Last week I saw a nice art project posted over on Engadget: Matt Richardson’s “Fade Away”. It uses a servo-powered pan-tilt arrangement and laser to write out tweets that contain the text “Fade Away” on a phosphorescent sheet.
It is a very clever and beautiful piece of art, and struck me because it is so much better than a laser pan-tilt rig I am building an attempt to solve a more practical problem: The 
biblical plague of crows that invade campus each night in the fall and winter, and then spend the next few months defecating on everything in sight.
One of my discoveries a while back is that I could drive off the crows from my lab window using my green laser pointer. The crows really don’t like it.
At one point we tried developing a Rube-Goldberg style apparatus using a laser pointer and mirror ball as a crow deterrent, to no avail. The result was to diffuse, and I was worried an errant light pluse might blind a motorist in the parking garage. It was a bit random and clumsy, not to mention ineffective.
My solution to the moved by a pair of servos all under the control of an Arduino microcontroller. I went with a nice red laser since it had TTL control in case I ever wanted to do something crazy like make a laser-based communication device out of it. This will be important later.
I had the rig together for sometime, and was able to test it out. I was able to make the laser draw a simple grid and using a darkened room and a long exposure to catch the progression of the laser. Note, each line is a fixed number of degrees of pan an tilt, the stretching at the top
is because I’m not projecting on the surface of a sphere or breaking out the trig to correct the spacing. Anyhow, it worked okay, but the quality of the servos and mount don’t allow me the type of precise control Richardson is getting- you can see alignment problems at the left hand size and top right, even though those lines should be overlapping.
You can watch a video of it in motion here: MVI_0906. I think it is kind of adorable.
So, after playing with it at home, it was time for a proper field test. One change that had to be made were the proper safety warnings of course.
And that is where the problem arises. Turns out the crows, while were easily perturbed my the green laser pointer, were essentially immune to the red laser pointer. Here’s why:
The red laser I purchased from Sparkfun emits at 654.5 nm, which is probably good if you are trying to light up Cy5, but turns out not to be so hot for the wavelengths absorbed by bird’s eyes (see http://www.uq.edu.au/~uqnhart/Hart_finches.pdf, figure 1a). In fact, based on that paper the cone cells of birds eyes should be orders of magnitude better at absorbing green light as opposed to red light. Which explains the difference: green laser= irritated departure of crows, red laser= indifference.
Not that human eyes are actually all that great at absorbing light at 654.5 nm either. Humans are most sensitive to green wavelengths light. Which is why the night vision light amplification systems work in the green spectrum: our eyes are better at picking up photons in the green spectrum than any other, and are these more sensitive.
Birds do have UV sensitive cones…. which might be a useful alternative. Bother the birds, but invisible to humans. Time to go look up UV lasers and their potential risks to human vision….