Physical characteristics of lamps
Principles of operation
A fluorescent lamp generates light from collisions in a hot gas (‘plasma’) of free accelerated electrons with atoms– typically mercury – in which electrons are bumped up to higher energy levels and then fall back while emitting at two UV emission lines (254 nm and 185 nm). The thus created UV radiation is then converted into visible light by UV excitation of a fluorescent coating on the glass envelope of the lamp. The chemical composition of this coating is selected to emit in a desired spectrum.
A fluorescent lamp tube is filled with a gas containing low pressure mercury vapour and noble gases at a total pressure of about 0.3% of the atmospheric pressure. In the most common construction, a pair of filament emitters, one at each end of the tube, is heated by a current and is used to emit electrons which excite the noble gases and the mercury gas by impact ionisation. This ionisation can only take place in intact light bulbs. Therefore, adverse health effects from this ionisation process are not possible. Furthermore, lamps are often equipped with two envelopes, thus dramatically reducing the amount of UV radiation emitted.
Electrical aspects of operation
A special electronic circuitry is needed to start the lamp and maintain currents at adequate levels for constant light emission. Specifically, the circuitry delivers high voltage to start the lamp and regulates the current flow through the tube. A number of different constructions are possible. In the simplest case only a resistor is used, which is relatively energy inefficient. For operation from alternating current (AC) mains voltage, the use of an inductive ballast is common and was known for failure before the end of the lamp lifetime inducing flickering of the lamp. The different circuits developed to start and run fluorescent lamps exhibit different properties, i.e. acoustic noise (hum) emission, lifetime (of the lamp and the ballast), energy efficiency and light intensity flicker. Today mostly improved circuitry is used, most especially with compact fluorescence lamps where the circuitry can not be replaced before the fluorescence lamps. This has reduced the occurrence of technical failures inducing effects as those listed above.
The part of the electromagnetic spectrum that comprises static fields, and fields up to 300 GHz is what is here referred to as electromagnetic fields (EMF). The literature on which kinds, and which strengths of EMF that are emitted from CFLs is sparse. However, there are several kinds of EMF found in the vicinity of these lamps. Like other devices that are dependent on electricity for their functions, they emit electric and magnetic fields in the low-frequency range (the distribution frequency 50 Hz and possibly also harmonics thereof, e.g. 150 Hz, 250 Hz etc. in Europe). In addition, CFLs, in contrast to the incandescent light bulbs, also emit in the high-frequency range of the EMF (30-60 kHz). These frequencies differ between different types of lamps.
All lamps will vary their light intensity at twice the mains (line) frequency, since the power being delivered to the lamp peaks twice per cycle at 100 Hz or 120 Hz. For incandescent lamps this flickering is reduced compared to fluorescence lamps by the heat capacity of the filament. If the modulation of the light intensity is sufficient to be perceived by the human eye, then this is defined as flicker. Modulation at 120 Hz cannot be seen, in most cases not even at 50 Hz (Seitz et al. 2006). Fluorescent lamps including CFLs that use high-frequency (kHz) electronic ballasts are, therefore, called “flicker free”.
However, both incandescent (Chau-Shing and Devaney 2004) and “flicker free” fluorescent light sources (Khazova and O’Hagan 2008) produce hardly noticeable residual flicker. Defective lamps or circuitry can in some cases lead to flickering at lower frequencies, either only in part of the lamp or during the start cycle of some minutes.
Light Emission, UV radiation and blue light
There are characteristic differences between spectra emitted by fluorescent lamps and incandescent lamps because of the different principles of operation. Incandescent light bulbs are tuned in their colour temperature by specific coatings of the glass and are often sold either by the attribute ‘warm’ or ‘cold’ or more specifically by their colour temperature for professional lighting applications (photographic studios, clothing stores etc.). In the case of fluorescent lamps, the spectral emission depends on the phosphor coating. Thus, 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. There are internationally recognized exposure limits for the radiation (200-3000 nm) emitted from lamps and luminaries 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 254 nm, which is not the case for incandescent lamps (Khazova and O´Hagan 2008). Experimental data show 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 (Moseley and Ferguson 2008).
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