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TRANSDUCER UNIT, AIRWAY ADAPTER AND GAS ANALYZER 5 BACKGROUND In a clinical context it may be crucial to monitor the CO2 level in the exhaled air of a patient, in particular, the end-tidal CO2 level (ETCO2). ETCO2 reflects the efficiency with which CO2 10 is carried in the blood back to the lungs and exhaled. Non-invasive methods for ETCO2 measurement include capnometry and capnography. Capnometry provides a numerical value for ETCO2. In contrast, capnography delivers a more comprehensive measurement that can be displayed in both graphical (waveform) and 15 numerical form. Capnometry or capnography devices are typically configured as either sidestream or mainstream. In a sidestream configuration, the monitoring device with the CO2 sensor is at a distance from the patient. The exhaled CO2 is diverted from the airway into the device via a sampling tube, which is attached to the breathing circuit fitted to the patient. In the case of a mainstream configuration, the CO2 sensor and a sampling cell are 20 integrated into a small device that connects directly at the airway. The sampling cell can be configured as an airway adapter, having connectors for connecting one end to a patient interface such an endotracheal tube and the other end to a ventilator. Capnometry or capnography devices are available as hand-held portable devices or as a module or component integrated into other medical equipment, such as defibrillators, anesthesiology 25 machines, and patient-monitoring systems. In order to determine the level of CO2 in the exhaled breath of the patient, it is known to pass infrared radiation through a breath sample and to measure the absorption of the infrared radiation. A large variety of devices for determining the level of CO2 in a breath 30 sample based on this principle have been proposed in the prior art. US4859858A discloses a portable, hand-held gas analyzer designed to monitor the concentration of carbon dioxide in the exhalations of a medical patient. The gas analyzer comprises a transducer unit and an airway adapter. The airway adapter is a disposable 35 one-piece unit. It is employed to connect, e.g., an endotracheal tube inserted into the patient's trachea to the plumbing of a mechanical ventilator. The airway adapter confines the expired gases to a flow path with a precise transverse dimension and furnishes an optical path between an infrared radiation emitter and an infrared radiation detector in the transducer unit. Apertures are formed in a central section of the airway adapter to allow infrared radiation to enter and exit the airway adapter. To keep the gases flowing through 5 the airway adapter from escaping through the apertures, the apertures are sealed by sapphire windows. US5067492A discloses an airway adapter having optical windows formed of the same material as the adapter body, e.g., polyethylene or polypropylene. 10 The optical windows of the airway adapter are generally thin compared to the wall sections of the airway adapter in which the apertures are formed to minimize unwanted absorption of infrared light. In the above prior-art examples, the optical windows are flush with an exterior surface of the airway adapter. However, it may be desirable that the windows are 15 flush with an interior surface that delimits a sampling passage for the breath sample. On the one hand, condensation is reduced in this manner because the flow of the breath sample overflows the windows. On the other hand, it may be easier to manufacture the windows in one piece with a base body of the airway adapter by injection molding if the windows are flush with an interior surface of the airway adapter and set back from an exterior surface. 20 The transducer unit typically also has optical windows to allow infrared radiation to pass from the transducer unit into the airway adapter and vice versa. When the transducer unit is coupled to the airway adapter, each window of the transducer unit is typically located at a certain distance to an adjacent window of the airway adapter. Due to this distance, a dead 25 space is formed between each window of the transducer unit and the adjacent window of the airway adapter. When the transducer unit is connected to the airway adapter, CO2 can be trapped between these windows. This can lead to measurement errors when CO2 concentration in the sampling passage is measured. It may therefore be necessary to put the airway adapter with the transducer unit coupled thereto away from any potential sources 30 of CO2 such as exhaled breath and to wait a considerable amount of time until the CO2 in the dead space has diffused away from the dead space before any baseline measurements are initiated. SUMMARY OF THE DISCLOSURE 35 It is an object of the present disclosure to reduce the effects of CO2 trapped in the dead space between a window of the transducer unit and a window of the airway adapter. A transducer unit for a gas analyzer is provided, comprising: a housing comprising a base and first and second lateral wall structures extending 5 away from the base in a coupling direction and being arranged at a distance to each other with respect to a lateral direction that is transverse to the coupling direction, the first and second lateral wall structures defining a receiving space between them for receiving an airway adapter, the receiving space being open in the coupling direction; a radiation emitter; and 10 a radiation detector; wherein the first lateral wall structure has a first guide surface that faces the receiving space, wherein a first optical window is provided in the first lateral wall structure at the first guide surface, 15 wherein the radiation emitter is received in the housing to emit infrared radiation towards the receiving space through the first optical window , wherein the second lateral wall structure has a second guide surface that faces the receiving space, wherein a second optical window is provided in the second lateral wall structure at 20 the second guide surface, wherein the radiation detector is received in the housing to detect a portion of the infrared radiation that has traversed the receiving space and the second optical window along the lateral direction, and wherein at least one of the first and second optical windows has a protruding 25 window portion that protrudes from the first or second guide surface, respectively, inwardly to...