Laser Safety falls under the ANSI 136.1 standard in the United States. Lasers are categorized by the ANSI Z136.1 standard into the following categories (NOTE: The category alone is not enough to work out which eye protection is required).

Class	Definition	Pulse Length
1	Non-hazardous	Eyewear not required.
1M	Eye safe visible laser (400-700nm) if used without magnifying optics.	Eyewear not required unless used with magnifying optics.
2	Eye safe visible laser (400-700nm) (safe within the blink reflex of 0.25s).	Eyewear not required.
2M	Eye safe visible laser (400-700nm) (safe within the blink reflex of 0.25s) if used without magnifying optics.	Eyewear recommended.
3R	Likely unsafe for intrabeam viewing. Maximum Permissible Exposure (MPE) is up to 5 times class 2 limit for visible lasers of 5 times class 1 limit for invisible lasers.	Eye hazard; eyewear is recommended.
3B	Eye hazardous for intrabeam viewing. Limited diffuse hazard.	Eye hazard; eyewear is recommended.
4	Eye and skin hazard for direct and diffuse exposure. Fire and burn hazard.	Eye protection and other personal safety equipment is required.

ANSI Z136.1 requires specification of laser safety eyewear according to optical densities (OD), and allows a Nominal Hazard Zone (NHZ) to be calculated, outside which diffuse viewing eyewear is allowed.

Optical Density (OD) is a measure of the attenuation of energy passing through a filter. The higher the OD value, the higher the attenuation and the greater the protection level. In other words, OD is a measure of the laser energy that will pass through a filter. OD is the logarithmic reciprocal of transmittance, expressed by the following: D? = -log10 T ? (where T is transmittance)

OD (Optical Density)	Transmission in %	Attenuation Factor
0	100%	1
1	10%	10
2	1%	100
3	0.1%	1000
4	0.01%	10,000
5	0.001%	100,000
6	0.0001%	1,000,000
7	0.00001%	10,000,000

Modes of Operation Lasers operating at different modes have different power density characteristics and often different eyewear requirements.

Mode	Definition	Pulse Length
D	Continuous Wave (cw) with consistent average power	greater than 0.25 second
I	Pulsed: short single or periodic energy emission	> 1 µs to 0.25 s
R	Giant Pulsed: very short single or periodic energy emission	1 µs to 1 ns
M	Mode Locked	< 1 ns (pico and femtosecond)