The highest luminous efficacy of a white LED
In recent years, lamp efficiency has increased notably thanks to the use of white LEDs in architectural lighting. Retailers claim a massive leap in luminous efficacy is coming, but such assertions lack credibility.
What drives luminous efficacy, and how efficient can white LEDs really be? In recent years, manufacturers have competed over their LED products' luminous efficacy. Yet, no consensus has been reached on a uniform definition of luminous efficacy or standard operating conditions-often leading to interpretations that confuse lighting planners and designers.
DIAL has used mathematical methods to determine the theoretical maximum luminous efficacy of various light spectra.
Theoretical Maximum Luminous Efficacy of White LEDs
The human retina has around 7 million color-perceiving receptors called cones, split into red, green, and blue types. Green cones make up roughly 60% of the total, so humans perceive green as much brighter than red or blue-even when all have the same physical radiant power.
For eyes adapted to bright conditions, maximum relative spectral sensitivity occurs at 555 nm. Green light at this wavelength creates the strongest brightness perception, with a theoretical maximum luminous efficacy of 683 lm/W-known in the industry as the photometric radiation equivalent, Km. In practice, though, this value is unreachable, as it would require converting 1 W of physical radiant power into visible light with no losses.
Even if monochromatic green light were the most efficient, it's unsuitable for most lighting needs. Planners prefer white light, which comes in various color temperatures and offers optimal color rendering. But adding more wavelengths within the visible range (380–780 nm) to the spectral distribution will lower the theoretical maximum luminous efficacy.
Even if monochromatic green light were the most efficient, it's unsuitable for most lighting needs. Planners prefer white light, which comes in various color temperatures and offers optimal color rendering. But adding more wavelengths within the visible range (380–780 nm) to the spectral distribution will lower the theoretical maximum luminous efficacy.
The dependency of luminous efficacy on the spectrum
The following table shows the theoretical maximum luminous efficacy of different spectra as determined mathematically by DIAL:
Beyond LEDs with varying color temperatures, there are also examples of temperature radiation and gas discharge lighting. The system efficiency and luminous flux of the lamps or modules featured in this table were tested and measured in DIAL's own accredited photometric laboratory-and these measurements form the basis for calculating system luminous efficacy. Using the relative luminous sensitivity curve for photopic vision (denoted as V(λ)), the theoretical maximum luminous efficacy was computed for each light spectrum.
From the table, it's evident that the typical spectrum of a warm white LED achieves a theoretical module luminous efficacy of approximately 320 lm/W. However, this calculation assumes lossless conversion of physical radiant power into the wavelengths of the spectrum; in reality, the achievable module luminous efficacy is far lower. Looking ahead, it may be possible to reach system luminous efficacy in the range of 200–250 lm/W.
Additionally, the overview includes the energy conversion efficiency of the tested lamps-a metric that describes what proportion of the input power is converted into visible light. In this regard, efficient LEDs outperform conventional lamps by a clear margin. For instance, incandescent lamps have an energy conversion efficiency of just 10% to 20%, while today's highly efficient LEDs achieve values between 40% and 50%. Even so, this still means 50% to 60% of the power is lost as heat.
These figures, however, should not obscure the fact that many LEDs currently on the market have significantly lower system luminous efficacy-and their energy conversion efficiency is correspondingly poor.
Goodbye to the Idea of Huge Advances in Luminous Efficacy
In the years ahead, the increase in achievable luminous efficacy is unlikely to match the rapid gains seen in the early years after white LEDs entered mass production. The curve tracking the maximum luminous efficacy of newly developed LED products is gradually flattening out.
That said, the average luminous efficacy of LED luminaires will undoubtedly continue to improve. This is because there are still many luminaires on the market with a luminous efficacy of 50–70 lm/W-figures that leave ample room for optimization. DIAL's measurements, conducted in its accredited laboratory over the past few years, clearly demonstrate this trend.
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