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Re: [OM] LED Stage Lighting and UV

Subject: Re: [OM] LED Stage Lighting and UV
From: <timhughes@xxxxxxxx>
Date: Fri, 9 Dec 2016 09:26:36 +0000 (UTC)
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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