What Does
Optical Density Mean?
Optical density (OD) refers to a material’s ability to absorb the power of a given light source (known as its “radiant power”) as it passes through the material. It is defined as a ratio between the incident radiant power (the power of the light as it hits the material) and the transmitted radiant power (the power of the light as it exits the material).
In other words, optical density is the ability of a material to block light. Although it is calculated using criteria that have units of measurement, optical density itself is dimensionless and is not associated with any scientific unit.
The term “optical density” is considered an obsolete scientific term. However, it remains in common usage in occupational health and safety (OHS), where it is used to describe the amount of protection necessary for a person to work with certain types of lasers.
Scientific organizations such as the International Union of Pure and Applied Chemistry (IUPAC) recommend the use of “absorbance” instead of “optical density.”
Safeopedia Explains Optical Density
Optical density can be used in OHS contexts to determine how well protective lenses will protect the user from exposure to a specific light source. As absorbance measures the attenuation of light through a material, OD numbers are an effective way to consider the amount of exposure that a given set of PPE eyewear would allow.
Calculating Optical Density
Optical density is a logarithmic measurement, calculated as log10(PowerIn/PowerOut) or -log10(PowerOut/PowerIn).
For instance, if an incident power of 10 units entered through a given protective lens and left the lens as a transmitted power of 1 unit, the OD of that lens would be 1, given by Log1010. A lens with an OD of 1 can absorb 90% of incident energy, an OD of 2 would absorb 99%, an OD of 3 would absorb 99.9%, and so forth.
Optical Density and Protective Eyewear
Lasers used in industry, which include Class 3b and Class 4 lasers, require the use of OD-rated protective eyewear to avoid injury. Class 4 lasers used in research and manufacturing are powerful enough to cause immediate and permanent eye damage if viewed without protection, even if the viewer sees only a diffuse element of the laser.
The OD of a given piece of protective eyewear pertains solely to the specific wavelengths of light that it has been rated for. Lasers emit light at specific wavelengths; a given piece of eyewear will do a poor job blocking the energy from a laser that is emitted at a different wavelength than the one which the eyewear is rated for.
OSHA standard 1926.102 requires the use of appropriate OD-rated eyewear for all work around lasers.
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