Welcome to Laser Pointer Forums - discuss green laser pointers, blue laser pointers, and all types of lasers

Laser Pointer Store

# Pointing from a distance

#### Atomicrox

##### Well-known member
That experiment sounds iffy:
-8ft after 5.2km is less than 0.5mRad. Very unlikely for a regular green pointer.
-An 8ft dot of 5mW power should be very, very dim. Unfortunately I don't have a focusable one here to test but I seriously doubt it'd be painful to look at.

It's possible they measured wrong and had a way overspec laser, though.

Last edited:

##### New member
And if you look at the picture with the beam on the ground..that was taken in daylight how can a 5Mw laser do that? And the the other pictures seem to be taken at nighttime.

#### steve001

##### Well-known member
And if you look at the picture with the beam on the ground..that was taken in daylight how can a 5Mw laser do that? And the the other pictures seem to be taken at nighttime.
That one photo was not taken in daylight. Scrutinize it again.

##### New member
That one photo was not taken in daylight. Scrutinize it again.
Looks likes blue sky though the trees to me but I could be wrong.

#### Atomicrox

##### Well-known member
Sorry for the late reply, didn't have time to read the whole PDF till today.

The safe distance formula on the PDF is slightly different (and gives different results) than other NOHD formulas out there. I'm not sure how much we can actually trust those formulas :/

It also has an interesting formula to compute "The flux of energy through a point at a given distance and angle from a diffusely reflecting surface":

H = Pλ*Q*cos(Φv) / π*r^2

where Pλ is the reflectance at the wavelength in question, Q is the energy of the incident beam, Φv is the angle of observation with respect to the normal, and r is the distance from the point of reflection to the point of observation. For safety calculations this equation can be greatly simplified by assuming Pλ = cos Φv = 1.

According to their example it's safe to view a diffuse reflection of a 2W visible laser from 40cm away for the duration of the blink reflex (0.25s).

If we do the calculation in W using the MPE for continuous exposure (1mW/cm^2) the minimum safe distance for a visible 5W laser would be 40cm. If this is true looking at the diffuse eflection of even the highest power visible lasers from at least that distance should be safe. I'd take the calculation with a grain of salt because the equation is for energy (J), not power (W). It would make sense to assume it holds for power but since this is a safety calculation it would be better to confirm. Also beware that most surfaces also have at least a bit of specular reflection.

Edit: that also means using lasers to burn stuff from short distances without goggles isn't safe even if the surface is completely matte.

Last edited:

Matt.

#### Attachments

• 52.1 KB Views: 987

#### opr

##### New member
This is a great chart. Thanks.
I thought buying a blue laser. But according to this chart, there is no point if the purpose is to point on distant objects.