Daylight quality lighting using Remote Phosphor Technology
Photographers know that the best lighting conditions are clear skies at dawn and dusk when the colour correlated temperature (CCT) of the light is 3500K and 2500K respectively, by mid-day or in cloud the CCT of the light will be greater than 6000K, at these temperatures colours towards the red part of the spectrum can look washed out.
This is the reason Jewellers and Quality fashion retailers use Tungsten Halogen Dichroic lamps which have excellent light quality with CCTs of 3000-3500K. The downside is that although Tungsten Halogen technology has improved over the years, efficiency and reliability figures are still only 20 lumens/Watt and 5000 hours.
Phosphor based white LED technology is seen as a perfect replacement for Halogen lighting offering a threefold increase in efficiency and tenfold increase in reliability. Unfortunately suppliers of these LED products have been misleading customers by making false claims that their products are direct replacements for Halogen lamps; most of these high efficiency LED lamps have CCTs of around 6000K with Colour Rendition Indexes (CRIs) in the region of 70, the LED lamps look brighter than they really are and the illumination from these lamps makes colour discrimination problematic!
White LED suppliers are trying to redress the situation by
introducing warm-white products with CCTs of 3500K. They are achieving
this by using new mixtures of phosphor coatings for the blue LED chips. These
coatings are not as efficient at the longer wavelength emissions (red spectrum)
so the LEDs run at higher temperatures which can degrade the phosphor over
time. CCT tends to vary over production batches and over lamp life-times. The CRIs are improving but still only in the region of 80.
Technology for LED lamps promises to overcome the problems of generating high
quality white-light from blue LED chips by moving the phosphor to a remote
location. The diagram below shows blue LED chips mounted on a heat-sink and
shining onto a remote phosphor coated plate. The blue light is absorbed by the
phosphor and re-emitted in the broadband photopic spectrum. Some of the light
is reflected off the phosphor plate back into the mixing chamber which has a
reflection index of 99% so very little light is absorbed in the chamber but
ejected through the phosphor plate. Phosphor layers can be deposited separately starting
with the red phosphor, then the green phosphor; this leads to optimum fluorescent efficiency.
European EMC Products Ltd. Saffron Business Centre. Saffron Walden. CB10 2NL.