In ,
Larry Fishel wrote:
On May 4, 2:30*pm, Frank wrote:
Googled this up at HD:
http://tinyurl.com/4xx897u
Interesting that the stated power consumption for this bulb is
actually HIGHER that for a 40W equivalent CFL...
A lot of light being wasted in a filter to improve the color?
According to the above link, this LED bulb consumes 8.6 watts to
produce 429 lumens (49.9 lumens/watt).
That is not much less efficient than a 40W-equiv. CFL, which produces
450-500 lumens from 9 watts (50-55.5 lumens/watt).
As for why this LED achieves about 50 lumens/watt while there are now
LEDs achieving 100-120 lumens/watt: I doubt the reason is filters:
1: My guess is that this LED bulb is a warm white one rather than a
cool white one. Warm white LEDs produce less green spectral content
and more red spectral content than cool white ones do. Not only is
human vision less sensitive to red than to green, but also phosphors
have higher "Stoke's loss" in producing red than in producing green
light from the same LED chip.
2: The LEDs in this bulb may be high color rendering index ones. There
is a recent trend for "warm white" LEDs used in lighting to have
color rendering index of at least 80. Cool white LEDs of extremely
high efficiency have CRI only 70 to low 70's, with some having
extreme lumen/watt phosphors only achieving CRI in the 60's.
Higher CRI requires the phosphor's spectral band to be wider,
to include a fair share of wavelengths from slightly bluish green
to mid-red. This is as opposed to a narrower phosphor band
concentrating on wavelengths from slightly yellowish green to
slightly orangish red. The wider band has more spectral content
at deeper red wavelengths that human vision is less sensitive to,
resulting in less lumens per watt.
3: The LEDs in this bulb may be less efficient than the most efficient
ones available of a given color and color rendering index. The
most efficient available LEDs have higher cost.
4: The LEDs in the LED bulb require a "ballast" or "driver circuit",
for similar reasons to the ones why a CFL requires a ballast
(included in usual screw-base CFLs). The ballast has some loss.
When a 120V AC-powered "LED driver" has 91 or 92% efficiency, that
is getting to be something to boast about. Merely fullwave-
rectifying 120 volts AC to DC has a loss around 1.5%.
--
- Don Klipstein )