An apparatus for measuring the light absorbance of a substance in a solution, comprising: - at least one sample cell arranged to contain said solution that is at least partially transparent to light of a predefined wavelength spectrum; - at least two light passages (305,305') through said at least one sample cell each of said light passages (305,305') having a known light path length; - at least two photodetectors (41,42) and - an LED light source arrangement comprising at least two UV LEDs (21, 22) each arranged to emit a light output with a UV wavelength different to the other UV LED or LEDs within said predefined wavelength spectrum, wherein each of the at least two UV LEDs (21, 22) includes an associated optical fiber bundle, the optical fiber bundle comprising plural optical fibers, one for each light passage (305,305'), each optical fiber bundle providing a plurality of discrete light guides from each respective UV LED (21, 22) to each of the light passages (305,305'), wherein the apparatus is configured to pass light from the LED light source arrangement through each of the light passage (305,305') to be received by respective photodetectors (41,42), wherein the LED light source arrangement further comprises at least one optical filter (50, 51) for each UV LED (21, 22), said optical filter being arranged between the UV LED (21, 22) and the optical fiber bundle, wherein said at least one sample cell is a dual path length flow cell comprising said at least two light passages (305,305'), and wherein each of said light passages has a path length differing from each of the other light passages, and wherein glass rods are provided and inserted into the sample cell to create a first and second light passage (305, 305') therein and where the first light passage has a smaller path length than a path length of the second light passage. - at least one sample cell arranged to contain said solution that is at least partially transparent to light of a predefined wavelength spectrum; - at least two light passages (305,305') through said at least one sample cell each of said light passages (305,305') having a known light path length; - at least two photodetectors (41,42) and - an LED light source arrangement comprising at least two UV LEDs (21, 22) each arranged to emit a light output with a UV wavelength different to the other UV LED or LEDs within said predefined wavelength spectrum, wherein each of the at least two UV LEDs (21, 22) includes an associated optical fiber bundle, the optical fiber bundle comprising plural optical fibers, one for each light passage (305,305'), each optical fiber bundle providing a plurality of discrete light guides from each respective UV LED (21, 22) to each of the light passages (305,305'), wherein the apparatus is configured to pass light from the LED light source arrangement through each of the light passage (305,305') to be received by respective photodetectors (41,42), wherein the LED light source arrangement further comprises at least one optical filter (50, 51) for each UV LED (21, 22), said optical filter being arranged between the UV LED (21, 22) and the optical fiber bundle, wherein said at least one sample cell is a dual path length flow cell comprising said at least two light passages (305,305'), and wherein each of said light passages has a path length differing from each of the other light passages, and wherein glass rods are provided and inserted into the sample cell to create a first and second light passage (305, 305') therein and where the first light passage has a smaller path length than a path length of the second light passage.