What Does Optical Density (OD) Mean?
Optical density (OD), also referred to as absorbance, is a property that describes a material’s ability to absorb the power of a given light (called “radiant power”) that is passed through that 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 a particular light. Although it's 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 commonly used in occupational health and safety situations, 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 (OD)
Optical density can be used in OHS contexts to determine the amount of potential exposure to a specific light source that would be encountered by an individual wearing protective lenses (personal protective equipment (PPE)) of a certain optical density. 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.
OD is a logarithmic measurement, calculated as log10(PowerIn/PowerOut) or -log10(PowerOut/PowerIn). This means that 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. From this example, it can be taken that a lens with an OD of 1 absorbs 90% of incident energy, while an OD of 2 would absorb 99% of incident energy, an OD of 3 would absorb 99.9%, and so forth.
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, even if the viewer sees only a diffuse element of the laser. They can also cause skin burns and start fires.
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 that which the eyewear is rated for. OSHA standard 1926.102 requires the use of appropriate OD-rated eyewear for all work around lasers.