Light Sources for Optical Gas Sensors

As it was shown before, optical IR, NDIR, and UV gas sensors need light sources for the excitation of the gas molecules in the wavelength range for the particular gas (see Table 14.4). Of course, for optimal gas sensor operation these light sources should correspond to the following requiranents 

Rugged construction High emissivity Long lifetime Low cost Small size 

High pulse rates and, hence, a low thermal time constant for light modulation to offset thermal 

However, we need to note that the use of LEDs and laser diodes instead of bulky, high-power-consuming light sources such as thermal IR sources, tungsten-halogen, or deuterium lamps (Johnston 1992 Reich 2005) is the common tendency in manufacturing gas sensors, especially 

High-power output with high rehability Higher pulse rates of up to 200 Hz, modulation depth of 50 %, often used with reflectors to direct entire radiation out of package 

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A survey of the optical bandgap, excitonic recombination properties under low excitation and electron hole plasma recombination in AlxGai.xN has been given. Demand for UV applications, i.e. gas sensors or monitors, flare sensors, medical applications, chemical and biochemical applications and light sources for phosphors increases rapidly, which will surely lead to the further improvement of the quality of the AIN containing nitrides, and thus give us much more information about their luminescence properties. 

Optical methods are especially useful for the selective detection of CO and C02 concentrations. In low-priced sensors, a simple miniature light bulb is used as IR-source. The radiation emitted enters an absorption chamber, through which the flue gas is pumped. An added interference filter lets only the absorption spectra of the target gas pass. The IR detector determines the reduction of the light intensity, which is then transformed into an electrical signal. The correlation between the source intensity and the received intensity is given in the Lambert-Beer equation. 

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