Compact Fluorescent Lamp or CFL is the energy saving lamp designed to provide adequate light at the expense of low current.CFL uses less power around one fifth of an ordinary bulb and has long life.CFL is designed to emit Soft white light using the phosphor formulations.CFL comes in two forms like integrated and non integrated CFLs. In integrated type, the tube combines with an electronic ballast as a single unit while in non integrated type, ballast and tube are separate units so that the tube can be replaced.AC and DC CFLs are available.CFL generally has a life span of 6000 to 20000 hours.
Inside the CFL
In structure, the CFL has a gas filled tube and an electronic ballast. The electronic ballast has a circuit board with rectifier diodes, filter capacitor, switching transistors and a high frequency DC to AC inverter. The inverter transformer along with the wave generating switching transistors produce high frequency waveform around 40kHz or higher. This high frequency AC is applied to the terminals of the tube. The resonant converter in the CFL, stabilize the tube current over a range of input voltages so that the CFL will not responds to lamp dimmer circuits.
The tube in the CFL uses the high frequency AC from the resonant converter to excite the mercury vapor present inside the tube. The excited mercury atoms generate ultraviolet light which produces fluorescence in the phosphor coating of the tube. This gives the visible light. That is, the light from CFL is secondary light emitted from the phosphor.
Some CFL Physics
The physics behind the light production in CFL is the inelastic scattering of electrons. When the cathode terminals of the CFL heat up by the high frequency AC, they emit electrons (through thermionic emission) which collide with the atoms of the mercury vapor. If the hitting electron has enough energy, it transfers the energy to the outer orbit of the mercury atom. This causes the outer electrons of the mercury atom to jump into a high energy level. Loss of energy occurs during this collision; therefore this type of collision is called inelastic scattering.
The mercury atom shifted to the high energy state is highly unstable and it returns to the ground state by releasing the acquired energy in the form of Ultraviolet light. The UV rays thus emitted will have a wavelength of 253 and 185 nano meters. Since the UV rays are invisible to human eye, Fluorescence is used to emit visible light. When the UV photons hit the phosphor coating of the tube, energy transfer occurs and the phosphor atoms emit photons in the visible range.
Usually the CFL has longer life span, but it will be reduced by many factors like voltage spikes, mechanical shock, lamp orientation, operating voltage, frequency of AC applied etc. In areas of low voltage in power lines, CFL will have very short life. Another important factor that affects CFL is frequent on and off. If the CFL is switched on and off at an interval of 5-6 minutes, the life span greatly reduces. CFL requires a start up time to attain full brightness. The light colour is slightly greenish at power on which changes to blue white after the warm up period of a few minutes.
Light output from CFL
Light output of Lamps is measured in terms of Lumens which is the power of light perceived by the human eye. The luminous efficacy of the lamp is the number of lumens produced per watt. The luminous efficacy of the CFL is around 70 lumens per watt while the ordinary bulb has only 12-18 lumens per watts. Light output from CFL depends on its wattage. In general, 8 watts CFL gives minimum 450 lumens while 36 watts CFL gives 2600 lumens.
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