An arterial blood monitoring device operative to monitor arterial blood in a body part of a person during exercise, the device being suitable for wearing on the body part; and the device comprising: at least two light emitters; at least two sensors; the at least two light emitters arranged such that light from the at least two light emitters is capable of passing through the body part to arrive at each of the at least two sensors, wherein light from each of the at least two light emitters arrives at each one of the at least two sensors in a direction different from that of light from the other of the at least two light emitters to the same sensor, and also in a direction different from that of light from the same emitter to the other one of the at least two sensors; and the at least two light emitters being operative to emit light of the same wavelength; wherein: the device is configured to cause the at least two light emitters to emit light of the same wavelength in consecutive order to obtain at least three sets of observations from the body part; characterised in that the at least three sets of observations are summed to reduce a noise signal, hence increasing a signal to noise ratio. the device being suitable for wearing on the body part; and the device comprising: at least two light emitters; at least two sensors; the at least two light emitters arranged such that light from the at least two light emitters is capable of passing through the body part to arrive at each of the at least two sensors, wherein light from each of the at least two light emitters arrives at each one of the at least two sensors in a direction different from that of light from the other of the at least two light emitters to the same sensor, and also in a direction different from that of light from the same emitter to the other one of the at least two sensors; and the at least two light emitters being operative to emit light of the same wavelength; wherein: the device is configured to cause the at least two light emitters to emit light of the same wavelength in consecutive order to obtain at least three sets of observations from the body part; characterised in that the at least three sets of observations are summed to reduce a noise signal, hence increasing a signal to noise ratio. at least two light emitters; at least two sensors; the at least two light emitters arranged such that light from the at least two light emitters is capable of passing through the body part to arrive at each of the at least two sensors, wherein light from each of the at least two light emitters arrives at each one of the at least two sensors in a direction different from that of light from the other of the at least two light emitters to the same sensor, and also in a direction different from that of light from the same emitter to the other one of the at least two sensors; and the at least two light emitters being operative to emit light of the same wavelength; wherein: the device is configured to cause the at least two light emitters to emit light of the same wavelength in consecutive order to obtain at least three sets of observations from the body part; characterised in that the at least three sets of observations are summed to reduce a noise signal, hence increasing a signal to noise ratio. the device is configured to cause the at least two light emitters to emit light of the same wavelength in consecutive order to obtain at least three sets of observations from the body part; characterised in that the at least three sets of observations are summed to reduce a noise signal, hence increasing a signal to noise ratio.