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Transmission Spectroscopy
Incident light interacting with a sample experiences reflection and absorption losses (Fig.1) which are minimized when the incidence angle qi = 0. Standard measurements of transmission are performed using normal incidence when the incident light beam is positioned strictly perpendicular to a sample surface.
Figure 1. Light transmission in the presence of
Reflection and Absorption losses.
Transmittance
Transmittance, or the coefficient of transmission, is a portion of incident light at a certain wavelength l passing through a sample:
Tl = I/I0,
Where I0 is the intensity of the incident light at l; I is the intensity of transmitted light at l.
For practical convenience, transmittance is usually expressed in percentage:
Tl% = I/I0 100%
Transmittance has close relations with Absorbance (in the absence of Reflection):
Al = 2 – log10 (Tl%) = - log10 (Tl)
According to the energy conservation law, the total energy of reflected, absorbed, and transmitted beams equals the energy of the incident beam.
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