##### New member
According to Sam's FAQ, humans can see something as dim as 8*10[sup]-16[/sup] W/cm[sup]2[/sup] (8*10[sup]-12[/sup] W/m[sup]2[/sup]). This would mean that a 100 mW laser with a 1 mm beam diameter and a divergence of 1 mrad would be visible at 63,000 km!

However, this would only be the case in a perfectly dark environment. Realistically, there are many other factors involved, and the light from the laser would probably not be visible beyond a few hundred miles at most.

Ignoring the curvature of the Earth, does anyone know how far the light from the following lasers would be visible, assuming that the air is very clean and there is no light pollution (like in the desert at night)?

1. A 150 mW green laser with a beam diameter of 0.8 mm and a beam divergence of 0.85 mrad (such as an "Alpha 125" laser from NOVAlasers)

2. A 550 mW green laser with a beam diameter of 14 mm and a divergence of 0.08 mrad (like a "Hercules" being used with a beam expander)

3. A 15 W laser with a beam diameter of 3.5 mm and a divergence of 1.7 mrad

(Yeah, I know they're starting to look like problems from a textbook, LOL.)

#### moond0ggie

##### New member
A 15 W laser with a beam diameter of 3.5 mm and a divergence of 1.7 mrad
You could put a 2 meter dot on the moon!

#### Murudai

##### New member
By the time you got to the moon, it would be several km wide.

And this as been gone over lots of times It takes a laser of scientific research grade to get to the moon, and even then you need very special high sensitivity detectors and mirrors already on the moon to 'see' it. There is no way, even with the most powerful lasers, for you to see the dot with the naked eye.

If you had a laser powerful enough to see a visible dot on the moon, the moon would probably explode

#### FireMyLaser

##### Well-known member
But what if one was on the moon, would the laser light be visible to him coming from the earth?

I mean city lights are visible from the space station, so a high power laser must be spectacular!

#### Maven

##### Member
ROFL what do you think NASA would do if they found you shining laser at the space station?

you would be seized and held as a terrorist lol

... think shining at a helicopter X 1,000,000

hehe

#### rathat

##### New member
What if you made it so a lens focuses it so the focal point is exactly half way there?

#### sdfrycz

##### New member
What if there was an eclipse and the moon was dark/er
lol, still doubt it i guess

#### Beren

##### New member
About the moon thing:  "I used to be able to sparkle off the new moon with my YAG at full power and full convergence. It takes some doing but you can see the sparkle from the Sea of Tranquillity with the naked eye off the corner cube reflector, aka: retroreflector left there in 1969 by the astronauts."  -Steve Quest

http://www.repairfaq.org/sam/laserlia.htm#lialls2

When he says "YAG," he's talking about a green, of course.

#### ixfd64

##### New member
Wow, that's really cool! It probably requires a very high power, though.

Anyone here have a 50 W greenie?