I have seen some internet electronic forums claming that low quality cool white LEDs emit more UV light than CFLs. Is there any truth in this claim?
Electronic – Do cool white LEDs emit UV light
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You get more lumens per watt from phosphor converted LEDs. You can tune the color temperature with phosphor composition. Additionally, blue diode used in these diode is very stable with temperature variations so when the temperature of the die increases you do not loose much lumens.
RGB mixed white LEDs do not have high efficacy (lm/W). The color point is not stable because the red diode used in the chip is very temperature dependent. When the temperature of the die rises, the color point of the whole LED will shift towards cyan and the overall efficacy drops.
These two structures will have different specra and this it the cause for different efficacy figures. Eye has its peak sensitivity around 555nm. Take a look at V(λ) function:
and compare it to the spectrum of phosphor converted LED:
and RGB LED:
You can clearly see the PC white LED has much bigger overlap.
One more advantage of phosphor converter LED over RGB ones is color mixing. RGB diode will have three separate dies for generating red, green and blue light. If you use this type of LEDs in a lamp the overall light will be a mix of three colors originating from different places. This may give a color shadow effect which is sometimes undesired.
LEDs will emit steady light if given steady current. The question then becomes what kind of current waveform LEDs are driven with in a light assembly.
The exact answer depends on the circuit in the light. LEDs run on just a few volts. The much high line voltage has to be converted to the lower LED voltage somehow. Most likely, this will include the cheapest, dumbest, and most stripped down switching power supply possible.
I see other answers mention a transformer, but I think that is quite unlikely. There is no need for isolation in a sealed unit with no external connections other than the power line. Even if there is a transformer, it's not going to be fed directly from the line frequency. The extra savings in cost and size of a transformer that works at 10s to 100s of kHz far outweighs the cost of the components to produce that frequency.
Most likely, there is a full wave bridge to rectify the AC line voltage directly. That will then be chopped thru a inductor to drive the LEDs. Depending on how cheap the lamp is, it might chop at a fixed duty cycle, which would make the LED brightness vary with power voltage. Even a little current feedback would keep the LED current reasonably constant over most of the line cycle, perhaps dipping only briefly at the power line zero crossings. A small cap would reduce that, but caps cost money and take space, so may not be included.
LED lamps are made in large volumes, so serious manufacturers probably develop custom ICs just for this purpose. In such a case I'd expect at least some attempt at regulation, so the LED brightness will be largely constant, perhaps with short dips at the power line zero crossings.
However, all this is speculation. Why not just look? Put the lamp at the end of a extension cord and swing it around in a otherwise darkened room. Whether it flickers or not will be immediately obvious.
Best Answer
Truth, No.
It is possible there is a tiny bit of UVA and UVB. Amounts so small they would be very difficult to measure.
Occasionally there are some papers that report a couple of spikes of UV from the yellow phosphors used to convert blue to white. The reason they are reported is they are hoping to find phosphors that convert down to UV wavelengths and when they find a hint of UV it will likely be reported. Sample of such a paper. I do not know any of these phosphors that have been reported are used in commercial lighting.
I have some measurements from a horticulture research project at the University of Florida. I used a StellarNet BLUE-Wave Spectrometer,
a decent $15,000 spectrometer, to measure the Photon Radiance of one of my LED grow lights. This fixture uses the standard color LEDs which includes the deep blue (Cree XP-E) that white LEDs are made from.
I was able to measure a well formed peak as low as 310nm, two wavelengths adjacent at 288 and 289nm and one spike at 280nm.
The UVA and UVB wavelengths were very low reading, but well above the noise level. So there are probably some UV emitting from a cool white LED, but they cannot be read with my spectrometer.
I do not use "low quality" LEDs, only Cree and Phillips Luxeon.
The day before I had measured a fixture with Luxeon LXML-PWC2 4000K LEDs. Not a cool blue. The lowest measurable wavelength for the 4000K was at 409nm @ 0.00077778. For comparison the peak at 310nm on the graph below, was 0.0034645 (4.45x larger)
These are the readings I got from the spectrometer from 280 to 400nm.
first column is wavelength, second is relative to micromoles/m2/s. By relative, I mean, I forgot what aperture was used. Readings were taken October 7, 2016 in a grow tent at my house. The current flowing through the LEDs was 350 mA.
This graph was made from the reading above plus the wavelengths from 400 to 1099nm.
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Photo of the tent with the white 4000K fixture and the spectrometer in my home lab.