transmittance, T, τ in mass spectrometry

https://dx.doi.org/10.1351/goldbook.T06484
The ratio of the transmitted radiant power(P λ) to that incident on the sample (P λ 0):
T = P λ P λ 0
Internal transmittance refers to energy loss by absorption, whereas the total transmittance is that due to absorption, reflection, scatter, etc.
Source:
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2281
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 996