A NIRS sensor assembly for non-invasive monitoring of blood oxygenation levels in a subject's body, the assembly comprising: at least one light source operable to emit light signals of a plurality of different wavelengths; at least one light detector operable to detect light emitted by the light source and passed through the subject's body tissue, which at least one light detector has an active area through which light signals may be detected; electromagnetic interference shielding disposed around at least a portion of the at least one light detector, wherein the electromagnetic interference shielding includes an electrically conductive substrate that is optically transparent, which optically transparent substrate is aligned with the active area of the at least one light detector; comprising: a flexible electrical circuit including a patterned arrangement of printed wiring mounted to a flexible base material, the flexible circuit being in electrical communication with the at least one light source and the at least one light detector which are mounted on the flexible circuit; and an encapsulation material disposed in contact with the at least one light detector in a manner that encapsulates portions of the at least one light detector exposed to the light, wherein the encapsulation material is configured to encapsulate a connection between the at least one light detector and the flexible circuit, and wherein the electromagnetic interference shielding is attached to a surface of the encapsulation material exposed to the light. at least one light source operable to emit light signals of a plurality of different wavelengths; at least one light detector operable to detect light emitted by the light source and passed through the subject's body tissue, which at least one light detector has an active area through which light signals may be detected; electromagnetic interference shielding disposed around at least a portion of the at least one light detector, wherein the electromagnetic interference shielding includes an electrically conductive substrate that is optically transparent, which optically transparent substrate is aligned with the active area of the at least one light detector; comprising: a flexible electrical circuit including a patterned arrangement of printed wiring mounted to a flexible base material, the flexible circuit being in electrical communication with the at least one light source and the at least one light detector which are mounted on the flexible circuit; and an encapsulation material disposed in contact with the at least one light detector in a manner that encapsulates portions of the at least one light detector exposed to the light, wherein the encapsulation material is configured to encapsulate a connection between the at least one light detector and the flexible circuit, and wherein the electromagnetic interference shielding is attached to a surface of the encapsulation material exposed to the light. a flexible electrical circuit including a patterned arrangement of printed wiring mounted to a flexible base material, the flexible circuit being in electrical communication with the at least one light source and the at least one light detector which are mounted on the flexible circuit; and an encapsulation material disposed in contact with the at least one light detector in a manner that encapsulates portions of the at least one light detector exposed to the light, wherein the encapsulation material is configured to encapsulate a connection between the at least one light detector and the flexible circuit, and wherein the electromagnetic interference shielding is attached to a surface of the encapsulation material exposed to the light.