A detection arrangement comprising: a blood culture bottle incorporating a colorimetric sensor subject to change of color due to change in pH or CO 2 of a sample medium within the blood culture bottle ; a sensor LED illuminating the colorimetric sensor ; a reference LED illuminating the colorimetric sensor ; a control circuit configured to selectively and alternately activate the sensor LED and the reference LED ; and a photodetector , the photodetector measuring reflectance from the colorimetric sensor during the selective and alternating illumination of the colorimetric sensor with the sensor LED and the reference LED and generating intensity signals; wherein the reference LED has a peak wavelength of illumination of between 750 and 950 nm such that the intensity signals of the photodetector from illumination by the reference LED are not substantially affected by changes in the color of the colorimetric sensor ; said detection arrangement further comprising a computer receiving the intensity signals, the computer including a memory storing a calibration relationship between intensity signals for the reference LED as a function of distance of the bottle from a home position in relation to the detection arrangement, and wherein the memory further stores a calibration relationship between intensity signals for the sensor LED as a function of distance of the bottle from the home position and wherein the computer compensates for a drop in intensity signals from the sensor LED due to the bottle being positioned a distance away from the home position in accordance with calibration relationships for the sensor LED and the reference LED . a blood culture bottle incorporating a colorimetric sensor subject to change of color due to change in pH or CO 2 of a sample medium within the blood culture bottle ; a sensor LED illuminating the colorimetric sensor ; a reference LED illuminating the colorimetric sensor ; a control circuit configured to selectively and alternately activate the sensor LED and the reference LED ; and a photodetector , the photodetector measuring reflectance from the colorimetric sensor during the selective and alternating illumination of the colorimetric sensor with the sensor LED and the reference LED and generating intensity signals; wherein the reference LED has a peak wavelength of illumination of between 750 and 950 nm such that the intensity signals of the photodetector from illumination by the reference LED are not substantially affected by changes in the color of the colorimetric sensor ; said detection arrangement further comprising a computer receiving the intensity signals, the computer including a memory storing a calibration relationship between intensity signals for the reference LED as a function of distance of the bottle from a home position in relation to the detection arrangement, and wherein the memory further stores a calibration relationship between intensity signals for the sensor LED as a function of distance of the bottle from the home position and wherein the computer compensates for a drop in intensity signals from the sensor LED due to the bottle being positioned a distance away from the home position in accordance with calibration relationships for the sensor LED and the reference LED . <EOS>