LED efficiency comparison · CAP 2019 #23
Physics · optics
Problem
The following graphs show the energy emitted by two LED's as a function of wavelength. These are both 0.2W LED's (they take 0.2 W of electricity when connected). Which one is the most efficient visible light source and what is approximately its efficiency. [Figure: Two graphs showing energy vs. wavelength for Diode 1 and Diode 2] a) Diode 1, 50% b) Diode 2, 40% c) Diode 1, 25% d) Diode 2, 20% e) Both have the same efficiency, 50%Intuitive explanation
The key is comparing how much of each LED's total 0.2 W output falls within the visible spectrum (roughly 400–700 nm). An efficient visible light source converts as much of its input power as possible into light we can actually see, rather than wasting energy on infrared or ultraviolet. Look at each graph to estimate what fraction of the total emitted power lies in the visible range—the LED with the larger visible fraction is more efficient at producing usable light.
Intuitive explanation
The key is comparing how much of each LED's total 0.2 W output falls within the visible spectrum (roughly 400–700 nm). An efficient visible light source converts as much of its input power as possible into light we can actually see, rather than wasting energy on infrared or ultraviolet. Look at each graph to estimate what fraction of the total emitted power lies in the visible range—the LED with the larger visible fraction is more efficient at producing usable light.