Title: transmittance
Long Title: IUPAC Gold Book - transmittance
DOI: 10.1351/goldbook.T06484
Status: current
Index: quantity

Definition
The ratio of the transmitted radiant power (\(P_{\lambda }\)) 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.

Related Terms
- Beer–Lambert law: https://goldbook.iupac.org//terms/view/B00626
- absorbance: https://goldbook.iupac.org//terms/view/A00028
- attenuance: https://goldbook.iupac.org//terms/view/A00513
- radiant power: https://goldbook.iupac.org//terms/view/R05046

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)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/T06484/html
- json: https://goldbook.iupac.org/terms/view/T06484/json
- xml: https://goldbook.iupac.org/terms/view/T06484/xml

Citation: Citation: 'transmittance' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.T06484
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Accessed: 2026-04-18T19:42:06+00:00