How does the CARBOCAP®, NDIR (non-dispersive infrared), single beam, dual wavelength sensor work?

The Vaisala CARBOCAP® Sensor has three major components, a light source, an interferometer, and an IR detector. The light source is positioned to shine at the IR detector so that the light travels a fixed distance to the detector, where the intensity of the light is measured. A Fabry-Perot Interferometer (FPI) is positioned just in front of the IR detector. The FPI is a tunable filter which allows only certain wavelengths of light to pass through to the detector. Carbon dioxide absorbs certain wavelengths of light and not others, so the FPI is designed to pass light at a CO2 absorption wavelength (4.26 μm) and a nearby,
non-absorbing wavelength. When the sensor is operating, the FPI is regularly tuned back and forth between the two wavelengths. At the CO2 absorption wavelength, the intensity of
detected light is reduced in proportion to the concentration of CO2 in the optical path. The light intensity measured at the nonabsorbing wavelength serves as a baseline for comparison (see Illustration 1).

As the concentration of CO2 varies, the difference in light intensities varies. The exact relationship between IR light intensity and CO2 volume concentration is determined when the instrument is calibrated using pure nitrogen (0 ppm CO2) and a known concentration of CO2.

The CARBOCAP® Sensor design is simple and robust, using only one light source and one IR detector. This eliminates errors caused by slight differences in the multiple components of dualbeam sensor designs. The FPI used in the CARBOCAP® Sensor is
micromachined from silicon and has no moving parts, providing much higher reliability than mechanical “chopper wheel” designs.