An arrangement for detecting colour changes in fluids is proposed that comprises an optical arrangement for detecting the intensities of certain colour frequencies. The optical arrangement includes a white light emitter (410) and a detector (420) arranged to generate signals representing the detected intensity of red, blue, green and white light. A processor (440) receives the signals and calculates values representing the ratio of detected red to white light, blue to white light and green to white light. The processor further compares these ratios with reference values and to generate a signal when the results of the comparison indicate that the ratios differ from the reference values. By generating the ratio of intensity of each received colour to the intensity of received white light, the values are rendered virtually self- compensating and thus largely insensitive to variations in temperature and in intensity that may occur over time with aging of the components.
