Date: 2005-03-03 22:48:00
Tags: toys
green laser pointer fun
So I have a little bit of a weakness for certain kinds of random electronic gadgets. Right now it's laser pointers. I've been curious about the green ones ever since the news reports a few months ago about people accidentally illuminating the inside of airplane cockpits.

The other day I ordered a green one through ebay. This thing appears to be every bit as powerful as they claim! It's Class IIIa (normal for laser pointers), so it's supposed to have a 5 mW maximum output, but this thing is bright. It almost hurts to look at the spot on a white sheet of paper. In a dark room, you can see the beam in the air as it reflects off all the particulate matter in the air. Although it's the same power output as a normal red laser pointer, the human eye is much more sensitive to the green area of the spectrum, so it appears brighter.

There's one situation where this might actually be useful instead of just an amusement. If you're out stargazing with more people than just yourself, it's really hard to point out a certain star without resorting to directions like: "Okay, see that bright one there, and the slightly dimmer one down and to the right? Go down from there two more brightish ones and then look between that and the one over that way that has an orangeish tint." At night you can see the beam for at least hundreds of feet, so it's a great way to point at specific places in the sky.

There are mods you can make to this pointer to increase its power output, everything from simply changing a potentiometer accessible under the pushbutton, to opening it up and replacing the diode element. I don't have any particular need for more power, or desire to tear apart the pointer, so I'll keep it the way it is.

Anyway, it's sure a fun toy!
What do you think about those news reports - bogus pseudoscience and stupid reporting, or is it actually possible to blind pilots in an airplane flying past with a laser pointer?

Stuff that seems potentially fishy to me is: if the plane is over you, are you even able to see much if any of the cockpit glass from the ground? And can you accurately track the laser pointer on a fast-moving target that's high up in the sky?
I was thinking about some back-of-the-envelope calculations regarding this.

First, accurately aiming the laser pointer while holding it in your hand is hard. Being conservative, assume that you can aim within 1 cm of a fixed target at 10 m distance (this is based on me shining the beam on an opposite wall and guessing). An aircraft on final approach would probably be no closer than 1000 m, giving a target proximity of no better than 1 m. That's a bit larger than the size of an airliner cockpit window, so at the absolute best the beam would be moving around in and out of the cockpit (certainly not aiming at a fixed point). Also of course the aircraft is moving so the necessary tracking would make the targeting even more difficult.

Second, although the laser beam is well-collimated, it is not perfect. The specification for this laser claims less than 1.2 mrad (milliradians) beam divergence. This means that at distance d, the beam diameter is approximately 2*0.0012*d (the factor of 2 is there because the divergence measures from the center of the beam to the edge). At our example target distance of 1000 m, the beam diameter is approximately 2.4 m. This certainly wouldn't project a high intensity dot inside the cockpit.

I have no doubt that the laser beam is highly visible to the pilots in the aircraft. Pilots regularly use ground-based navigation lights that are much less intense than a laser (at that distance). However, due to the beam divergence, it is unlikely to cause eye damage because only a very small percentage of the beam energy would actually fall upon the pilot's pupil.

As you mention, the laser would also have to come from a reasonably oblique angle because aircraft have notoriously bad visibility in the "down" direction.

So, I believe that there have certainly been cases of pilots seeing laser pointer lights while in flight (most likely during approach to land). I think the likelihood of damage to the pilots' vision is very low, if any at all. The brief media attention given to this issue probably increased awareness to the point that it's not very likely to happen again.
BTW, I believe at least some of these cockpit-lasing incidents are due to pilots over-flying much more powerful ground based scientific lasers. I recall reading something about this in a recent issue of the AOPA magazine. IIRC, the article was aimed towards private pilots, telling them that they might want to re-think some of those advisory areas they thought it was safe to fly through.
5 Killowatts by May . . .
Greg Hewgill <>