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Pointing from a distance

O

opr

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I want to test how the laser looks from a distance.
Can pointing directly on someone from 1Km with 200mw Green laser do any damage? (Lets say I mesured the distance with Google earth before trying).

Same for 1w Blue laser?
 





ARG

ARG

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Depends on the power density! If you have a large beam, and a large divergence it will likely not exceed the MPE (maximum permissible exposure) If you have a really small beam with an amazing divergence it will likely exceed the MPE.

Without knowing more about the lasers I cannot say.
 
Atomicrox

Atomicrox

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Interesting.

If that's correct you can keep staring at even the most powerful handheld visible lasers if the dot is larger than say 80cm (1mW/cm^2 @ 5W). What's the catch?

I've never looked at my 1100mW's dot indoors without goggles even while defocused but this is getting me tempted. What about the whole blue light hazard thing?
 
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opr

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Ok. I did some research and learning. And here are my conclusions:

Let’s talk about a super high quality laser as an example, with the following spec:
Green laser
-200 mw
-Beam diameter - 1mm (Typically it is 1.5mm and above)
-Divergence 0f 0.5mrad (Typically it is 1 and above for Green laser)
-1000 Meter distance.
-Assuming air do not make any loses

Calculation will be done in cm:
Diameter of the beam will be 100000*tan(0.0005)+0.1 = 50cm
Beam coverage area: (50/2)^2*PI=1963 cm2 (Assuming a round beam, which is the most powerful)
Power per cm2: 200mw/1963cm2=0.1mw/cm2

According to the following graph (File:IEC60825 MPE W s.png - Wikimedia Commons)
This amount of power is completely safe
and in fact you can look at the laser directly
without any problems.

Blue laser
-1000 mw
-Beam diameter - 1mm (Typically it is 1.5mm and above)
-Divergence 0f 0.4mrad (Typically it is 1 and above for Blue laser)
-1000 Meter distance.
-Assuming air do not make any loses

Calculation will be done in cm:
Diameter of the beam will be 100000*tan(0.0004)+0.1 = 40cm
Beam coverage area: (40/2)^2*PI=1256 cm2 (Assuming a round beam, which is the most powerful)
Power per cm2: 1000mw/1256cm2=0.8mw/cm2

Still, you can look at it from this distance without any damage.

For fun, here is the calculation for a distance of 5m:
Green laser
-200 mw
-Beam diameter - 1mm (Typically it is 1.5mm and above)
-Divergence 0f 0.5mrad (Typically it is 1 and above for Green laser)
-5 Meter distance.
-Assuming air do not make any loses

Diameter of the beam will be 500*tan(0.0005)+0.1 = 0.35cm
Beam coverage area: (0.35/2)^2*PI=0.096 cm2 (Assuming a round beam, which is the most powerful)
Power per cm2: 200mw/0.096cm2=2078mw/cm2

SUPER DANGEROUS.
IMMEDIATE DAMAGE!!!!
 
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opr

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Hi Atomicrox.

I thought I miscalculated something, so I delete the message. But eventually it was ok.
I added a short distance calculation.
There is a huge difference as you can see.
Short distance is very dangerous.
 
RA_pierce

RA_pierce

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opr said:
Green laser

Power per cm2: 200mw/1963cm2=0.1mw/cm2

Blue laser

Power per cm2: 1000mw/1256cm2=0.8mw/cm2
Click to expand...


Using these numbers we can calculate approximately how much light is entering the eye.
According to this: The Pupils - Clinical Methods - NCBI Bookshelf
The minimum pupil diameter is 2-4mm and maximum is 4-8mm.
Using the average of these numbers gets us 3mm min. and 6mm max.

Then the total amount of light (mW) entering the eye would be the power per cm^2 you calculated above multiplied by the area of the pupil.

Green laser: 0.1mW/cm2 = 0.01mW/mm2

Minimum pupil diameter 3mm (radius 1.5mm): 3.14 x 1.5^2 x 0.1 -->
7.06 x 0.01 = 0.07mW total entering the eye

Maximum pupil diameter 6mm (radius 3mm): 3.14 x 3^2 x 0.1 -->
28.26 x 0.1 = 0.28mW total entering the eye

Blue laser: 0.8mW/cm2 = 0.08mW/mm2

Minimum pupil diameter 3mm (radius 1.5mm): 7.06 x 0.08 = 0.56mW total entering the eye

Maximum pupil diameter 6mm (radius 3mm): 28.26 x 0.08 = 2.26mW total entering the eye

If we use the figure of 8mm diameter (4mm radius) for maximum pupil dilation that results in:
Maximum of 0.5mW entering the eye from 200mW green laser
Maximum of 4.01mW entering the eye from 1000mW blue laser

Considering the fact that the lens of the eye focuses light on the retina, there still is a chance of some damage with higher power lasers. At the very least it would not be comfortable to look at with dark-adapted eyes.

Does this seem correct?
 
Last edited:
O

opr

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Unfortunately, you have a mistake in a factor of 10.
0.1mw/cm2 = 0.001mw/mm2 (You have to divide by 100)
Lets stay in cm.
According to Wikipedia, when the pupil is max opened it is 4-9 mm. Let’s take 9 mm which is 0.9cm.
So the power entering the eye with blue laser is:
(0.9/2)^2*pi*0.8=0.5mw.
 
RA_pierce

RA_pierce

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opr said:
Unfortunately, you have a mistake in a factor of 10.
0.1mw/cm2 = 0.001mw/mm2 (You have to divide by 100)
Lets stay in cm.
According to Wikipedia, when the pupil is max opened it is 4-9 mm. Let’s take 9 mm which is 0.9cm.
So the power entering the eye with blue laser is:
(0.9/2)^2*pi*0.8=0.5mw.
Click to expand...

Oops. :eek:

So at 1km distance it's relatively safe to view the beam directly.
Might I ask what you plan to do with this information?
 
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opr

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I was just curious to see how a laser looks from a large distance.
Can it be seen? Can it be seen from 5 km?
All this laser stuff is not very practical. It is just pure fun to experiment with it.
 
RA_pierce

RA_pierce

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opr said:
I was just curious to see how a laser looks from a large distance.
Can it be seen? Can it be seen from 5 km?
All this laser stuff is not very practical. It is just pure fun to experiment with it.
Click to expand...

I agree.
There are some videos on youtube of people shining lasers over a few miles (not sure exactly how far).
I've been able to put a spot on a mountain about 10 miles (about 16km) from where I live. It was just visible with a telephoto lens on my camera and good visibility (it's usually smoggy).
The laser was >650mW 532nm with a 10x beam expander. At that distance the beam should have been about 2 meters across.

I'm sure that if someone was on the other end they would have had no problem seeing it.
 
Atomicrox

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Wikipedia said:
The maximum permissible exposure (MPE) is the highest power or energy density (in W/cm2 or J/cm2) of a light source that is considered safe, i.e. that has a negligible probability for creating damage. It is usually about 10% of the dose that has a 50% chance of creating damage[8] under worst-case conditions. The MPE is measured at the cornea of the human eye or at the skin, for a given wavelength and exposure time.

A calculation of the MPE for ocular exposure takes into account the various ways light can act upon the eye. For example, deep-ultraviolet light causes accumulating damage, even at very low powers. Infrared light with a wavelength longer than about 1400 nm is absorbed by the transparent parts of the eye before it reaches the retina, which means that the MPE for these wavelengths is higher than for visible light. In addition to the wavelength and exposure time, the MPE takes into account the spatial distribution of the light (from a laser or otherwise). Collimated laser beams of visible and near-infrared light are especially dangerous at relatively low powers because the lens focuses the light onto a tiny spot on the retina. Light sources with a smaller degree of spatial coherence than a well-collimated laser beam, such as high-power LEDs, lead to a distribution of the light over a larger area on the retina. For such sources, the MPE is higher than for collimated laser beams. In the MPE calculation, the worst-case scenario is assumed, in which the eye lens focuses the light into the smallest possible spot size on the retina for the particular wavelength and the pupil is fully open. Although the MPE is specified as power or energy per unit surface, it is based on the power or energy that can pass through a fully open pupil (0.39 cm2) for visible and near-infrared wavelengths. This is relevant for laser beams that have a cross-section smaller than 0.39 cm2. The IEC-60825-1 and ANSI Z136.1 standards include methods of calculating MPEs.[4]
Click to expand...

Since the MPE is calculated at the cornea there's no need to compute the light that goes through the pupil.

This is also a worst case calculation so in reality lasers should be a lot safer... This is leading me to believe long range/defocused laser danger is highly overestimated around these forums and by the media in general.

I'd really like one of the more experience members to shine a light here, especially if we're missing something!
 
RA_pierce

RA_pierce

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I think that a number for how much light is entering the eye is easier to relate to than a number for power density.
Just meant to give a different perspective rather than just going by the charts.
 
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