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?
Summary: CFLs can emit a small amount of UV light. Only very specialist white phosphor LEDs will emit any UV at all. These are specialised and unlikely to be encountered in practice - certainly not at the low cost / low quality end of the market.
However, even correctly operating good quality white phosphor LEDs can emit blue (near to UV) light at levels which are potentially eye-hazardous.
I have 200 mW DC in LEDs which are rated as marginally eye hazardous in the blue end of the spectrum. ANY say >= 500 mW LED is liable to be potentially eye hazardous to some degree.
As several people have noted, obtaining UV from phosphor LEDs is extremely unlikely - and will only occur wity the very specialist UV emitter versions that Tony mentions. You are extremely unlikely to ever encounter these.
HOWEVER - the blue light from the LED which excites the phosphor in most white LEDs can be a notional eye hazard at quite low power levels.
I had a formal eye hazard assessment carried out for some cool white LEDS* (6000 K) which were very high efficiency (as the time) and rated at 65 mA, Vf = 2.95V so power was only 190 mW (giving 25+ lumen at about 70 degrees double half-cone angle). (* Nichia - 'as good quality as any').
The regulations break the areas of interest into a number of wavelength ranges. (3 AFAIR). These LEDs were assessed as being formally hazardous in the deepest blue part of the test ranges. Actually achieving eye damage would have required gross stupidity or having somebody tape an LED to your eye BUT it was interesting to see what low levels are required.
Is there any truth in this claim?
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,
- 200-1150nm wavelength
- Signal to noise = 1000:1
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.
280.00 1.1290E-002 281.00 0.0000E+000 282.00 0.0000E+000 283.00 0.0000E+000 284.00 0.0000E+000 285.00 0.0000E+000 286.00 0.0000E+000 287.00 0.0000E+000 288.00 1.1274E-002 289.00 9.0670E-003 290.00 0.0000E+000 291.00 0.0000E+000 292.00 0.0000E+000 293.00 0.0000E+000 294.00 0.0000E+000 295.00 0.0000E+000 296.00 0.0000E+000 297.00 0.0000E+000 298.00 0.0000E+000 299.00 0.0000E+000 300.00 0.0000E+000 301.00 0.0000E+000 302.00 0.0000E+000 303.00 0.0000E+000 304.00 0.0000E+000 305.00 0.0000E+000 306.00 0.0000E+000 307.00 1.4569E-002 308.00 1.4059E-002 309.00 2.4614E-002 310.00 3.4645E-002 311.00 2.5260E-002 312.00 6.2953E-003 313.00 3.6099E-003 314.00 0.0000E+000 315.00 0.0000E+000 316.00 0.0000E+000 317.00 7.2641E-003 318.00 1.4983E-002 319.00 9.9858E-003 320.00 3.7277E-003 321.00 5.7936E-003 322.00 4.5364E-003 323.00 2.1041E-003 324.00 1.3479E-002 325.00 9.7610E-003 326.00 2.4578E-003 327.00 3.9432E-003 328.00 7.8247E-003 329.00 3.6119E-005 330.00 0.0000E+000 331.00 0.0000E+000 332.00 0.0000E+000 333.00 5.3355E-003 334.00 6.3256E-003 335.00 1.6233E-003 336.00 9.1491E-003 337.00 5.3881E-003 338.00 7.1098E-004 339.00 3.9042E-004 340.00 9.4603E-004 341.00 0.0000E+000 342.00 5.8628E-003 343.00 1.9500E-003 344.00 0.0000E+000 345.00 0.0000E+000 346.00 0.0000E+000 347.00 1.8100E-003 348.00 8.4942E-003 349.00 1.1109E-003 350.00 0.0000E+000 351.00 0.0000E+000 352.00 0.0000E+000 353.00 0.0000E+000 354.00 0.0000E+000 355.00 7.3287E-004 356.00 6.0877E-003 357.00 1.8842E-003 358.00 0.0000E+000 359.00 0.0000E+000 360.00 0.0000E+000 361.00 0.0000E+000 362.00 2.4273E-003 363.00 4.5285E-003 364.00 3.7282E-003 365.00 1.7190E-003 366.00 4.4021E-003 367.00 1.1359E-002 368.00 1.2333E-002 369.00 1.9218E-003 370.00 0.0000E+000 371.00 1.8477E-003 372.00 5.1049E-003 373.00 8.0459E-003 374.00 1.9174E-003 375.00 0.0000E+000 376.00 0.0000E+000 377.00 0.0000E+000 378.00 0.0000E+000 379.00 1.3191E-003 380.00 8.7230E-003 381.00 6.5227E-003 382.00 3.3852E-003 383.00 4.2065E-003 384.00 6.1745E-003 385.00 8.8286E-003 386.00 4.2646E-003 387.00 0.0000E+000 388.00 5.2739E-003 389.00 8.0110E-003 390.00 4.6509E-003 391.00 5.4275E-004 392.00 3.7508E-003 393.00 1.0168E-002 394.00 1.0653E-002 395.00 9.2924E-003 396.00 7.9877E-003 397.00 1.0248E-002 398.00 1.2157E-002 399.00 1.1787E-002 400.00 1.2336E-002
This graph was made from the reading above plus the wavelengths from 400 to 1099nm.
Photo of the tent with the white 4000K fixture and the spectrometer in my home lab.
It depends. Both cool and warm white LEDs can be built from blue LEDs + luminescent material or UV LEDs + luminescent material.
Those made from blue LEDs are in the far majority because they are much cheaper. Their UV emission is zero. The problem with these is the luminescent material is aging and so they are getting bluer with time, because a higher fraction of the light is passing the luminescent material unaltered.
Those made from UV LEDs don't have that aging problem because all light passing the luminescent material unaltered is invisible. They are only getting darker with time. So, yes, these LEDs do emit UV. And they are not cheaper but more pricey because of their better color stability over time.
BUT, in a light bulb there is a glass in front of the LEDs. And ordinary glass filters UV nearly perfectly. (You can't get a suntan behind a window.) That's the same as for CFLs. If you need the UV, e.g. in a solarium lamp, the CFLs or LEDs need to use quartz glass or a transparent plastic for the casing.
Blue and White LEDs made from Blue do not have any significant level of UV.
However high levels of Blue can cause damage to eyes and also Art Galary paintings. This can occur due to the relative percentage level of blue and the higher energy of shorter wavelength visible Blue` LEDs compared to Metal Halide makes LED rejected by Lighting Experts for Art galleries to prevent energetic photon related aging.