A remote sensor device comprises at least one sensor that produces an output signal indicative of the value of a measure, and a reflective display that is positioned such that it is visible along a line of sight from a remote location, the reflective display comprising at least two retro-reflective assemblies, each of which is arranged to reflect a variable fraction of radiation incident upon the reflective assembly back towards a source in response to respective control signals applied to the reflector assemblies. Each of the reflector assemblies preferentially reflects one wavelength of the incident radiation or preferentially removes at least one wavelength of the incident radiation that is not preferentially reflected or removed by the other reflector assembly. The device further comprises modulating means for modulating the control signals applied to the reflector assemblies as a function of the value of the output signal from the sensor so as to modulate the fractions of incident light reflected by the display.