NOVELTY - The sensor for detecting environmental changes using optical properties of a sensing surface, comprises a surface (1) having a sensing material (2) that is responsive to a change in an environmental condition when illuminated, a source to illuminate the surface, a unit to receive light emitted from the surface, a unit for detecting a change in light received at the receiver, a variable reference voltage at the detector, a positive feedback loop, and a unit to indicate a change in light received at the receiver.    USE - The sensor is useful for detecting environmental changes using optical properties of a sensing surface, and for detecting hazardous substances e.g. gases.    ADVANTAGE - The sensor is a small module thus simply snapping into a place in the overall collection system, can be easily replaced during routine maintenance, and is rapid to prevent hazardous situations.    DETAILED DESCRIPTION - The sensor for detecting environmental changes using optical properties of a sensing surface, comprises a surface (1) having a sensing material (2) that is responsive to a change in an environmental condition when illuminated, a source to illuminate the surface, a unit positioned to receive light emitted from the surface, a unit for detecting a change in light received at the receiver, a variable reference voltage at the detector, a positive feedback loop, and a unit to indicate a change in light received at the receiver. The detector comprises a comparator. The surface comprises a coating having the sensing material. The environmental condition is temperature, pressure and atmospheric composition. The change in environmental condition is a change in atmospheric oxygen content. The indicator comprises a colored light. An emission spectrum or intensity of the sensing material changes in response to the change in the environmental condition when illuminated. The sensing material comprises a dye in a polymer matrix, and a polymer having pair detector compounds forming a fluorescence resonance energy transfer pair or forming an exciplex, where the dye is incorporated into a polymer material. An INDEPENDENT CLAIM is included for a method for detecting a change in an environmental condition.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of a sensor.    Surface (1)    Sensing material (2)    Emitted light (4)    Illumination source (5)    Receiver. (6)