Mike, I think when they do use UV Leds to create white Leds with
phosphors, they almost always mean near UV leds.
It is only relatively recently that deeper UV leds have become available at all
and there is an inherent quantum efficiency deficit the bigger the difference
in wavelength between pumping wavelength and phosphor emission wavelength.see
this excellent old article discussing LED wavelengths and phosphors from 2011:
"Selecting conversion phosphors for white light-emitting diodes"
https://biblio.ugent.be/publication/1234547/file/3145679Relevant quote about UV
led: "as long as the peak wavelength of the UV pumping LED is not too short, as
this would create too large Stokes losses"
more detailed discussion:>>Hence, what are the advantages of using ultraviolet
pumping LEDs compared to blue ones? First of
all, if the electrical to optical power conversion is more efficient in UV than
in blue LEDs, shifting to
UV LEDs can yield an overall more efficient design. Secondly, it is
questionable whether good color
rendering in combination with a low color temperature can be obtained using a
blue LED and a single
conversion phosphor. If two phosphor materials have to be used anyway,
including one with a small
Stokes shift to cover the emission spectrum around 500nm, one might consider
the full phosphor
approach with ultraviolet pumping LEDs. This also has the advantage that the
emission spectrum can
be more stable with respect to the driving current and the temperature of the
LED chip. In this case,
spectral shifts of the pumping LED are not reflected in spectral or intensity
changes in the phosphor
emission, on condition that the excitation spectrum of the phosphor is
sufficiently ‘flat’ around the
emission of the pumping LED. When a blue pumping LED is used, shifts in the
emission spectrum of
the LED will induce a color shift of the white LED.
Consequently, both approaches seem useful, as long as the peak wavelength of
the UV pumping LED
is not too short, as this would create too large Stokes losses and an
inherently lower electrical-to-
optical conversion efficiency of the device.
<<The same article also discusses the CRI issues and different improved CRI
test methods.
The Companies producing thin rubber products like condoms and some medical
products, have to be very careful not to expose them to flourescent tubes
during manufacture because even modest exposures greatly shortens the life of
the rubber.
The amount of energy which will be saved as the latest generation of leading
edge LEDs (~200lm/W) gets adopted in the next 10years is amazing, the DOE is
estimating the cost saving in the US will be about 50B$ per anum, and related
energy savings enough to close about 140 typical power plants. The ultimate
theoretical white light limiting efficacy, is something like ~300lm/W so we
likely will see only incremental improvement at very much above the 200lm/W
level.
It is interesting looking at the history of flourescent tube
coatings:http://www.lamptech.co.uk/Documents/FL%20Phosphors.htm
The article discusses the phosphor stability and temperature issues and that is
a real problem for color temperature of LED monitors for graphic art and
photography applications.There is quite a bit of gamesmanship too as vendors
often measure LED efficacy and CRI while LED is new and it can change quite a
bit in the first few hundred hours of burn in.
Tim
From: Mike Gordon via olympus <olympus@xxxxxxxxxxxxxxxxx>
To: olympus@xxxxxxxxxxxxxxxxx
Cc: usher99@xxxxxxx
Sent: Friday, November 18, 2016 3:02 PM
Subject: Re: [OM] LED Stage Lighting and UV
Never heard of UV filter helping but the experiment is not hard to do. Seems
this should be a largely solvable issue. (I am not an engineer and no nothing
about stage lights except can confirm the photographic issues) The power
spectral distributions of LED's are notoriously peaky and with valleys
despite the usual
phosphors used. The CRI uses R1 to R8 and I have LED's in our bathroom that
make skin tones look putrid--at least I hope that is the problem. They have a
CRI in the low 90's but clearly bad R9 value as deficient in red.
Some recent ultra high CRI LED's use violet LED instead of blue and R, G, B
phosphors for a CRI up to 98 or so. The luminous efficiency is down to 65-85
lm/W though.
Spuriously sallow skinned? Mike
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