How much light comes from a firefly?

In the middle of a forest, surrounded by a flickering insect constellation, your first thought is likely, “This feels magical,” followed by questions like, “Why do fireflies glimmer, anyway?” or “How does bioluminescence work?”

For David Silver, the question is much more specific: Precisely how many photons does a firefly emit in each flash?

This is surprisingly nuanced, with the most often quoted value — on the order of 10¹³-10¹⁴ photons — being much larger than what we actually see.

“Real fireflies look much dimmer than LEDs of comparable apparent brightness,” Silver said.

A firefly’s lantern consists of hundreds of thousands of photocytes, which act as the workshop for creating luminescence. Each photocyte holds about a million copies of luciferase, an enzyme that burns fuel called luciferin when it interacts with oxygen. Silver wanted to follow a single firefly photon end to end, from the luciferin’s quantum excitation, through the firefly’s reflective layer, and out into the wild as a courtship signal.

“To do that, you have to put numbers on every layer,” Silver said. “The textbook brightness didn’t match the biochemistry by two to three orders of magnitude, and a number off by that much for a century is worth knowing about.”

With a photographer and a $30 lux meter in a quiet field site, Silver sought to answer the question once and for all. Comparing the field measurements with a calculation of the upper bound of the quantum yield and a reanalysis of early work, including the original measurement, Silver put a new cap on fireflies’ photon output: 10¹⁰-10¹¹.

“Even if every enzyme fires once and every reaction makes a photon, you cap out near 10¹¹,” he said, noting that quantum limits reduce this even further. “There is simply no biochemical room for 10¹⁴.”

Because fireflies luminesce as a courting behavior, their signal is likely naturally tuned to the minimum that works. With the new cap in hand, Silver suggests that other early bioluminescence measurements might need a revisit, too.

Source: “How bright is a firefly? resolving a century of overestimation,” by David H. Silver, American Journal of Physics (2026). The article can be accessed at https://doi.org/10.1119/5.0325834 .