Leonard Troland and the story of the photon's name

posted by @ulaulaman via @peppeliberti #photon #physics #LeonardTroland
Gilbert Newton Lewis was a physical chemist who used the name "photon" in order to describe the light(1, 2).
Probably unknown to Lewis and almost all contemporary physicists, the word "photon" can be found in the scientific literature as early as 1916. It was coined by the American physicist and psychologist Leonard Thompson Troland, who used it as a unit for the illumination of the retina. Although little known today, and if known at all then for his work in experimental psychology, at the time he was considered one of America's most promising scientists. When he died tragically and prematurely in 1932 by a fall from the summit of Mount Wilson in California, his death was mourned in obituaries in Science (vol. 76, pp. 26-27) and American Journal of Psychology (vol. 44, pp. 817-820).
Troland introduced the "photon" in 1916 in the article On the measurement of visual stimulation intensities
(...) as a unit for physiological stimulus intensity, defining it as follows [Troland 1917, p. 32]:
A photon is that intensity of illumination upon the retina of the eye which accompanies the direct fixation, with adequate accommodation, of a stimulus of small area, the photometric brightness of which ... is one candle per square meter, when the area of the externally effective pupil ... is one square millimeter. The physiological intensity of a visual stimulus is its intensity expressed in photons. The photon is a unit of illumination, and hence has an absolute value in meter-candles. The numerical value of the photon, in meter candles, ... will obviously be subject to some variation from individual to individual.
Troland first suggested the photon in a presentation given to the tenth annual meeting of the Illuminating Engineering Society in Philadelphia 18-20 September 1916. "I have," he said, "found it very convenient to express all intensity measures in terms of a unit retinal illumination which I have called the photon"(3). In the discussion following his talk, he mentioned as an advantage of the new unit that "the photon unit does not require so much mathematics, and I have been interested primarily in helping the psychologists, many of whom are studying vision somewhat at random."
It seems that also Joly used, in 1921, the name "photon" before Lewis, but the story of Mr. Troland it seems really interesting, and you can read his whole story on Photon: New light on an old name by Helge Kragh.
(1) Lewis G.N. (1926). The Nature of Light., Proceedings of the National Academy of Sciences of the United States of America, 12 (1) 22-29. PMID:
(2) Lewis G.N. (1926). The Conservation of Photons, Nature, 118 (2981) 874-875. DOI:
(3) Troland, L. T. (1916). Apparent brightness; its conditions and properties. Transactions of the Illuminating Engineering Society (archive.org), 11, 947-975

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