Re: All helicopters should be grounded

Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:

M.I.5¾ wrote:
"Mike Scott" <usenet.11@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:nR09k.110753$NN3.105146@xxxxxxxxxxxxxxxx
stillnobodyhome@xxxxxxxxx wrote:
...
"Captain Westwood told he court he had suddenly been dazzled by an
intense green light that filled his cockpit.
Intense & filled the cockpit - from a handheld laser pen??? And green?
I've never seen a green laser pen advertised.

Then you haven't looked very hard. There are 4 models of green laser
pointer to chose from in the first catalogue that I grabbed.

I checked the CPC catalogue: 6 pens, 5 red and 1 unstated colour.

And there are some truly scary ones being offered by Chinese websites
for a price (talking here serious damage to plastics and paper at short
range).

The description of the effect is entirely consistent with being dazzled by a
laser. Because the beam doesn't significantly diverge, the light intensisty
per square milimetre is very high. Also the lens of the eye cocusses it
into almost an infintesimal spot. Both of these effects tend to make the
beam look intensely bright, and if it is powerful enough can cause retinal
damage to the eye, so don't try this at home.

The laser pens in the CPC catalogue have the notice "class II lasers
are not considered dangerous."
Otoh, the laser diode modules (also class II) have an eye protection
warning.
"Class II" seems to mean "visible, <1mW."
This suggests the pens have something that the modules don't.

"The overall effect was temporary blindness," he said. "I lost outside
visual reference and could not see the instrumentation displayed on
the aircraft.

"We continued to be zapped or illuminated by this laser a number of
times".

A hand-held laser? Given the low spread, the pointing has to be very
accurate - suppose a range of only 1/4 mile, cabin window, what, 10 feet
across? That's an accuracy of half a degree. Moving target - handheld????
I found it hard enough even seeing a red spot just down the length of our
garden - say 60ft. In free air, how would he see the beam to adjust
targetting?

I'm tempted to say I don't believe the account.

A green laser pointer beam is clearly visible in the air most nights.

Accuracy of half a degree is no harder than pointing a pair of
binoculars at the moon, or following a race horse.

You look through binoculars, which makes it easier.
(It's easier to aim a rifle than a pistol.)

Now here, I agree. It would be extremely difficult to direct a hand held
laser to the required accuracy. Aiming for an eye of (assumed) 5mm iris
diameter over an (assumed) quarter of a mile, works out at less than 1
second of arc (Divergance of the beam ignored, but would be of significance
from a laser pointer). I doubt that anyone could hit the target once let
alone multiple times. It should also be remembered that the laser beam from
a pointer is rarely co-axial with the pointer body and even at a quarter
mile, it would be quite impossible to see where the laser spot landed even
if it was on the helicopter.

Green lasers look significantly brighter for a given amount of power.
The eye is a lot more sensitive in the green. They are favoured by
astronomers for pointing out constellations (a legitimate use) at public
events since you can see the beam clearly due to backscatter from dust
in the air.

Yes, but can you read a book by the light of that backscatter?
Being able to percieve that light is a long, long, long, long, long
way, from it causing you difficulties. Think about driving a car at
night. Getting dazzled is common, and having the interior of the
car lit up by exterior sources is incessant.

Red ones by comparison are much harder to see.

However since the green laser is at a very well defined wavelength of
532nm the simplest fix for the police helicopter pilots is to give them
laser googles with a notch filter against that wavelength. Beam
divergence is typically 1-2 mrad so at 1000 feet distance the main beam
bright spot would be of the order of 1-2 feet across. It would
illuminate a fair chunk of the cabin - the pilots description is quite
plausible.

For a 1 mW, 5mm wide laser beam....

(inches) ( inch^2) (mW/inch^2)
Radius Area intensity[1]
0.1 0.03 33.3
0.2 0.12 8.3
0.4 0.5 2
0.8 2.0 0.5
1.6 8.0 0.125
3.2 32 0.03
6.4 128 0.008
12.8 515 0.002

So[2], a 1-2ft spot would be 4,000 - 16,000 times less
intense[1] than the near-pen beam.

[1] not strictly the proper term.
[2] given that I've got my sums right, which is rather unlikely.

But this is all academic. Surely the prosecution could take the
jury down to an airport [3], sit them in a 'copter (on the ground) and
demonstrate the effect (or lack thereof) of the laser pen. No?

[3] or bring a 'copter to a suitable site.

--
Sleepalot aa #1385

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