An apparatus and method for detecting the concentration of a substance in a fluid. A light source 12 generates and directs light onto a housing 10, which contains the fluid 20. The intensity of light transmitted through the housing is detected. The housing is configured to focus light passing through 22, and may be a tube structure; hollow, with a curved surface acting as a lens. The housing may be connected to pipes allowing fluid to flow through. The apparatus may be used in a water purification system to control the concentration of ozone, iron oxide or manganese in water, compared to a target concentration (figures 11 & 12). Other uses may be to detect iron, zinc, sulphur in petrol, diesel, oil. The housing may be made from quartz. The light source may be an ultraviolet LED or laser. The detector 14 may be a photosensitive device.
