Which class of laser is unsafe for direct and specular viewing, but usually not unsafe for diffuse viewing?

The class of laser that is unsafe for direct and specular viewing, but usually not unsafe for diffuse viewing, is Class 3B. This classification indicates that these lasers can produce hazardous levels of exposure under certain conditions. Direct or specular viewing means looking directly at the beam or a reflective surface that can direct the beam back to the observer, which can cause eye damage or injury. However, diffuse viewing encompasses situations where the laser light is scattered rather than directly viewed. In this scenario, the scattered light is less intense and generally considered to pose a lower risk to the eyes.

In contrast, Class 1 lasers are considered safe under all conditions because they emit very low levels of radiation, while Class 2 lasers, which emit visible light, can be safe for brief exposure, as the human blink reflex offers some protection. Class 4 lasers are the most hazardous and can cause severe eye damage from direct, specular, or even diffuse viewing, thereby necessitating stringent control measures.