## Wikipedia - Transmission (Physik) (de) Wikipedia - Transmittance (en) Wikipedia - Trasmittanza (it) transmittance, $$T$$, $$\tau$$

Also contains definition of: internal transmittance
https://doi.org/10.1351/goldbook.T06484
The ratio of the transmitted @R05046@ ($$P_{\lambda }$$) to that incident on the sample ($$P_{\lambda }^{0}$$): $T=\frac{P_{\lambda }}{P_{\lambda }^{0}}$ Internal transmittance refers to energy loss by absorption, whereas the total transmittance is that due to absorption, reflection, scatter, etc.
See:
absorbance
,
attenuance
,
Beer–Lambert law
Sources:
Green Book, 2nd ed., p. 32 [Terms] [Book]
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2281 [Terms] [Paper]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 996 [Terms] [Paper]