The US Food and Drug Association classifies lasers according to their potential hazard to the eyes or skin. The danger has a fairly complex, non-linear dependence on wavelength and exposure time, but predominantly depends on power output of the laser. The classes are defined as follows:
- Class I – Inherently safe lasers that will either cause no damage by themselves (even in combination with optical instruments) or are enclosed in a housing that prohibits exposure during use (e.g. CD players or laser printers)
- Class II – (below 1 milliWatt) Emit visible light that will not cause eye damage unless intentionally kept from blinking
- Class IIa – Can be ultraviolet (UV) or infrared (IR) but whose effect would not be damaging unless exposed directly in excess of 1000 seconds continuously
- Class IIIa – (1 – 5 milliWatt; power density less than 2.5 mW/cm2) (e.g. laser pointers) Can be dangerous if combined with optical instrumentation that changes beam size, and therefore power density
- Class IIIb – (5 – 500 milliWatt) Can permanently damage the eye with millisecond exposure. Primary reflections can also be dangerous. Can burn skin and be a fire hazard. Safety glasses necessary for anyone in immediate vicinity
- Class IV – (above 500 milliWatt) May cause serious, permanent damage without optical magnification. Even diffuse reflections can be dangerous in the immediate vicinity. Requires safety glasses (specifically for wavelength used), interlock switch, and emergency shutdown button.
People often refer to "hot" or "cold lasers. Historically, this distinction was defined as the difference between surgical (hot) and non-surgical, therapeutic (cold) applications. Lower level laser companies often skew this definition to be just above the power they are able to produce, so that they can claim to be "the most powerful cold laser", mostly to accuse any more powerful laser of burning patients.
Notice there is no upper-limit on the power limitation of Class IV. So the statement, "Class IV lasers can burn patients" should be a very obvious one, since there are some lasers that fall into Class IV that can put holes in metal.The K-Laser has treated HUNDREDS OF THOUSANDS of people and pets, without a SINGLE case of even a 1st degree burn.