Lasers are classified relevant to their output and the wavelength of the laser beam in a special set-up corresponding to the United States Center for Devices & Radiological Health (CDRH) or the International Electrotechnical Commission (IEC). The new classification standard EN60825-1 was introduced in 2011.
Generally the higher a laser's safety class, the greater the risk of eye injury. As a precaution, it is always advisable to wear laser safety goggles. Even "eye-safe" lasers such as laser pointers should never be intentionally aimed at people. The only exceptions are medical applications where for example, lasers might be used to show a blood clot in the eye and are covered by strict rules.
Laser classification depends on the wavelength and the power output. The class can change if a lens spreads or fans the beam. The fan angle, in degrees, denotes the beam spread. This has the effect of reducing the power that could be intercepted by the eye.