An apparatus for detecting a target substance in a pixel array is provided. The apparatus has a light source to emit a light suitable for exciting the target substance in the array, a total internal reflection (TIR) member, and a light detector. The TIR member has a TIR surface on which the array is located. The light from the light source passes into the TIR member and is reflected by the TIR surface. The array is within an evanescent field region at the TIR surface. The light detector is adapted to detect light emitted from the array as a result of evanescent excitation of the target substance.