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.