What happens to the efficiency of an incandescent lamp when it is dimmed?

When an incandescent lamp is dimmed, the efficiency decreases primarily due to the way incandescent lamps function. These lamps operate by passing an electric current through a filament, which heats up and produces light. The efficiency of a light source is often measured in lumens per watt, indicating how much light is produced for each watt of electrical power consumed.

When dimming the lamp, particularly through phase-cut dimming methods (like TRIAC dimmers), the voltage and current supplied to the filament are reduced. While it might seem that the total energy use decreases, the reduction in brightness is not directly proportional to the amount of energy saved. The filament will actually operate at a lower temperature, which results in a significant drop in luminous efficacy. Essentially, the amount of light produced per watt of power consumed decreases when the lamp is dimmed, leading to reduced efficiency.

In contrast, other lighting technologies, such as LEDs, may demonstrate different characteristics when dimmed, often maintaining or enhancing their efficiency under lower power conditions. However, with incandescent lamps, the physics of heat generation and light output clearly indicate a decrease in efficiency when one attempts to dim them.