Re: Stargazer arrested for using green laser

From: canopus56 (canopus56_at_yahoo.com)
Date: 01/07/05 

Date: 7 Jan 2005 14:26:35 -0800

Larry Stedman wrote:
<snip> Something about distance and beam diffusion, etc.
> It'd be interesting to hear from those who posted that what they make
of
> all this.

My primary interest in the green laser threads was to better
understand how to compute the size of hazard zones and the likely
vision effects within those zones when using consumer Class IIIA lasers
at star parties. The following computations may be a useful
quanitification on the effects zones for consumer Class IIIA lasers
with respect to aircraft dicussed in this and other threads.

In summary, with respect to a star party, the flash blindness zone
for a Class IIIA laser probably is around 270 feet; the glare annoyance
effect zone (applicable to your aircraft question) probably ends at
around 1200 ft. Additionally, other star party attendees will
reflexively blink if their eyes are struck by a consumer Class IIIA
laser up to 50ft away. (For the aircraft scenario, I seem to recall
that FAA visual flight regulations require aircraft to maintain an
altitude of at least 1000 ft above of the ground at all times, except
during landing and take-off.)

This estimate is based on my informal amateur calculations for a
Class IIIA laser ( 5mWatts< and 1 milliradian dispersal) and is
consistent with FAA published effects for the biological effects of
consumer lasers.

A FAA biological laser effect study lists two key emission levels of
concern by consumer laser users who want to avoid annoying community
members:

1) Flash-blindness and after images can occur to luminosity levels
of 1 x 10-4 Watts^cm.

2) Annoyance glare occurs to 5 x 10-6 Watts^cm.

The following table is the source for these effects and emissions
levels. (The MPE Level column in the following table is relevant to the
following discussion. The Nominal Hazard Zone in this reference
applies to non-consumer Class IIIB and IV lasers.)

================================================
Table 1 - Excerpt from Rockwell "Safety Recommendations of Laser
Pointers"

Rockwell, Jr., James R., and Ertle, William J., Rockwell Laser
Industries, Moss, C. Eugene, National Institutes of Occupational Safety
& Health. Undated. Safety Recommendations of Laser Pointers. <<
http://www.rli.com/resources/pointer.asp undated, accessed 1/5/2005 >>

The NHZ associated with open-beam Class IIIB and Class IV laser
installations can be useful in assessing area hazards and implementing
controls. The summary in Table 1 shows the magnitude of the NHZ's for a
visible frequency laser pointer emitting 5mW:

Table 1

Nominal Hazard Zones for Visible Diode Pointers
Power: 5 mW - Divergence: 1 mr - beam size: 2 mm
Based on: FAA 74002D Outdoor Laser/High Intensity Light Demonstrations

Bioeffect MPE Level * Nominal Hazard Zone
Condition (W/cm2 ) (feet)

BLINK REFLEX 2.6 x 10-3 51

FLASH-BLINDNESS
/AFTERIMAGE 100 x 10-6 262

GLARE 5 x 10-6 1,171

"NO EFFECT" 50 x 10-9 11,707

[ MPE = Maximum premissible exposure ]

The different visual effect "MPE" criteria used in the NHZ computations
in Table 1 were are based upon the U.S. Standard for the Federal
Aviation Administration: FAA 74002D Outdoor Laser/High Intensity Light
Demonstrations which provide a numerical basis for the various
bioeffect criteria [6].

(6) FAA 74002D Outdoor Laser/High Intensity Light Demonstrations: From
Chapter 34: Outdoor Laser/ High Intensity Light Demonstrations, Federal
Aviation Administration

=================================================

Class IIIA consumer lasers emit less than 5 milliWatts and have a
dispersion of 1 milliradian. The following table is my informal
computation showing the diameter of the beam and the energy density at
various distances. These numbers overstate the hazard and annoyance
zones, since the initial beam diameter assumed to be 0 mm. (1 or 2 mm
initial beam size would be a more reasonable assumption.)

The key columns in the following table at the distance to the target
(Dt ft) and the brightness of the emission at that distance (W^cm).
Compare the W^cm column with the FAA biological effect limits.

========================
Table 2 - Informal computation of laser brightness at distance

Brightness computation for Class IIIA lasers
5mW< initial power and 1 milliradian dispersion

Dt ft Dt m Bdiain Bdiacm BAreain BAreacm W^cm Per dlt_mag
50 15 0.6 2 0.3 1.8 2.75E-03 55.08% 8.1
100 30 1.2 3 1.1 7.3 6.89E-04 13.77% 9.7
175 53 2.1 5 3.5 22.2 2.25E-04 4.496% 10.9
200 61 2.4 6 4.5 29.0 1.72E-04 3.443% 11.2
275 84 3.3 8 8.6 54.9 9.10E-05 1.821% 11.8
300 91 3.6 9 10.2 65.4 7.65E-05 1.530% 12.0
500 152 6 15 28.3 181.6 2.75E-05 0.551% 13.1
825 251 9.9 25 77.0 494.3 1.01E-05 0.202% 14.2
1000 304 12 30 113.1 726.2 6.89E-06 0.138% 14.7
1100 334 13.2 33 136.8 878.7 5.69E-06 0.114% 14.9
1500 456 18 46 254.5 1634.0 3.06E-06 0.061% 15.5
2000 608 24 61 452.4 2904.9 1.72E-06 0.034% 16.2
3000 912 36 91 1017.9 6535.9 7.65E-07 0.015% 17.0
4000 1216 48 122 1809.6 11619.5 4.30E-07 0.009% 17.7
5000 1520 60 152 2827.4 18155.4 2.75E-07 0.006% 18.1
6000 1824 72 182 4071.5 26143.8 1.91E-07 0.004% 18.5
7000 2129 84 213 5541.8 35584.6 1.41E-07 0.003% 18.9
8000 2433 96 243 7238.2 46477.8 1.08E-07 0.002% 19.2
9000 2737 108 274 9160.9 58823.5 8.50E-08 0.002% 19.4
10000 3041 120 304 11309.7 72621.6 6.89E-08 0.001% 19.7
11000 3345 132 334 13684.8 87872.1 5.69E-08 0.001% 19.9

Dt ft = Distance feet
Dt m = Distance meters
Bdiain = Beam diameter inches at distance
Bidacm = Beam diameter centimeters at distance
BAreain = Beam area inches at distance
BAreacm = Beam area centimeters at distance
mW^cm = milliWatts per square centimeter
Per = change in milliWatts per square centimeter from
distance zero (5mW) to distance
dl_mag = same as Percent, expressed in changes in magnitude
delta_mag (m1 - m2) =

2.5 * log_10 * ( mW^cm / 5mW at zero distance )

(Note delta_mag is not the apparent magnitude, since
magnitude at distance zero is not known)

========================

With respect to an inadvertent aircraft strike from using a
consumer laser at a star party, at 1500 ft, a consumer Class IIIA laser
has already dropped to well-below the emission level needed to cause an
FAA measured flash blindness/after image effect (1 x 10-4 W^cm) and is
below the FAA glare effect level (5 x 10-6 W^cm).

As others have noted in the various green laser threads in this
usenet group, the 5mWatt emission of a Class IIIA laser is an upper
limit. Many consumer Class IIIA lasers used by amateur astronmers
operate at much less than 5mWatts. If your consumer laser pointer or
other astronomical laser product (like a laser collimator) has a tag on
it that says "CAUTION", it operates at less than 2.5 mWatts. If your
consumer laser pointer has a tag on it that says "DANGER", is operates
between 2.5 mWatts and 5 mWatts.

For comparision purposes, the following is a recomputation of Table 2
for a Class IIIA consumer laser emitting 2.5 mWatts:

========================
Table 3 - Informal computation of laser brightness at distance

Brightness computation for a Class IIIA laser emitting 2.5m Watts
initial power and 1 milliradian dispersion

Dt ft Dt m Bdiain Bdiacm BAreain BAreacm W^cm Per dlt_mag
50 15 0.6 2 0.3 1.8 1.38E-03 55.08% 8.1
100 30 1.2 3 1.1 7.3 3.44E-04 13.77% 9.7
175 53 2.1 5 3.5 22.2 1.12E-04 4.496% 10.9
200 61 2.4 6 4.5 29.0 8.61E-05 3.443% 11.2
275 84 3.3 8 8.6 54.9 4.55E-05 1.821% 11.8
300 91 3.6 9 10.2 65.4 3.83E-05 1.530% 12.0
500 152 6 15 28.3 181.6 1.38E-05 0.551% 13.1
825 251 9.9 25 77.0 494.3 5.06E-06 0.202% 14.2
1000 304 12 30 113.1 726.2 3.44E-06 0.138% 14.7
1100 334 13.2 33 136.8 878.7 2.85E-06 0.114% 14.9
1500 456 18 46 254.5 1634.0 1.53E-06 0.061% 15.5
2000 608 24 61 452.4 2904.9 8.61E-07 0.034% 16.2
3000 912 36 91 1017.9 6535.9 3.83E-07 0.015% 17.0
4000 1216 48 122 1809.6 11619.5 2.15E-07 0.009% 17.7
5000 1520 60 152 2827.4 18155.4 1.38E-07 0.006% 18.1
6000 1824 72 182 4071.5 26143.8 9.56E-08 0.004% 18.5
7000 2129 84 213 5541.8 35584.6 7.03E-08 0.003% 18.9
8000 2433 96 243 7238.2 46477.8 5.38E-08 0.002% 19.2
9000 2737 108 274 9160.9 58823.5 4.25E-08 0.002% 19.4
10000 3041 120 304 11309.7 72621.6 3.44E-08 0.001% 19.7
11000 3345 132 334 13684.8 87872.1 2.85E-08 0.001% 19.9

========================

For the 2.5mWatt laser, the flash-blindness zone (1 x 10-4 W^cm)
ends around 200ft; the glare effect zone (5 x 10-6 W^cm) around 825ft.

I hope the above provides some illumination ( minor pun intended -:)
) on this discussion, and promotes safe consumer Class IIIA laser use
at star parties.

- Canopus56

P.S. - I have no special physics training or background.

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