A detection method comprising: providing a surface plasmon resonance, SPR, device; attaching a replaceable modular sensor comprising a first planar metallic layer to the SPR device, wherein the SPR device comprises: a housing; a detector array; a sensor prism comprising: a double faceted input side and a double faceted output side, wherein each facet of the input side is angled to internally reflect a respective input beam to a first side of the first planar metallic layer; and wherein each facet of the output side is angled to internally reflect a respective output beam from a second side of the first planar metallic layer to the detector array to provide output reflected beam signals; the method further comprising: sequentially disposing a reference fluid and a testfluid in contact with the first planar metallic layer; providing a first input beam and a second input beam to the first metallic layer, respectively, via the input facets (402, 404); delivering to the detector array: two reflected beam signals when the top side of the metallic layer is in contact with the reference fluid and two reflected beam signals when the top side of the metallic layer is in contact with the test fluid; and processing said reflected beam signals to measure a spacing, at the detector array, between a reference SPR signal and a test SPR signal. providing a surface plasmon resonance, SPR, device; attaching a replaceable modular sensor comprising a first planar metallic layer to the SPR device, wherein the SPR device comprises: a housing; a detector array; a sensor prism comprising: a double faceted input side and a double faceted output side, wherein each facet of the input side is angled to internally reflect a respective input beam to a first side of the first planar metallic layer; and wherein each facet of the output side is angled to internally reflect a respective output beam from a second side of the first planar metallic layer to the detector array to provide output reflected beam signals; a housing; a detector array; a sensor prism comprising: a double faceted input side and a double faceted output side, wherein each facet of the input side is angled to internally reflect a respective input beam to a first side of the first planar metallic layer; and wherein each facet of the output side is angled to internally reflect a respective output beam from a second side of the first planar metallic layer to the detector array to provide output reflected beam signals; a double faceted input side and a double faceted output side, wherein each facet of the input side is angled to internally reflect a respective input beam to a first side of the first planar metallic layer; and wherein each facet of the output side is angled to internally reflect a respective output beam from a second side of the first planar metallic layer to the detector array to provide output reflected beam signals; sequentially disposing a reference fluid and a testfluid in contact with the first planar metallic layer; providing a first input beam and a second input beam to the first metallic layer, respectively, via the input facets (402, 404); delivering to the detector array: two reflected beam signals when the top side of the metallic layer is in contact with the reference fluid and two reflected beam signals when the top side of the metallic layer is in contact with the test fluid; and processing said reflected beam signals to measure a spacing, at the detector array, between a reference SPR signal and a test SPR signal.