Class 3B laser products are medium power devices for which direct ocular exposure is unsafe, but for which the diffuse reflection is safe under certain conditions (i.e. a minimum viewing distance of 130 mm and a maximum viewing time of 10 seconds). The beam also presents a minor skin hazard, especially when focused. For a CW laser in the visible or infrared wavelength range the Class 3B AEL is 500 mW.
Class 4 laser products are high power devices for which even the diffuse reflection is generally hazardous. They may also cause skin burns and may present a fire hazard. There is no AEL for Class 4.