Page 59 - Sterilization Training Manual

Alan H. Petersen Jr.
Sensor Electronics Corporation
1Mid IR Optical Sensing
Some gas molecules with a dipole moment can be excited to higher energy states by Mid Infrared photons. (Wavelength=3-8um). These are fundamental inter-molecular motions. Atoms in the molecule move with respect to each other. These are quantum energy levels and the energy contained in the photon must match the energy required to excite that particular motion in that molecule. As the molecule is excited the photon is gone. Longer and shorter wavelength photons pass right through without effect. The molecule will stay excited until it loses the energy by emitting a photon or transferring the energy to another molecule.
An optical system can be constructed where a broadband infrared optical source is illuminating photo detectors sensitive to certain wavelength bands. If the target gas (analyte) is present between the source and detector, light will be absorbed in certain bands and not others. By measuring relative absorption, we can identify and quantify the analyte gas.
The optical absorption is approximated by the following expression:
I = Ioe-(acl)
Solving for gas concentration c = -(ln(I/Io))/al
Since the amount of absorption is a function of the number of analyte molecules in the optical path, the concentration is expressed as density (typically mg/l).
Where I = amount of light after absorption
Io = amount of light before absorption
a = coefficient of absorption c = gas concentration
l = length of light path
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