What's Optical Densities (OD)?

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

Optical Density (OD) is a measure of the radiation permitted to pass through a filter, or a measure of the attenuation of energy passing through a filter and it's determined by the 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. I.e. and OD of 2.0 allows 1/100 of the laser light energy to pass through the filter. If you eyewear are marked with an OD of 4 or greater at 755 nm and your laser requires OD of 6 or greater at 755 nm your laser glasses will not provide enough protection from your laser and you could sustain a serious injury. OD is determined by several factors and should only be calculated by a expert.

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%	1,000
4	0.01%	10,000
5	0.001%	100,000
6	0.0001%	1,000,000
7	0.00001%	10,000,000