The relative efficacy of lamps refers to their efficiency in converting electrical energy into visible light, which is measured in lumens per watt (lpW). Lumens measure the total light output, and watts measure the power consumed, so lpW indicates how much light is produced per unit of energy, a key metric for comparing lamp efficiency. Option A (luminous flux, lm) measures total light output but does not account for energy use, so it’s not a measure of efficacy. Option C (foot-candles or lux) measures illuminance (light intensity on a surface), not lamp efficiency. Option D (light reflectance value, LRV) measures how much light a surface reflects, unrelated to lamp efficacy.
Verified Answer from Official Source:
The correct answer is verified using NCIDQ IDFX content on lighting design and technology.
Exact Extract:TheNCIDQ IDFX Reference Manualstates, “The relative efficacy of lamps is measured in lumens per watt (lpW), indicating the efficiency of light output per unit of energy consumed.”
The NCIDQ IDFX curriculum includes lighting principles, emphasizing lumens per watt as the standard measure for lamp efficacy, which is critical for sustainable design and energy efficiency.
Objectives:
Understand lighting metrics and their applications (IDFX Objective: Building Systems and Technology).
[References:, NCIDQ IDFX Reference Manual (Section on Lighting)., IESNA Lighting Handbook (Lamp Efficacy Metrics)., ]
