Interchangeable sensor device far a near-infrared functional (fNIR) spectroscopy system intended to detect changes in the concentration of haemoglobin species on a body surface; wherein such interchangeable sensor device comprises: - a unit of measurement which in turn comprises: - a plurality of transmitters intended to emit a first luminous signal on the body surface, - a plurality of receivers intended to receive a second luminous signal from the body surface, - a control unit intended to control the unit of measurement , wherein the interchangeable sensor device comprises a plurality of said unit of measurement , wherein each unit of measurement is elastic, interchangeable and intended to be adapted to different parts of the body surface, and wherein each unit of measurement further comprises: - an elastic base comprising a plurality of transmitting compartments wherein the transmitters are located and a plurality of receiving compartments wherein the receivers are located, - light guides located in correspondence with the receiving compartments through which the receivers receive the second luminous signal, - a measuring base plate linked to the transmitters and receivers , comprising a first connector , - a receptacle resting on the elastic base and comprising a hole intended to receive the first connector in such a way that the receptacle protects the transmitters and receivers from ambient light, - a first clamping strap which at least covers the transmitters and receivers and which is intended to be linked with at least a first clamping mechanism to adjust the interchangeable sensing device to the body surface, and/or - a second clamping strap comprising a hole allowing the passage of the first connector , wherein the second clamping strap surrounds the receptacle and is intended to be linked to at least a second clamping mechanism far adjusting the interchangeable sensing device to the body surface, and wherein the control unit comprises a second connector intended to be connected with the first connector of the units of measurement . - a unit of measurement which in turn comprises: - a plurality of transmitters intended to emit a first luminous signal on the body surface, - a plurality of receivers intended to receive a second luminous signal from the body surface, - a plurality of transmitters intended to emit a first luminous signal on the body surface, - a plurality of receivers intended to receive a second luminous signal from the body surface, - a control unit intended to control the unit of measurement , - an elastic base comprising a plurality of transmitting compartments wherein the transmitters are located and a plurality of receiving compartments wherein the receivers are located, - light guides located in correspondence with the receiving compartments through which the receivers receive the second luminous signal, - a measuring base plate linked to the transmitters and receivers , comprising a first connector , - a receptacle resting on the elastic base and comprising a hole intended to receive the first connector in such a way that the receptacle protects the transmitters and receivers from ambient light, - a first clamping strap which at least covers the transmitters and receivers and which is intended to be linked with at least a first clamping mechanism to adjust the interchangeable sensing device to the body surface, and/or - a second clamping strap comprising a hole allowing the passage of the first connector , wherein the second clamping strap surrounds the receptacle and is intended to be linked to at least a second clamping mechanism far adjusting the interchangeable sensing device to the body surface, <EOS>