Infrared thermometers are devices used to remotely measure temperature in situations where it is not possible to be in physical contact with the object being measured. This includes objects that are very hot, very small or very far away. Infrared thermometers are also well suited for measuring objects that are especially prone to minute changes in temperature, or which cover vast areas making use of conventional thermometers impractical.
Variable in design and size - ranging from something that would fit in your palm to a 200 lbs. telescope-shaped device - infrared thermometers take advantage of the fact that, above absolute zero, all objects emit electromagnetic radiation or energy. By measuring the energy given off by objects in two different wavelength regions of the infrared portion of the spectrum, infrared thermometers internally compare the different readings in a ratio which corresponds to a known set of values linking energy distribution with wavelength to temperature.
Infrared temperature measurements were made as early as the mid-19th century, but they proved less-than-reliable. Accuracy did not improve until physicist Max Planck (1858-1947) speculated around the turn of the century that radiation was not emitted in a continuous wave across the spectrum, as had been widely assumed. Instead, he found it was emitted in whole number multiples of 6.625 x 10-34 joule-sec - now referred to as "Planck's Constant" - making it necessary to revise the technique of measuring. Two readings are required because certain properties like reflectivity, texture and wavelength sensitivity can degrade accuracy.
In taking an infrared temperature reading of the moon, a simplified example would use a telescope hooked up to an infrared detector, which converts infrared radiation into an electrical current or voltage. As two different infrared filters are placed sequentially over the lens, the detector registers two different readings. One reading is then divided into the other, and the number that results corresponds to a temperature that can be found in existing tables of values for Planck's Equation. It should be noted that in trying to ascertain the temperature of a large, distant body like the moon, one must take care to ensure that its entire image fills the area of infrared detector's sensor, lest the cold void of space influence the reading.