Spectroscopic Nomenclature

The spectroscopic nomenclature must comply with the conventions recommended by the International System of Units (SI) and the International Union of Pure and Applied Chemistry (IUPAC). A brief summary of acceptable terms and their accurate use appears periodically on the back page of Applied Spectroscopy and is available on the Journal’s website.

(1) Define acronyms and abbreviations on first use, followed by the acronym or abbreviation in parentheses.

(2) Please do not begin a paragraph with an acronym or abbreviation and avoid their use in headings where possible and practical.

(3) Provide names of manufacturers in parentheses for instruments, equipment, and materials.

(4) Latin terms (i.e., et al., in vivo, ca.) should not be italicized.

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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 Ti, τi 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 αi

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).

Internal absorbance Ai

Negative logarithm to the base 10 of the internal transmittance:

Ai = –log10(Ti).

Absorptivity α

Internal absorbance divided by the product of sample path length, ℓ, and mass concentration, ρ, of the absorbing material.

α = Ai/ρℓ.

SI unit: m2 kg−1.

Common unit: cm2 g−1; L g−1 cm−1.

(Not used: absorbancy index, extinction coefficient, or specific extinction.)

Molar absorptivity or Molar (decadic) absorption coefficient ε

Internal absorbance divided by the product of sample path length, ℓ, and mole concentration, c, of the absorbing material.

ε = Ai/cℓ.

SI unit: m2 mol−1.

Common unit: L mol−1 cm-1; cm2 mol−1. (Not used: molar absorbancy index or molar extinction coefficient.)

Naperian absorbance, Ae

The absorbance calculated in base e, i.e., ln (1/T)

Linear absorption coefficient, α

The Naperian absorbance divided by the path length,

α = Ae/ℓ

Beer–Lambert law

Absorptivity of a substance is constant with respect to changes in path length and concentration of the absorber. Often called Beer’s law when only changes in concentration are of interest.

Net absorption cross-section σnet

The absorptivity per molecule, i.e., the molar absorptivity, ε, divided by Avogadro’s number, usually corrected to base e through σnet = 2.303ε/NA.

SI unit: m2.