BIRDS & LIGHTING - Flourescent Light Principles
BIRDS & LIGHTING - Flourescent Light PrinciplesFLOURESCENT LIGHT FUNDAMENTALS
Fluorescent tubes operate on a completely different principle from the incandescent, or metal halide systems. Whereas the incandescent system relies on light emission from thermal characteristics, the fluorescent system operates through the principles of electron flow and photo-phosphorescence. To understand the relationship of light characteristics of differing products for avian illumination, we must first look at how fluorescent tubes are designed and operate. A diagram of the device is shown in Figure 1.FIGURE 1 - THE FLOURESCENT TUBE
The design of the fluorescent lamp is really quite simple. This design remains the same whether the form is a straight tube, circular, or convoluted as in compact fixtures. A fluorescent tube is constructed in its most simple form with a filament similar to an incandescent light bulb at either end, and a coating of a phosphorescent material on the inside of the glass envelope. The tube itself vacuum evacuated, a small amount of vaporized mercury is added to the tube, , and then it is injected with a small quantity of argon gas.
When a current is applied to the filaments at both ends of the tube, the filaments become what are called “cathodes”, meaning that they provide an intense source of positively charged electrons. This energizes the argon gas to a 'plasma state' which "excites" the metallic mercury. At this point, the flooding of positive electrons cause the electrons in the shell of the mercury atoms to “jump” (move outward) from a neutral or “ground state” and become “excited”. This pushes electrons outward, filling an “empty” orbital ring with an new electron. The atom then releases its excess electron as the atom attempts to return to its neutral state, and through this process the mercury gas becomes “energized”, driving the excess energy off in the form of photons that lie within the ultraviolet range. The external ballast device serves to limit the amount of current which is delivered to the plasma in this process, maintaining a consistent and uniform source of electrical flow to the cathodes.
This same atom which has just released a photon then picks up another from the cathode flow, continuously repeating the process for as long as the cathode is attached to a source of current. The argon gas (known as a ‘noble’ gas) imparts nothing to the spectral/photon process, as it is neutral from the electron shell perspective, all of it’s eight orbits filled already with electrons. Argon is added solely as a stable medium for the plasma reaction to occur. This is important as the emitted photons must have a special wavelength to make the tube function properly. These special wavelengths are know as spectral lines.
In the case of mercury, this element emits a very strong line at 2537 Angstroms, well into the far ultraviolet range (UVC). By its own properties, this wavelength is dangerous as none of it ever penetrates to the earth, and life is not prepared to deal with these wavelengths of radiation. But this frequency is helpful in two ways. If no phosphors were added to the tube, this lamp would be the kind that is found in UV sterilizing equipment (such as in barber shops for combs and scissors, and in bowling alleys for shoes), as it will kill all living organisms exposed to it after a time. But this is not the case in consumer fluorescent tubes. This spectral line of photons strike a suitably doped (meaning selected impurities are added) calcium halophosphate coating inside the tube itself, which causes two things to happen. First, the coating filters out the dangerous UVC radiation, and then converts the energy to a different spectral range, mostly that of the visible spectrum.
Depending on the blend of doping materials combined with the calcium halophosphate phosphor, the output range of the tube will vary, and thus a fluorescent device can be custom tailored to produce certain ranges or specifications of light output. This is how various manufacturers produce lighting devices which have different output characteristics. While there are several thousand doping materials which have the potential to change the spectral output of a lamp, only a hundred or so produce useable wavelengths. Characteristics and properties of pertinent lighting devices may be found in the Fluorescent Tubes section.
Copyright 1999 by Patrick Thrush
