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,