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How is direct chemiluminescence different from indirect chemiluminescence?

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time-lapse photo of fireflies — Bioluminescence Time-lapse image of fireflies in the Catskill Mountains, New York. Firefly light is a cold light, with approximately 100 percent of the energy given off as light and only a minute amount as heat. Only light in the visible spectrum is emitted.

The primary difference between direct and indirect chemiluminescence lies in how the energy produced by an oxidation reaction is converted into light.12

Direct Chemiluminescence

In direct chemiluminescence, the high-energy intermediate molecule formed during an oxidation reaction emits a photon directly as it returns to its ground state. This process is exemplified by the forensic use of luminol, which emits a blue glow when oxidized by hydrogen peroxide in the presence of a catalyst.2

Indirect Chemiluminescence

In indirect, or sensitized, chemiluminescence, the oxidation energy is not emitted directly by the intermediate. Instead, the energy is transferred to a [fluorophore]—a molecule capable of absorbing and emitting light—which then releases the radiation as it returns to its ground state. A common example is the glow stick, where a high-energy intermediate transfers its excitation energy to a fluorescent dye to produce light.12

While bioluminescence is a specialized form of chemiluminescence, it is characterized by a direct conversion of chemical energy to radiant energy that is nearly 100 percent efficient.23