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In 1802, Davy showed that artificial light was produced by passing electrical current through a platinum wire. Although simpler than the open arc lamp, it was not as bright. Nevertheless, in 1820, De La Rue turned Davy’s observation into the first incandescent light bulb. In 1879, Swan enhanced the brightness by using a thin carbon filament instead of platinum wire. The same year, Edison patented an incandescent lamp based on a thin cotton filament encased in a partially evacuated tube. His lamp burned brighter and longer (50 hours) than any other incandescent lamp, and it soon replaced arc lamps as the most popular form of artificial lighting. In 1906, Coolidge invented the tungsten light bulb. Tungsten is more malleable than other metals allowing it to be coiled; with more wire, it burned brighter and longer than other incandescent bulbs. Tungsten also emits a broader spectrum than carbon-based filaments yielding a whiter (and more UV-intensive) light.Another significant development in photophysics was the invention of devices for quantifying radiation. In 1829, Nobili invented the thermopile, and it was improved in 1852 by Melloni. In 1876, Crookes invented the rotating vane radiometer, and in 1878 Langley invented the bolometer. All three inventions used blackened metal to absorb radiation, but each device differed as to how the radiation was quantified. The thermopile used a stack of tightly packed metal plates to amplify the photoelectric signal. The radiometer measured light intensity by the number of revolutions induced over time, and the bolometer measured a decrease in electrical resistance upon absorption of radiation. Each provided an effective means of measuring radiation throughout the UV-visible-infrared spectrum.
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Early in the 20th century, new discoveries in photochemistry and photophysics improved both theoretical and empirical understandings of the behavior of electromagnetic radiation. In 1900, Planck theorized that radiation is comprised of tiny packets of energy called “quanta.” In 1905, Einstein theorized that Planck’s quanta were massless particles of energy (named “photons” in 1928 by Lewis) that are released from atoms and molecules upon absorption of light. In 1913, Bohr proposed that electrons absorb the light energy and re-emit it at wavelengths that correspond to the electron’s energy. In 1926, Schrödinger developed a theory of wave mechanics that treated electrons as waves rather than particles. These theories provided a new conceptual framework for studies of radiation.About the same time, experimentalists were devising new ways to measure the extent of UV radiation. In 1903, Schumann used a carbon spark discharge lamp and fluorite prism placed in a vacuum chamber (called a “vacuum spectrograph”) to detect the emission of hydrogen at 120 nm. In 1906-08, Lyman used the vacuum spectrograph to detect emission of helium at 50 nm. He also demonstrated that oxygen, but not nitrogen, absorbs radiation between 127-176 nm. In 1920, Millikan used a high intensity nickel spark lamp in a vacuum spectrograph to measure the emission of hydrogen at 20 nm. He also detected the emission of weak X-rays indicating that there was no natural cut-off between UV and X-rays.
