As noted, any CW visible laser (400-700 nm) that has an output power <1.0 mW is termed a Class 2 (low-risk) laser, and could be considered more or less equivalent in risk with staring at the sun, at a tungsten-halogen spotlight, or at other bright lights that can cause a photic maculopathy (central retinal injury). Only if one purposely overcomes their natural aversion response to bright light, can a Class 2 laser pose a real ocular hazard. An aiming beam or alignment laser operating at a total power above 1.0 mW would fall into hazard Class 3, and could be hazardous even if viewed momentarily within the aversion response time. A subcategory of Class 3, termed Class 3R (formerly 3A in the United States), consists of lasers from 1-5 mW in power, and these lasers pose a moderate ocular hazard under viewing conditions, where most of the beam enters the eye. Class 3b is when the subcategory that comprises, among certain pulsed lasers, CW visible lasers that emit 5-500 mW output power. Even momentary viewing of Class 3b lasers is potentially hazardous to the eye.
Only lasers that are totally enclosed or that emit extremely low output powers fall under Class 1 and are safe to view. Any CW laser with an output power above 0.5 W (500 mW) falls under Class 4. Class 4 lasers are considered to pose skin or fire hazards as well as severe eye hazards, if not properly used. The purpose of assigning hazard classes to laser products is to simplify the determination of adequate safety measures, that is, Class 3a measures are more stringent than Class 2 measures, and Class 4 measures are more stringent than Class 3b measures. Virtually, all surgical lasers fall into Class 4, although the ophthalmic Nd:YAG photodisruptor is one example of a Class 3B surgical laser. Low-Level Laser Therapy (LLLT) lasers and LED arrays that employ nonthermal effects to stimulate or “biomodulate” cells in the dermis and epidermis are frequently Class 3B—or even Class 3R laser products, or Risk Group 2 lamp products.