Gas analyzer using infrared radiation to determine the concentration of a target gas in a gaseous mixture

A carbon dioxide gas analyzer for determining a concentration of a carbon dioxide gas in a respiratory gas of a subject, comprising:

• a detachable airway adaptor for introducing a respiratory gas into the analyzer, and comprising a plurality of windows through which light passes, said plurality of windows having an anti-fogging characteristic;
• an infrared light source for emitting infrared radiation which passes through said airway adaptor;
• a beam splitter for reflecting and transmitting infrared radiation impinging thereon, after the infrared radiation has passed through said airway adaptor;
• first detecting means for detecting the infrared radiation reflected by said beam splitter;
• second detecting means for detecting the infrared radiation transmitted through said beam splitter;
• a gas cell filled with a gas including carbon dioxide, and located between said beam splitter and one of said first and said second detecting means;
• two bandpass filters for allowing only an infrared radiation having wavelength of approximately 4.3 μm to transmit through, said bandpass filters being respectively located between said first detecting means and said beam splitter and between said second detecting means and said beam splitter, such that said first and said second detecting means detect only an infrared radiation having wavelength of approximately 4.3 μm;
• and
• processing means for processing a concentration of carbon dioxide gas in a respiratory gas based on a ratio of output signals from said first and said second detecting means;
• wherein said respective infrared radiation detected by said first and said second detecting means is from infrared radiation that has passed through said airway adaptor along only a single beam passageway.