What does luminous efficacy depend on and how efficient can a white LED actually be? So far, no attention has been paid to a uniform definition of luminous efficacy and operating conditions.

DIAL has mathematically determined the theoretical maximum luminous efficacy of various spectra. First, let’s take a look at the retina of the human eye. It has about seven million receptors: red, green, and blue ones. These perceive color and are called cones. However, approximately 60 percent of them are green receptors. Therefore, humans perceive the color green as much brighter than red or blue although the physical radiant power is the same.

Of course, monochromatic green light is not suitable for most lighting purposes even if it is the most efficient. The designer prefers to use white light with different color temperatures and optimal color rendering quality. But simply filling the spectral distribution with further wavelengths in the visible field (380 nm to 780 nm) will lead to a drop in the theoretical maximum luminous efficacy.

Consequently, there is no single numerical value for the maximum luminous efficacy of white LEDs because it always depends on the spectral distribution.

Table courtesy of DIAL
Table courtesy of DIAL

The table to the right shows the theoretical maximum luminous efficacy of different spectra.

Besides LEDs, you can also find several examples of temperature radiation and gas discharge lamps in the table. The system efficiency and the luminous flux of the lamps were measured in DIAL's accredited photometric laboratory in Lüdenscheid/Germany. These values are the basis of the system luminous efficacy. Regarding the relative luminous sensitivity curve for photopic vision V (λ), the theoretical maximum luminous efficacy was calculated for each spectrum.

You can see that the typical spectrum of a warm white LED achieves a theoretical module luminous efficacy of approx. 320 lm/W. However, since the assumption is that there is loss-free conversion of physical radiated power into the wavelengths of the spectrum, the module’s actual realized luminous efficacy is much smaller. In the future, it may be possible to achieve system luminous efficacy in the range of 200 to 250 lm/ W.

In addition, the overview shows the energy conversion efficiency of the lamps examined, which describes how much of the power is converted into visible light. In this respect, efficient LEDs with both a high energy conversion rate and a high luminous efficacy (lm/W) are clearly well ahead of conventional lamps. While energy conversion efficiency of incandescent lamps is between 10 and 20 percent, highly efficient LEDs currently achieve values between 40 and 50 percent. Nevertheless, this means that 50 to 60 percent of the power is lost as heat.

In the coming years there is not likely to be an increase in the achievable luminous efficacy comparable to what happened in the first years after white LEDs went into serial production. The curve of the maximum luminous efficacy of newly developed products is slowly levelling off. A note for designers: Look carefully and always use your common sense.

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