-----this article original from lasersafetyfacts
Lasers are classified for safety purposes based on their potential for causing injury to humans’ eyes and skin.
Most laser products are required by law to have a label listing the Class. It will be listed either in Arabic numerals (1 2, 3R, 3B, 4) or in Roman numerals (I, II, IIIa, IIIb, IV). At this website, we primarily use the Arabic numerals, for convenience.
For visible-beam consumer lasers, there are four main classes. Each is described in more detail here: Class 2, Class 3R, Class 3B and Class 4. The first two Classes are relatively safe for eye exposure; the last two are hazardous. The chart below shows how the eye injury hazard increases as the laser’s power increases.
The detailed information given below is for laser light that is visible -- between 400 and 700 nanometers -- and for an unintentional exposure of less than 1/4 second. Consult other sources for classifications of infrared and ultraviolet lasers, or other visible-light exposure durations.