William Herschel, born November 15, 1738, was known for many things. He doubled our awareness of the size of the known universe by finding and naming Uranus, he wrote more than two dozen symphonies and, of course, in 1800 he discovered the “dark heat”—or what we’ve come to know as infrared radiation.
Herschel’s famous experiment, measuring the temperature of the various colors of light after it passed through a prism, was devised to help him find filters for his telescope that would not be heated by the sun. But serendipity was at his side that day and his curious mind was ripe to recognize something totally unexpected. It quickly became clear there was more to light than anyone had imagined.
I recently had the thrill of visiting Herschel’s home in Bath (UK) and not only stood in the room where he conducted his work, but also saw the prism he actually used! It was not hard to imagine the sun coming through the slit in the shade, passing through the prism to cast the rainbow of colors over the thermometers. As he worked, the sun shifted until the invisible area beyond red caused the temperature to register warmer on one of the thermometers. The thermometers, unfortunately, are currently housed at another museum, but Professor Francis Ring (one of the founders of the Herschel Society) hopes they will soon return one day. He also hopes to be able to recreate the experiment—what a remarkable experience that will be!
I would encourage all readers to make their “thermal pilgrimage” to Bath to visit the Herschel Museum. At the same time, you can see the famous hot (46°C/115°F) spring baths from which the city derives its name. These were probably first used as a sacred site three thousand years ago or more. The Romans, occupying the area around 60 A.D., built an elaborate infrastructure around them, much of which has since been unearthed and can be seen in the remarkable museum at the site. Bath itself is a beautiful city of Georgian architecture filled with artistic and intellectual pursuits.
During the same trip, I counted myself very fortunate to be an invited speaker at the “Infrared 100” conference, sponsored jointly by The Royal Astronomical Society (Herschel was the first president) and the Imaging Science Group of the Royal Photographic Society. The 2-day event celebrated the 100th anniversary of the first infrared image taken in the near-IR band in 1910 by Robert Wood at Johns Hopkins. Several of us spoke about more recent developments in thermal imaging, including my presentation on the value it provides in understanding energy use in buildings. Professor Ring showed the first thermal image he made in 1959 of one of his patients suffering from arthritis in his knees. It took ten minutes to make a single, scanned image!
When you pick up your thermal imaging system later today, know that we have arrived at this point only due to the efforts of many great people who have preceded us, not the least of whom is Sir William Herschel. If he were here today, I’d wish him a “thermally pleasant” Happy Birthday—272 candles would make quite an interesting thermal image!
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner