"Laser" is an acronym that stands for "Light Amplified by the Stimulated Emission of Radiation" For the technical stuff regarding lasers, read Sam's Laser FAQ for a good idea of what lasers are and how they work. Don't worry, you don't need to know all of the technical info to get started in the laser hobby.

mW stands for milliwatt, a measure of energy. nm stands for nanometer, a measure of length. In lasers, mW is used as a unit to describe the power of lasers. Basically the higher the power, the brighter the dot and beam will be. Also, with more power comes more burning capability. nm refers to the wavelength of the laser. Wavelength can basically be thought of as a very specific color. Common wavelengths you will see here are 405nm (violet), 473nm (blue), 532nm (green), 593.5nm (orange/yellow), and 650-670nm (red). This is a good illustration of the visible spectrum.

As long as the laser's wavelength is over 400nm, there is a near-to-zero risk of cancer. It is disputed if 405nm 'Blu-Ray' diodes wander into the below 400nm ultraviolet spectrum. It is best to minimise skin exposure to the emmissions of these lasers.

Green diodes are very rare and hard to find. The industry doesn't mass-produce them, as they're very expensive and hard to build. In addition, they're only capable of reaching a few milliwatts, with extensive cooling. Most green lasers do not use a green diode, but use as system called DPSS (Diode Pumped Solid State).

IR, or Infrared light, is beyond the visible spectrum and appears as a dim red in the lower wavelengths and is invisible as the wavelength increases. It can be dangerous in high powers, but is not any more or less hazardous to the eyes as any bright light. Many green lasers that aren't sold by reputable companies don't have IR filters. Without a filter, excess IR will pollute the output of the 532nm light. Because many people don't know how DPSS lasers work, they don't expect the IR to be present, and therefore aren't prepared for it. Remember - it's still light, even if you can't see it.

Because, as previously mentioned, green DPSS lasers are built using an IR diode and crystals to convert it to green light, thus approximately 80% of the energy is absorbed by the crystals (poor quality ones absorb even more). As you might already know, no material is perfect and in consequence, some of the IR light goes through the crystals and is "fired" with the green light you see at the end of the laser. Despite the fact that you can't see IR doesn't mean you can't get hurt by it. Again, use safety glasses!

Red lasers use a diode to produce red light, there's no wavelength conversion, so there's no IR. This doesn't mean that you shouldn't wear safety glasses either; if you're using a high-powered red laser you can get your eyes injured same as any other wavelength.

Blue lasers are built with the same concept as green lasers, but with more complex conversions and crystals. Subsequently they are a lot more expensive than red and green lasers (yellow/orange and other colors are even more expensive because they have even more complex building processes). What else can I do with a laser besides burning stuff? Here are some great examples of other cool tricks you can do with lasers besides just burning stuff: Spirograph Scanner Liquid Sky Time Tunnel And this is just the beginning of what lasers can do!

Yes, technically for anything over 5mW you need goggles for. Goggles usually run around $50 a pair, but you have to realize that your eyes are worth more than whatever you pay for goggles. One unlucky hit in the eye with almost any laser and you will have a permanent blind spot. A saying you will hear around here is "You can only look into a high powered laser twice, once with each eye". This is because it only takes a tiny fraction of a second to blind yourself or severely damage your eyes. Can I use sunglasses or Welding Goggles for at least some laser protection? No. This is a common misconception. All sunglasses/welding masks will do is make your pupils dilate, which makes the potential for damage much greater. Need a demonstration? Watch this short clip.

OD means Optical Density. Basically, the higher the OD rating on the goggles, the less light will be allowed to pass through. OD3+ goggles are fine for lasers up to 125mWs. Further than that, you'll need goggles with progressively higher OD ratings. Specifically, the number is the power of ten reduction that the transmitted light of that wavelength is reduced. IE, OD3 passes 10-3 (1/1000) times the incident light.

Looking at the color wheel, you can see what colors are opposites of each other - IE, green is the opposite of red, and orange is the opposite of blue. If your laser is green, the opposite color (Red) will absorb more of the laser's light than any other color. Since the object is to reduce the amount of light reaching your eyes, the laser's light needs to be absorbed by the goggles. So, protective goggles for green lasers are red, goggles for blue lasers are usually orange (Because the wavelengths for green and blue are so similar, many goggles are simply orange - these serve very well at protecting from both wavelengths) and goggles for red lasers are either blue or dark green.