Spectral quantities transmissivity

Note that the spectral absorptivity, transmissivity, and emissivity of a medium are dimensionless quantities, with values less than or equal to l.The spectral absorption coefficient of a medium (and tlrus Ca, cta, and Ta), in general, vary with wavelength, temperature, pressure, and composition. 

Spectral absorption (transmission) lines are not monochromatic, due to which physical values characterizing transitions of the molecular system from one quantum state to another are also energetically diffused. Therefore, any spectral quantity F (absorption cross section, absorption coefficient, Einstein coefficients, and others) can be of three types F, is the spectral value, Fq is the maximum value corresponding to the frequency Hq, and F = 6F dn is the integral value for the spectral line. The integral and spectral values are related by the following relationship ... 

The radiation may be due to emissions from a hot source, or to the luminescence, fluorescence or phosphorescence of the sample. An emission spectrum consists of a number of generally very narrow peaks (called spectral lines) occurring at certain wavelengths which are characteristic of the materials contained within the source. The amplitudes of the peaks are related to the abundance or concentration of the materials present. Alternatively, radiation from a source is passed through a sample. In this case the quantity absorbed by the sample at a particular wavelength is again characteristic of the materials present in the sample. This is termed absorption spectrometry and produces spectral transmission lines in the form of equally narrow valleys—or peaks (Fig. 6.42) where the information is expressed in terms of absorbance (si) rather than transmittance (20<57>, and ... 

Similar directional spectral reflectivity and transmissivity quantities can also be defined. These properties are normally given the symbols p and r. However, from the above discussion, it s apparent that the sum of the reflected, absorbed and transmitted components must equal the irradian, and so we can write ... 

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