concept

Quantum yield

The ratio of light particles (photons) emitted to molecules (or photons) that react - a measure of how "efficient" a light-producing process is. A quantum yield of 1 means every fuel molecule that reacts yields exactly one photon, with no waste. Early measurements suggested the firefly reaction reached nearly that perfection; later, more careful measurements lowered the figure, but the conclusion holds: compared with any way of making light by getting hot, biological light is extraordinarily energy-cheap.