A capnoscope assembly for measuring carbon dioxide concentration, comprising: - a detection unit comprising an illumination arrangement and a camera configured to capture colours of an indicator surface illuminated by light emitted by said illumination arrangement, wherein said illumination arrangement and camera are closely mounted together, - an indicator holder configured to hold a colorimetric indicator having an indicator surface in front of the detection unit in a predetermined position in relation to said detection unit, - a processing unit configured to control light parameters, e.g. the colour and intensity, of the light emitted by the illumination arrangement by generating control signals, characterized in that the capnoscope assembly further comprises a calibration member, and that said indicator holder is configured to receive and hold said calibration member in front of the detection unit and in said predetermined position in relation to the detection unit, the calibration member comprises a calibration surface having a predetermined calibration colour corresponding to the colour of an ideal reference colorimetric indicator when subjected to a predefined CO2 concentration that is not zero, wherein, when said calibration member is arranged in front of the detection unit, the processing unit is configured to adjust the light parameters such that the camera detects the predetermined calibration colour, and to determine and store the control signal values required to achieve that the predetermined calibration colour was detected, as calibrated control signal values, wherein the processing unit is configured to apply a calibration procedure that comprises: - arranging a colorimetric indicator to be used in said indicator holder, - measuring the colour of the colorimetric indicator surface when illuminated by emitted light being generated by using said determined calibrated control signal values, when no CO2 is present, and storing said measured colour, as a zero CO2 colour value, and - determining a CO2 concentration relationship based upon said measured zero CO2 colour value, wherein said processing unit is configured to apply said CO2 concentration relationship during CO2 concentration measurements provided that the determined calibrated control signal values are applied when emitting light. - a detection unit comprising an illumination arrangement and a camera configured to capture colours of an indicator surface illuminated by light emitted by said illumination arrangement, wherein said illumination arrangement and camera are closely mounted together, - an indicator holder configured to hold a colorimetric indicator having an indicator surface in front of the detection unit in a predetermined position in relation to said detection unit, - a processing unit configured to control light parameters, e.g. the colour and intensity, of the light emitted by the illumination arrangement by generating control signals, characterized in that the capnoscope assembly further comprises a calibration member, and that said indicator holder is configured to receive and hold said calibration member in front of the detection unit and in said predetermined position in relation to the detection unit, the calibration member comprises a calibration surface having a predetermined calibration colour corresponding to the colour of an ideal reference colorimetric indicator when subjected to a predefined CO2 concentration that is not zero, wherein, when said calibration member is arranged in front of the detection unit, the processing unit is configured to adjust the light parameters such that the camera detects the predetermined calibration colour, and to determine and store the control signal values required to achieve that the predetermined calibration colour was detected, as calibrated control signal values, wherein the processing unit is configured to apply a calibration procedure that comprises: - arranging a colorimetric indicator to be used in said indicator holder, - measuring the colour of the colorimetric indicator surface when illuminated by emitted light being generated by using said determined calibrated control signal values, when no CO2 is present, and storing said measured colour, as a zero CO2 colour value, and - determining a CO2 concentration relationship based upon said measured zero CO2 colour value, wherein said processing unit is configured to apply said CO2 concentration relationship during CO2 concentration measurements provided that the determined calibrated control signal values are applied when emitting light. - arranging a colorimetric indicator to be used in said indicator holder, - measuring the colour of the colorimetric indicator surface when illuminated by emitted light being generated by using said determined calibrated control signal values, when no CO2 is present, and storing said measured colour, as a zero CO2 colour value, and - determining a CO2 concentration relationship based upon said measured zero CO2 colour value, wherein said processing unit is configured to apply said CO2 concentration relationship during CO2 concentration measurements provided that the determined calibrated control signal values are applied when emitting light.