Abstract: A sensor of a parameter such as temperature includes an indicator encapsulated within a rigid enclosure, wherein the sensor has a characteristic that varies with the parameter that is detectable upon illumination with electromagnetic radiation through a window of the enclosure that is transparent to the radiation. In a specific example, the indicator changes an optical characteristic such as its color as a function of its temperature, and may be of an irreversible type in order to indicate the peak temperature reached. The sensor may include a pattern of such indicators that have different peak temperatures to which they respond, so that the sensor gives a unique visual pattern at each temperature within its measurement range. This pattern may be viewed directly or processed by computer to compare the pattern with those which indicate known temperatures.