112 TECHNOLOGY / LED Table 1: Comparison of spectral light sources with the BLH and photopic functions.
at each wavelength. For example, in last month’s article we use the spectrum curves to determine the Correlated Colour Temperature (CCT) and Colour Rendering Index (CRI) of an LED light source. Depending on the characteristics of the light emitting system, the emitted spectrum can be broad or it can have sharp ‘peaks’ at certain wavelengths; the former is the case for natural sunlight and the latter is for incandescent, halogen and certain types of LED lamps where the spectrum will contain peaks of radiant intensity at certain wavelengths as shown in figure 2.
White Light Generation - Fluorescent Lamps A fluorescent lamp generates light from collisions in a hot gas (‘plasma’) usually containing mercury which emit photons at two UV emission lines (254nm and 185 nm). The created UV radiation is then converted into visible white light by UV excitation of a fluorescent coating on the inside of the glass tube. The chemical composition of this coating is selected to emit in a desired spectrum for example warm white lamps may use three part phosphors. For example, fluorescent lamps can be enriched for blue light (wavelengths 400-500 nm) in order to simulate daylight better in comparison to incandescent lamps. Like fluorescent lamps, CFL emit a higher proportion of blue light than incandescent lamps.
White Light Generation – LEDs The vast majority of LEDs use a similar principle but instead use a blue LED coated in a phosphor material to generate the white light. The advantage of LEDs as a light source is the pump wavelength is around the 470nm wavelength +/-20nm and therefore does not contain UVA, UVB or UVC wavelengths that are harmful.
UV Radiation and Blue Light With a few exceptions (most notably the formation of pre-vitamin D3), most photochemical reactions caused by UV radiation in the skin and eyes are detrimental as proteins and DNA can
become damaged and dysfunctional if exposed in a prolonged manner. There are internationally recognised exposure limits for the radiation (200-3000 nm) emitted from lamps and luminaires that are set to protect from photobiological hazards (International Electrotechnical Commission 2006). These limits also include radiation from CFLs.
The UV content of the emitted spectrum depends on both the phosphor and the glass envelope of the fluorescent lamp. The UV emission of incandescent lamps is limited by the temperature of the filament and the absorption of the glass. Some single-envelope CFLs emit UV-B and traces of UV-C radiation at wavelength of 254nm. Experimental data has shown that CFLs produce more UVA irradiance than a tungsten lamp. Furthermore, the amount of UVB irradiance produced from single- envelope CFLs, from the same distance of 20 cm, was about ten times higher than that irradiated by a tungsten lamp. For example in 2008 the Health and Safety Executive studied the emissions from various CFL lamps and found that some energy saving compact fluorescent lights can emit ultraviolet radiation at levels that, under certain conditions of use, can result in exposures higher than guideline levels. The HSE’s view was that open (single envelope) CFLs should not be used where people are in close proximity - closer than 30 cm or 1 ft - to the bare light bulb for over 1 hour a day. The Agency advised that for such situations open CFLs should be replaced by the encapsulated (double envelope) type or alternatively, the lamp should be moved so that it is at least 30 cm or 1 ft away.
The photochemical blue light hazard (BLH) can be evaluated on the basis of the standard EN 62471. The latter classifies light sources into risk groups 0, 1, 2 and 3 (from 0 = no risk through to 3 = high risk). The sun would be classified as being in the highest risk group.
Risks can be allocated to the risk groups according to different measurement criteria:
• Measure the distance at which an illuminance of 500 Lux is attained (a typical value for general lighting purposes). According to EN 62471, the 500 Lux criterion must be used for lamps intended for general lighting (including lamps for lighting offices, schools, homes, factories, roadways, or automobiles). • The second criteria measures photo biological safety from a distance of 200 millimetres. The 200 millimetre criterion is to be used for all other lamps (including for example lamps for such professional uses as film projection, reprographic processes, sun tanning, industrial processes, medical treatment and searchlight applications). Table 1 compares the portions of the spectra of various light sources that overlap with the blue light hazard function and also the CIE photopic function. The ideal light source would want a low BLH value and a high CIE photopic value to be a truly efficacious (and least hazardous) source. It is interesting to see the halogen lamp only generates 5.7% of its light within the BLH region and cool white (6500K) LEDs generate 24.5% of their light in the BLH region. Of course this is to be expected as halogen lamps are warm white and 6500K LEDs generate a colder bluer light. Previously it has been difficult to measure the BLH radiation of lighting fixtures as it required the use of expensive laboratory equipment however Blueside Photonics have developed the Blue Light Hazard estimator PC software which only needs the use of a standard lux meter providing you know the spectrum of the light source within the fixture. If you then measure the illuminance of the fixture with the light meter one can instantly estimate the hazard (provided you are aware of the source size ‘limitations’ that apply to the test). Figure 3 shows the example whereby a cool white LED fixture with a 500 lux point at 2 metres (equating to a 2000 cd source) is not hazardous by the GLS criteria referred to in the BLH standard, the fractional exposure of the allowed limit (for all day viewing) is 0.477: a 3000 K incandescent source with the same intensity metric delivers 0.217 of the daily dose.
Figure 4 shows a comparison of light sources against the blue light hazard as measured by CELMA and as shown the highest irradiance occurs with any light source as the CCT increases.
Focussing on LEDs we see that if they are measured using the 500 Lux criterion, none of the current LED products available (at any colour temperature) belong to risk group 2. However, a study by ANSES in 2010 found that three out of nine high-output
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