Spectroscopic Nomenclature

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Transmittance T, τ

Ratio of the radiant power transmitted by the sample to the radiant power incident on the sample. The incident radiant power is usually obtained as that transmitted by a non-absorbing reference material.
(Not used: transmittancy or transmission). (Footnotes 1 to 3).

Percent transmission    Transmittance ×100.

Absorptance α

Ratio of the radiant power absorbed by the sample to the incident radiant power. (Footnotes 1 to 3).

Reflectance ρ

Ratio of the radiant power reflected by the sample to the incident radiant power. (Footnotes 1 to 3).

Emittance ε

Ratio of the radiant flux emitted by the sample to that emitted by a black body at the same temperature. (Footnotes 3 and 5).

Internal transmittance T_i, τ_i

Ratio of the radiant power transmitted by the sample to the incident radiant power, fully corrected for reflection losses and any window absorption. (Footnotes 6 and 7).

Internal absorptance α_i

Ratio of the radiant power absorbed by the sample to the incident radiant power, fully corrected for reflection losses and any window absorption. (Footnotes 6 and 7).

Absorbance A

Negative logarithm to the base 10 of the transmittance:

A = –log₁₀(T).

(Not used: absorbancy, extinction, or optical density). (Footnote 3).