The efficiency of a lamp is expressed in which terms?

The efficiency of a lamp is expressed in terms of efficacy. Efficacy refers to the measure of how well a light source converts electrical energy into visible light, typically expressed in lumens per watt (lm/W). This metric is crucial as it helps in comparing different lighting options based on how much light they produce for the amount of energy consumed. A higher efficacy indicates a more efficient lamp, meaning that it provides more light output for each unit of energy used.

The other options do not accurately represent the concept of lamp efficiency. An efficiency rating could be a general term but lacks the specific context that efficacy provides. Illuminance refers to the amount of light falling on a surface, measured in lux or foot-candles, and does not directly relate to how effectively a lamp converts energy. Lumen output simply indicates the total amount of visible light emitted by the lamp, but does not communicate the efficiency of this process in relation to energy consumption. Thus, efficacy is the most precise term for describing lamp efficiency.