Which two classifications of lasers require the posting of a laser warning sign?

Class 3B and Class 4 lasers necessitate the posting of a laser warning sign due to their potential hazards. Class 3B lasers can cause eye injuries from direct exposure, and some may also pose a fire risk. Because of their ability to inflict harm, they require specific safety protocols, including signage to alert individuals to the potential dangers.

Class 4 lasers represent an even higher level of risk, as they can cause severe eye and skin injuries and can ignite materials. These lasers require stringent safety measures, including warning signs, because they pose significant hazards in the environments where they are used.

In contrast, other classes, such as Class 1, Class 2, Class 1M, and Class 2M, are generally considered safe for use and typically do not require warning signs, as they either present no risk under normal operating conditions or the risk is minimal. Class 3R lasers have a lower risk when it comes to harm, and while caution is advisable, they don't necessitate signage like Class 3B and Class 4 lasers do.