In the field of substance detection an apparatus can be provided for detecting the concentration of a substance in a fluid that allows detection of a substance to be made in situ, in real time. In some examples the apparatus includes a transparent housing arranged to contain the fluid, a source operable to generate and direct light through the housing containing the fluid, wherein the housing is operable to focus light passing therethrough; and a detector operable to detect the intensity of light transmitted through the housing.
