A sensor is described for use with at least one optical detector (5), the sensor comprising a substrate (11) with optical outlets, a porous membrane (12) and micro fluidic channels (13) for conducting analyte fluid towards sensing locations (10) of the porous membrane (12). The sensing locations (10) are adapted for at least restraining light variable molecules (23) which bind to analytes to be determined. The optical output of the light variable molecules changes when they are in close proximity to a target molecule. The micro fluidic channels (13) are shaped to reflect light emitted from the sensing locations (10) towards the optical outlets and the substrate has diffracting optical elements (15) aligned with microfluidic channels to diffract light towards the optical outlets. The diffracting optical elements can be lenses.