Heat from Invisible Rays
In 1800 William Herschel, a German astronomer and composer, while testing filters to observe sun spots passed sunlight through a prism, observed a heating effect in the visible red portion of the spectrum. Testing further, he detected the presence of the greatest amount of heat in the area just beyond the red end of the visible spectrum now called Near-Infrared (NIR). He wrote two papers detailing his findings and later that year, presented his discovery of invisible "Calorific rays" to the Royal Society of London. Thirty-five years later Andre-Marie Ampere employed the newly invented thermocouple to demonstrate that near-IR (NIR) radiation was in fact invisible light. It wasn't until 1900 that the term infrared was applied to the part of invisible spectrum contiguous to the red end of the visible spectrum that comprises wavelengths from 0.74 microns to 300 microns.