RONAR-SMITH^® LASER OPTICS & IR IMAGING

Transmission & Optical Density

Transmission (T) and Optical Density (OD) are two common ways to express the throughput of a filter or filter assembly. Throughput is the portion of the total energy at a given wavelength that passes through the filter. A throughput value is always a portion of unity (less than 1 and greater than 0).

When describing the transmitting performance of a filter usually when throughput is equal to between 1% and 99+%, the preferred expression is “transmission.” When describing the attenuating performance of a filter usually when throughput is less than 1%, the preferred expression is “optical density”.

Transmission can be expressed either as a percentage (e.g., 90%) or decimally (e.g., .90). Optical density is always expressed as the negative logarithm of transmission. Unit conversions are:

OD = -log¹⁰ T or T = lO^-OD

For example,  OD=10 à T=10^-8%