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Firefly brightness holds a cautionary tale about accepting older measurements
For over a century, the accepted value for a firefly's brightness has mostly stood, tracing its origins to experiments carried out in 1912. Through rigorous new analysis published in the American Journal of Physics, David Silver of Remiza AI in New York has discovered that this value has likely been vastly overestimated. His results provide a stark reminder of what can happen when widely accepted older measurements are converted into modern standard units.
A century-old measurement
Out of the hundreds of species of animals, fungi and bacteria that produce their own light, fireflies are the most widely studied. In the 1880s, experiments revealed that their flashing bioluminescence emerges from a catalyzed reaction between an organic compound named luciferin and an enzyme named luciferase.
In 1912, the brightness of these flashes was measured for the first time by William Coblentz—one of the founders of modern radiometry. "Coblentz reported that the flash of the firefly Photinus pyralis ranged from 1/50th to 1/400th the power of a candle, with 1/400 predominating," Silver describes.
"Later, with Herbert Ives, he did painstaking absolute photometry with the instruments of the day, and the value was faithfully recorded at the time."
Yet as the field progressed, expressing this brightness as an absolute photon count required a more rigorous unit than candlepower. After passing through successive textbooks and being converted to newly standardized units, its accepted value settled at roughly a milliwatt of visible light. This corresponded to around 1/40 candlepower—placing it firmly at the higher end of the brightnesses measured by Coblentz.
At the same time, the field of bioluminescence moved away entirely from describing brightness in terms of overall photon counts, turning instead to more robust metrics such as quantum yields and relative light units. As a result, the accepted value for absolute photon count sat unscrutinized for decades.
Fresh analysis
In his study, Silver revisited this value from four different angles. First, he calculated the photon count from first-principles estimates of enzyme abundance and the quantum yield of the luciferin–luciferase reaction. Second, he used a calibrated lux meter to measure the brightnesses of wild fireflies in Malaysia. Third, he carefully reread Coblentz's original report from 1912. Finally, he reanalyzed later measurements of the brightness value.
"My goal was to get several unrelated methods to converge," Silver describes. "All four lines land on roughly 10⁸ to 10¹¹ photons per flash. The canonical '1/40 candle' figure, converted with modern photometry, implies 10¹³–10¹⁴—two to four orders of magnitude too high."
Revisiting old assumptions
This vast overestimation uncovers an inaccuracy that has persisted for over a century, even in modern textbooks. For biologists, who are more concerned with absolute brightness than modern bioluminescence standards like quantum yields, Silver's discovery could lead to fresh insights into firefly courtship.
Since the insects' visual systems are tuned to the minimum amount of light they can detect, their courtship rituals—involving the exchange of flashing signals—could work on far less light than previously assumed.
More broadly, the result serves as a cautionary tale of the potential consequences of accepting long-established values without investigating their origins more closely.
"It's a reminder that absolute photon counts drifted out of the literature when the field switched to relative units, and other century-old values may deserve the same revisit," Silver explains. "It also makes a clean teaching example: the whole story can be reproduced with a quick estimate, a $30 lux meter and a careful reading of a historical paper."
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Publication details
David H. Silver, How bright is a firefly? Resolving a century of overestimation, American Journal of Physics (2026). DOI: 10.1119/5.0325834
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Citation: Firefly brightness holds a cautionary tale about accepting older measurements (2026, July 13) retrieved 13 July 2026 from https://phys.org/news/2026-07-firefly-brightness-cautionary-tale-older.html
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