An apparatus for measuring the absorbance of a substance in a solution, includes at least one sample cell arranged to contain the solution that is at least partially transparent to light of a predefined wavelength spectrum, at least two light passages through the at least one sample cell, each of the light passages having a known path length, an LED light source arrangement including at least two LEDs, each arranged to emit a light output with a wavelength within the predefined wavelength spectrum. A plurality of optical fibers, one for each light passage, is arranged at each LED for receiving the light output and guiding it to the light passages. A method for measuring the absorbance of a substance in a solution includes providing the LED light source arrangement with an associate fiber bundle for each LED.