Not your (academic) Papa’s Laser Pointer

Posted By Pamela on Jan 29, 2008 | 29 comments

laser1.jpglaser2.jpgOne 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.


  1. You are scaring me. Now I’m afraid of lasers.

  2. goober powerful? I’m going to have to write that one down.

    And the supply list for class:

    – Paper
    – Pens (or crayons for the engineers)
    – Welder’s goggles
    – Elmer’s glue
    – Scientific calculator… etc.

  3. My favorite laser warning sticker reads:

    “Do not look into laser with remaining eye.”

  4. 160MW! That -IS- powerful! You might be able to light a match with it…

    I bought a 35MW Wicked Laser last December so I could do star tours when the Moon is first quarter or full phase. My other green laser, an Orion

  5. (what happened to the last half of my post? Weird!)

  6. I can’t point to the specific law either, but I’ve certainly heard several cases of people being arrested for illuminating airplane or helicopter cockpits with lasers. It’s a very dangerous thing to do, so I’m glad you reminded your readers to NEVER, NEVER point a laser at an aircraft in flight!

  7. Where was I… My Orion 5MW just doesn’t cut it when the Moon is up, and the 35 does fine. And yeah, no pointing at aircraft at all, of course!

  8. I work for a company that makes a variety of lasers for the military. The smallest laser we make is 100mW. Our largest green right now is 250mW although we are working on one significantly bigger.

    We make IR lasers in the 100mW, 200mW, 1000mW, and 3W class.

    Talk about burning your eyes out!

  9. I’ve heard anecdotal evidence of people that pointed lasers towards or at police officers being charged with “menacing”.

  10. Taras; If they’re mildly unlucky, they could go for assault on a police officer (assault is defined as creating the threat or fear of injury). If they’re really unlucky they could go into a body bag.

    Dr. Gay; On cold nights, I can barely feel the warmth of my 5mW GLP on the back of my hand. I bet you could use the 125mW one as a hand warmer, or to defrost your car door lock.

  11. My uncle once upon a time brought me a proffesional laser (he used it to cut the steel :-)) ) It was not legal, but we’ve got such laser on the ‘black market’ for about a 500 bucks only. It’s some-kind of 0.5 MW 🙂 But as soon as i can’t plug it (380 volts are not common in our flats) – i have no exp. with it. Story about your 125 mW made me laugh. 🙂

  12. Another useful number to calculate would be how strong the light is in units of lux, which is basically an intensity per unit area measurement, which is proportional to how many photons per unit area per second, and also proportional to Watts per unit area. 1 candela (cd) is 1/683W for a spherical light intensity amount. Or we can also put things in a lumen measurement where 1 lumen is equivalent to 1 cd over one steridian or 1 cd sr which is ultimately equivalent to 1 lux m^2. Yeah, this gets complicated fast. I guess I’m trying to compare the light intensity of the laser with that of a new LED flashlight I purchased which has a parabolic total reflector with a backfiring LED which forms a highly collimated beam. It has a peak lux rating of approximately 7,000 @ 1 meter. I’ll get right on that calculation…

  13. I have sent all manner of lasers to my fellow astronomers and think you would be more informed and very surprised by what is happening in the R&D world of laser pointers. I have a blue Aquarius -10 a real favorite, and a red laser that is ~250 mW as well as many green lab and portable lasers. My most powerful personal laser is a XYMARK 7000 20 watt CO2. I am sure astronomers want to have the best dark apaption of the eeyes, so I reccomend about 7 to 35 mW for star pointing and of course care to avoid aircraft or any people/ vehciles on the ground. I have seen the brightest of minor planets, naked eye with the help of a 15 mW pointer on a computer guided telescope to tell exactly where to look! I sell lasers in Violet, Red, Yellow, Green and Blue and can save the buyer as compared to great suppliers such as You can’t even imgine what a 15 mW 405 nm laser is like without seeing it in person! If you want more information, my email is thanks! -Glenn

  14. You made one substantial error in your calculation of the number of photons of light reaching a certain distance from the laser vs. lightbulb. A 100 watt light bulb actually only outputs something on the order of less than 10 watts of visible light, but the 125 mw laser actually outputs 125 mw of green light. Light bulbs are rated at input power while lasers are rated at output power. That 125 mw laser actually has an input power of one watt or so.

  15. All lasers should be used with care and you only get two chances to look into a high power laser, one with each eye! The experiments and learning is only limited by your imagination though. I am now delivering violet lasers up to 200 mW. Many are ordered lately at 100 to 200 mW and I often use the >150 mW violet laser to search for fluorescent minerals and fungi at night. It turns out that scorpions and rattle snakes fluoresce as well. A typical 150 mW violet laser is sold for about $160 and will actually burn a hole in white computer paper! These will also ignite a match at 6 feet and pop a balloon at twice that when the focus is correct.

  16. Damn that’s cool! I love the scintillation characteristics of lasers! Very Cool!

    Is green the best colour for astronomy purposes? Because I’ve seen plenty of red ones too? So which is better?

  17. I’s been a long time since I posted, I really enjoy lasers, and I have a large astronomy club, with several pals who enjoy them too. I have lasers in all the colors, and I have foiunf better quality and price than most anywhere, so I obtain them for others all the time. Lately, I can even ge violet lasers that will pop a balloon from over 10 meters, (30 feet plus!). Too bad I can not post a picture here, but I can email them if you would like to be astounded! my email is I have 100% positive feedback on Ebay., astromart and Laser Pointer Forums, so I must be doing a good service to folks. Any questions about lasers? Best Wishes! -Glenn

  18. I heard techlasers belongs to wickedlasers,but also too expensive,I just bought a more cost-effective green laser pointer from

  19. wow, your laser pointer seems pretty good! I would like to have a cni laser pointer

  20. nice, the laser pointers above seems quite cool! while i know the laser pointer from Wicked are a bit expensive, can i afford it?

  21. Even though there might be less quality problem of wicked lasers, yet i still think they are selling in a high price. can they provide astronomy used laser pointer? i’d like to know.

  22. Love the sharing in your posts. Amaing information. ps: i love red laser most.

  23. Lasers are good stuff. But then if you like most powerful lasers i choose you to look the site of mine. Very powerful than others.

  24. I enjoy looking through a post that will make people think. Also, thank you for allowing me to comment!


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