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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%