Ratio of the transmitted spectral radiant flux Φout λ) to the incident spectral radiant flux Φin λ).
$\tau \left( \lambda \right) = \frac{\phi_{\text{out}} \left( \lambda \right) }{\phi_{\text{in}} \left( \lambda \right)}$
Fig. 50: Spectral transmission
In many cases, the transmittance is displayed by photometers in addition to the absorbance (formerly called extinction).