The invention provides a luminescent based sensor having a luminescent material optically coupled to a substrate, and adapted to be used in a medium or environment such as water or air. A detector is provided to detect light that is emitted into the substrate by the material. The substrate is adapted to redirect light that is emitted into the substrate at angles with the range θesc ? θ ? θlsc where θesc is the critical angle of the environment/substrate interface and θlsc is the critical angle of the luminescent layer/substrate interface. Examples of possible configurations are described.