10 Nov
2011
10 Nov
'11
1:20 p.m.
On 10 Nov 2011, at 11:49 AM, Jochen Kunz wrote:
Not exactely. Whilte LEDs are based on blue LEDs and a
fluorescent that
converts part of this blue light to longer wavelengts. I sugest you go
to the site of a major LED manufacturer like Cree or Nichia, donload
some data sheets and look at the spectrums. Usualy the spectrum has a
peek at around 450 nm, but not that narrow like a blue LED without
fluorescent. Often there is a gap at around 500 nm and then a broad
"hill" at 550 nm to 600 nm depending on color temperature. So the
spectrum is quite smoth.
I wouldn't be surprised to find that individual white emitters function in that
fashion.
Most whilte LEDs have a Color Rendering Index of 70%..80%, where
sun light is 100% and incandescent lamps are close to this. There are
high CRI LEDs with a CRI of around 95%. One of those is lightening my
living room at this very moment. It needs 14 W and produces as much
light as a 60 W incandescent lamp. It has a color temperature of 5000 K,
that fits to the 5500 K calibration of computer screens much better
then incandescent lamps. (2700 K or 3200 K for halogen)
I have a bunch (on the order of 100) CREE LR6 modules installed which have a 92+ CRI (not
that CRI means much). The visible spectrum seems quite flat and I've been very
pleased with the results with color film.
I'd sort of expected that these things worked as suggested -- by irradiating a
phosphor screen -- but I had one arrive damaged in shipment that put an end to that
notion. What I thought was a phosphor screen was just a diffusor lens; the guts of the
thing was a mix of colored SMD LEDs mixed with white XR-Es, so for (these admittedly
bulky) things they seem to have smoothed the spectrum by playing with the emitter mix.
Incandescent replacements based on LEDs are a poor choice. Incandescent
lamps work better the hotter they get. So the fixtures for incandescent
lamps are made in a way that isolates as much heat as possible. This is
absolutely contrary to the needs of LEDs. LEDs need to stay cool. They
need a loot of cooling to keep the junction temperature in a reasonible
range. They decay fast when hot and the efficieny drops. Unfortunately
real LED lapms, that are constructed according to the needs of LEDs,
are rare. Thats why I build LED lamps myself, as I am the geek, that I
am. ;-)
The LR6 modules are huge, bulky things that are a fairly tight fit into 6" IC cans
with most of the bulk being heat sink. Despite the 50K hour lifetime on the things I was
worried about temperature rise, but stuffing a thermocouple into the housing showed a
delta of only a few degrees C over ambient regardless of power setting (the things are
dimmable as well).
Bottom line is you can get LED replacements for certain classes of incandescent lighting
that has good color performance, but it's not cheap.
--
Dr. Christian Kennedy
chris at mainecoon.com AF6AP
http://www.mainecoon.com PGP KeyID 108DAB97
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"Mr. McKittrick, after careful consideration..."