Couldn't find the strength to read it all. But I'd be curious to know what
trick or cure a photog could resort to, particularly in mixed lighting ...
TIA
Philippe
Le 9 déc. 2016 à 10:26, <timhughes@xxxxxxxx> a écrit :
> 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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