pulsed or continuous. For laser classes 1 to 4, see the section on the revised system above. Class I Inherently safe; no possibility of eye damage. This can be either because of a low output power (in which case eye damage is impossible even after hours of exposure), or due to an enclosure preventing user access to the laser beam during normal operation, such as in CD players or laser printers. Class II The blink reflex of the human eye (aversion response) will prevent eye damage, unless the person deliberately stares into the beam for an extended period. Output power may be up to 1 mW. This class includes only lasers that emit visible light. Some laser pointers are in this category. Class IIa A region in the low-power end of Class II where the laser requires in excess of 1000 seconds of continuous viewing to produce a burn to the retina. Commercial laser scanners are in this subclass. Class IIIa Lasers in this class are mostly dangerous in combination with optical instruments which change the beam diameter or power density, though even without optical instrument enhancement, direct contact with the eye for over two minutes may cause serious damage to the retina. Output power does not exceed 5 mW. Beam power density may not exceed 2.5 mW/cm if the device is not labeled with a "caution" warning label, otherwise a "danger" warning label is required. Many laser sights for firearms and laser pointers commonly used for presentations are in this category. Class IIIb Lasers in this class may cause damage if the beam enters the eye directly. This generally applies to lasers powered from 5–500 mW. Lasers in this category can cause permanent eye damage with exposures of 1/100 of a second or more, depending on the strength of the laser. A diffuse reflection is generally not hazardous, but specular reflections can be just as dangerous as direct exposures. Protective eyewear is recommended when direct beam viewing of Class IIIb lasers may occur. Lasers at the high power end of this class may also present a fire hazard and can lightly burn skin. Class IV Lasers in this class have output powers of more than 500 mW in the beam and may cause severe, permanent damage to eye or skin without being focussed by optics of eye or instrumentation. Diffuse reflections of the laser beam can be hazardous to