An apparatus for the detection of a fluid-borne particle (10,110), the apparatus comprising: i) a scattering chamber; ii) a means for providing a sample of fluid, containing the fluid-borne particle (10, 110), in the form of a flow through an optically defined particle sensing zone wherein the optically defined particle sensing zone is contained in the scattering chamber; iii) a laser for generating a beam (12, 112) of radiation through the optically defined particle sensing zone; iv) a single reflector or refractor (14, 114), having a primary focus within the optically defined particle sensing zone and a secondary focus located outside the beam of radiation; v) a single scattered light detector (24, 124), to detect the scattered light emitted by the particles (10,110) in the optically defined sensing zone, said detector (24,124) being located at the secondary focus of the single reflector or refractor (14,114), the single scattered light detector (24,124) comprising a first photosensitive detection area and a second photosensitive detection area wherein the detection areas are integral and the first photosensitive detection area is located wholly inside the second photosensitive detection area and wherein the first and second photosensitive detection areas are electrically separate from one another; vi) a means for deriving data from the scattered light detected by the first photosensitive detection area and second photosensitive detection area of the scattered light detector (24,124) wherein the single reflector or refractor (14,114) is adapted to direct radiation scattered from the fluid borne particule (10,110) passing through the beam of radiation (12,112) within the optically defined particle sensing zone to the scattered light detector (24, 124) and wherein the optically defined sensing zone comprises a first zone, the first zone being the zone where the scattered light detected by the first photosensitive detection area is generated, and a second zone, the second zone being the zone where the scattered light detected by the second photosensitive detection area is generated, wherein the first zone is smaller than the second zone and wherein the first zone is located wholly inside the second zone. i) a scattering chamber; ii) a means for providing a sample of fluid, containing the fluid-borne particle (10, 110), in the form of a flow through an optically defined particle sensing zone wherein the optically defined particle sensing zone is contained in the scattering chamber; iii) a laser for generating a beam (12, 112) of radiation through the optically defined particle sensing zone; iv) a single reflector or refractor (14, 114), having a primary focus within the optically defined particle sensing zone and a secondary focus located outside the beam of radiation; v) a single scattered light detector (24, 124), to detect the scattered light emitted by the particles (10,110) in the optically defined sensing zone, said detector (24,124) being located at the secondary focus of the single reflector or refractor (14,114), the single scattered light detector (24,124) comprising a first photosensitive detection area and a second photosensitive detection area wherein the detection areas are integral and the first photosensitive detection area is located wholly inside the second photosensitive detection area and wherein the first and second photosensitive detection areas are electrically separate from one another; vi) a means for deriving data from the scattered light detected by the first photosensitive detection area and second photosensitive detection area of the scattered light detector (24,124) wherein the single reflector or refractor (14,114) is adapted to direct radiation scattered from the fluid borne particule (10,110) passing through the beam of radiation (12,112) within the optically defined particle sensing zone to the scattered light detector (24, 124) and wherein the optically defined sensing zone comprises a first zone, the first zone being the zone where the scattered light detected by the first photosensitive detection area is generated, and a second zone, the second zone being the zone where the scattered light detected by the second photosensitive detection area is generated, wherein the first zone is smaller than the second zone and wherein the first zone is located wholly inside the second zone.