Before the year 1800, the existence of the infrared portion of the electromagnetic spectrum wasn't even suspected. The original significance of the infrared spectrum (or simply “infrared,” as it’s often called) as a form of heat radiation is perhaps less obvious today than it was at the time of its discovery in 1800 by Sir William Herschel during his search for a new optical material.

Sir William Herschel (1738-1822), Royal Astronomer to King George III of England – and already famous for his discovery of the planet Uranus – was searching for an optical filter material to reduce the brightness of the Sun’s image in telescopes during solar observations.
While testing different samples of colored glass that gave similar reductions in brightness, he was intrigued to find that some of the samples passed very little of the Sun’s heat, while others passed so much heat that he risked eye damage after only a few seconds’ observation.

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