One of the cool things about my life is that I occasionally get asked to review things. Mostly, I get to read books I otherwise couldn’t afford, but sometimes some really cool technology crosses my desk too. Most recently, techlasers sent me an Infiniti 125mW Green (532nm) laser. (For reference the Federal Laser Product Performance (CDRH) Standard considers 0.385mW the max that should go into your eye! See here).
This is an OM#G bright laser. This is a laser that the laser safety officer on my campus (a good friend in the office across the hall from me), made me register and agree not to ever use as a laser pointer indoors infront of students ever ever ever. (Really. Do not use this thing on a movie/overhead screen.) This is a laser I want to use to build neat optical demos, and to point out constellations when I know I’m in an FAA no fly zone or when I have 4 spotters watching for air planes. This is a goober powerful laser that is the size of a sharpie. (see my image above left)
So here are some stats:
- In the battle of laser versus photosensor (many thanks to Jack Glassman for making this measurement!), the laser weighs in with a bolometric power (power across all wavelengths) of 160 +/- 5 mW. This is significantly more than the “Max Power Output < 125mW” listed on the laser. That said, these extra milli-Watts may be in some color other than green, so that “125mW” number may only apply to the 532nm light (green comes from doubling the energy in red photons). Jack didn’t have a spectrograph, and nor do I 🙁
- The laser beam is ~0.5cm in diameter at a distance of 2.4m. This corresponds to a spread of 0.12 degrees. This means that the beam will be ~2m in diameter at a distance of 1km, and ~845km in diameter on the surface of the moon.
Here is what this means. Let’s assume the laser gives off photons only in 532nm light (energy per photon = hc/532nm = 3.7×10^-19 J). This means the laser gives of 4.3×10^17 photons per second (# photons/sec = 160mW/(energy per photon)/sec). If you stood in front of the laser, 1 km away from the laser, ~1/1,000,000 of the laser’s light would enter the 7mm diameter opening in your eye, nailing your retina with ~0.0001mW of energy.
On the moon (ignoring atmospheric scattering, which is a HUGE thing to ignore), an astronaut’s eye would still intercept ~30 photons each second (area of beam at Moon/Area of Eye*#of photons)! For comparison, your eye only intercepts 300-400 photons from a 6th magnitude star each second! Kind of cool?
Now, you may be asking yourself how it is that this little laser, at just 160mW, can have such a large effect on the moon when a 100 W lightbulb won’t. The difference comes in how the light spreads out. The 100W from that bulb spreads in all directions. This means its photons, at the distance of the moon, are spread over 5×10^17m^2 instead of the 6×10^11m^2 area the laser spreads over. There is a difference of about 625 in power between the 2 sources, and a difference of 900,000 in the area the light is spread over. This means the lightbulb is 1500 times fainter at the moon than the laser!
Okay – so I’m having way to much fun with math, and really wish I was teaching physics for engineers this semester because these calculations would make a great test question 🙂
Needless to say, when using this in a star party, you can see the beam easily in the sky (USE SPOTTERS TO WATCH FOR PLANES – do not point anywhere near a plane! I can’t find any laws saying it’s illegal to use one, but – really, do you want to blind a pilot?) During a star party, you can actually say the beam is hitting the moon!
Anyway – the laser is way cool.
There are a few things I’d change though (all safety):
- It is possible (according to the insert, but I couldn’t find the option on the website quickly) to get the Infinite laser with an optional safety key that would allow you to make it so that silly little children and absent minded professors can’t turn it on and stare into the beam and blind themselves. The key probably shouldn’t be optional.
- The box should say in big letters “Use of goggles when using this laser indoors is highly recommended to prevent eye damage from reflected light.” Really, the back scatter off of just a white wall is amazing! In the picture above right (mine) the laser beam is hitting the cloth on a speaker cover because that was the least reflective thing I could find!
So, what can you do with one? Well, pointing out constellations, stars, planets, galaxies, satellites (NOT airplanes) are all good things. You can make every other astronomer on the star party field jealous. You, science teachers out there, can also build optical tweezers (which I plan to do when I get some spare time), measure the size of pits on a CD using diffraction, and make spectacular diffraction patterns for classroom demos (done at an angle that doesn’t allow scattered light to back scatter to the students in huge amounts).
I’m g0ing to have fun this semester.