optical filter wheel and
infrared detector
3.2 Principle of operation
The measurement of CO
2
in the breathing gas mixture is based on the fact that different gas
components absorb infrared light at specific wavelengths. A beam of invisible infrared light is directed
through the respiratory gas flow in the EMMA Airway Adapter. As the beam passes through the Airway
Adapter, some of the light is absorbed by the gas mixture. The amount of absorbed light is measured by
a miniaturized two channel spectrometer positioned to receive the infrared light beam.
The spectrometer incorporates a filter wheel fitted with two different optical "color" filters. The
wavelength ranges of these filters are chosen such that one filters out colors where carbon dioxide has
very strong absorption and the other filters out colors where carbon dioxide has no absorption.
The spectrometer also incorporates an infrared detector that converts the light beam to an electrical
signal. The electrical signal is converted to a digital value that is fed to a microprocessor. The ratio of the
light measured through the two filters is then used by the microprocessor to calculate the carbon dioxide
concentration in the breathing gas mixture.
Figure 2. Principle of operation
10(32)
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