The efficiency of lamps has increased considerably in recent years with the use of white LEDs in the field of architectural lighting. Retailers claim that we can expect a gigantic leap forward in luminous efficacy. Such claims cannot be taken seriously.
What does luminous efficacy depend on and how efficient can a white LED actually be? Manufacturers have been competing with each other in recent years with regard to the luminous efficacy of their LED products. However, no attention has been paid to agreeing on a uniform definition of luminous efficacy and operating conditions. Often we find interpretations which are not comprehensible to the planner or designer.
DIAL has mathematically determined the theoretical maximum luminous efficacy of various spectra.
There are about 7 million receptors in the retina of the human eye: red, green and blue receptors. These perceive colour and are called cones. However, most of them (approx. 60 %) are green receptors. Therefore humans perceive the light colour green as much brighter than red and blue although the physical radiant power is the same.
For the human eye adapted to brightness the maximum relative spectral sensitivity is at a wavelength of 555 nm. The highest perception of brightness experienced by the human eye is generated with green light on the 555 nm wavelength. The greatest luminous efficacy which can theoretically be achieved at 555 nm is, altogether, 683 lm/W. In this context the expert refers to the photometric radiation equivalent Km. However, in practice this value cannot be reached, since, if this were so, it would mean that 1 W of physical radiant power can be converted loss-free into visible light.
Of course, monochromatic green light is not suitable for most lighting purposes even it were the most efficient. The planner likes to use white light with different colour temperatures and optimal colour rendering quality. But simply filling the spectral distribution with further wavelengths in the visible field (380–780 nm) will lead to a drop in the theoretical maximum luminous efficacy.
Of course, monochromatic green light is not suitable for most lighting purposes even it were the most efficient. The planner likes to use white light with different colour temperatures and optimal colour rendering quality. But simply filling the spectral distribution with further wavelengths in the visible field (380–780 nm) will lead to a drop in the theoretical maximum luminous efficacy.
The following table shows the theoretical maximum luminous efficacy of different spectra as determined mathematically by DIAL:
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