A system for measuring an evanescent wave phenomenon at total internal reflection, the system comprising: a) a sensing surface comprising a plurality of areas of interest; b) an illumination sub-system comprising a light source, which illuminates each area of interest on the sensing surface over a range of angles of incidence; c) a detector which responds differently to an intensity of light received by it at different locations; and d) projection optics comprising primary optics and a plurality of secondary elements, the primary optics projecting an image of the illuminated sensing surface onto the secondary elements, which project their received light onto the detector in such a way that it is possible to determine, from the response of the detector, how much light is reflected from each area of interest, as a function of angle of incidence over the range of angles for that area.