quality quantification facet transmittance PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) A transmittance is a quality quantification facet that quantifies the ratio of the transmitted radiant power (P_λ) to that incident on the sample (P0_λ): T = P_λ/ P0_λ. [IUPAC] PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) transmittance