A reflective materials sensor for detecting remotely located reflective material (12,77,78) within an area, the reflective materials sensor comprising: a transparent window having first and second window surfaces (26 and 28); an operating parameters sensor, for example a temperature sensor, and a controller for executing an algorithm to determine a temperature of the transparent window, wherein the controller is in communication with the operating parameters sensor; a first radiation detector and at least one further radiation detector located away from the second window surface; first and second spaced apart radiation emitters (18 and 20) located on either side of radiation detectors (22, 72) and away from the second window surface, each radiation emitter (18, 20) being located to emit radiation along a first axis (32, 34), respectively, through the transparent window towards reflective material in the area located away from the first window surface, the radiation detectors (22, 72) being located to receive reflected radiation from the reflective material through the transparent window along a second axis (36,76), the first axes (32, 34) of the radiation emitters (18, 20) being angled towards the second axis (36,76) of the reflected radiation; wherein the sensor further comprises a first lens and one or more second lenses, and wherein the first lens combined with the one or more second lenses form an array arranged to focus reflected radiation from spatially separated reflective material elements (12, 77, 78) on an array of the first radiation detector and the one or more further radiation detectors, the radiation detectors (22,72) and the second axis being in between the first and second radiation emitters (18, 20) and their first axes (32, 34) to form a common plane comprising the first and second axes (32,34,36); and wherein the transparent window is positioned to form a non-perpendicular angle to the common plane, thereby directing transparent window reflection axes of radiation emitted along the first axes (32,34) and reflected at the window surface away from the second axis, in order to prevent radiation from radiation emitters (18,20) from being reflected to the radiation detectors (22,72) by the transparent window. a transparent window having first and second window surfaces (26 and 28); an operating parameters sensor, for example a temperature sensor, and a controller for executing an algorithm to determine a temperature of the transparent window, wherein the controller is in communication with the operating parameters sensor; a first radiation detector and at least one further radiation detector located away from the second window surface; first and second spaced apart radiation emitters (18 and 20) located on either side of radiation detectors (22, 72) and away from the second window surface, each radiation emitter (18, 20) being located to emit radiation along a first axis (32, 34), respectively, through the transparent window towards reflective material in the area located away from the first window surface, the radiation detectors (22, 72) being located to receive reflected radiation from the reflective material through the transparent window along a second axis (36,76), the first axes (32, 34) of the radiation emitters (18, 20) being angled towards the second axis (36,76) of the reflected radiation; wherein the sensor further comprises a first lens and one or more second lenses, and wherein the first lens combined with the one or more second lenses form an array arranged to focus reflected radiation from spatially separated reflective material elements (12, 77, 78) on an array of the first radiation detector and the one or more further radiation detectors, the radiation detectors (22,72) and the second axis being in between the first and second radiation emitters (18, 20) and their first axes (32, 34) to form a common plane comprising the first and second axes (32,34,36); and wherein the transparent window is positioned to form a non-perpendicular angle to the common plane, thereby directing transparent window reflection axes of radiation emitted along the first axes (32,34) and reflected at the window surface away from the second axis, in order to prevent radiation from radiation emitters (18,20) from being reflected to the radiation detectors (22,72) by the transparent window.