The ratio of the transmitted radiant power (P λ) to that incident on the sample (P λ 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 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2281 (https://doi.org/10.1351/pac199668122223)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 996 (https://doi.org/10.1351/pac199668040957)