Laser safety glasses are made of compounds that block specific wavelengths of light to protect your eyes from laser emissions.
The amount of protection is rated by the Optical Density (OD) of the safety glasses, which measures how much laser light is blocked. Mathematically, the amount of transmission is the light source to the negative power of the Optical Density (Transmission = Light Source-OD). So every 1 point increase of the Optical Density blocks transmitted light by another factor of 10.
Therefore, OD 4 glasses will reduce the transmitted energy of a 500mw laser to an equivalence of 0.05mw, which is barely visible. However, even if safety glasses are being worn, a laser should never be pointed at the eyes. High-power lasers are not toys –- they must be treated with respect and caution. Any retinal damage done is permanent.
OD 0 = 100%
(Note: since protective glasses block a laser’s wavelength, this means that a laser’s beam cannot be seen while wearing the glasses. So if a laser is being used for entertainment purposes, for example, then wearing safety glasses would defeat the purpose as none of the laser's visual effects could be seen. Safety glasses are best suited for high-power lasers in working environments. Consult your safety supervisor for appropriate precautions.)