Why CFL gives bluish white light? Fact File 17

Compact Fluorescent Lamp was developed with aim of energy conservation. 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. This bluish white colour is due to the presence of Ultraviolet rays from the  excited mercury vapour. The CFL has a resonant converter to generate high frequency AC to excite the mercury vapour filled inside the tube. The excited mercury atoms generate UV rays that produces fluorescence in the Phosphor coating of the tube. The phosphor coating then emits visible light with some UV fraction that gives bluish white colour to the light.

Unlike ordinary bulb, CFL generates secondary light. The light is not directly from the filament, but from the phosphor coating. Actually this secondary light is not friendly for the eyes since the human eye is adapted for natural direct lights such as sunlight and light from flame. UV ray is also not good for the eyes. Low quality CFL generates large amount of UV rays. This can cause eye strain if the light is viewed long time.

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.

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.

Some CFL Facts

1. 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. Good poisoning of CFL is the downward orientation from the roof. Fixing on the wall will reduce its life.

2. Do not hold the tube while inserting into the holder. This may detach the tube from the base.

3. In areas of low voltage in power lines, CFL will have very short life.

4. Another important factor that affects CFL is its frequent on and off. If the CFL is switched on and off at an interval of 5-6 minutes, the life span greatly reduces.

2 responses to “Why CFL gives bluish white light? Fact File 17

  1. please make an study of circuit and operation of this cfls,diagram,and operation,it will be very interesting,Thanks and God bless you