Why are CFL lamps not shaped like an incandescent lamp

Although like incandescent lamps, compact fluorescent lamps (CFL) are used for the same purpose of lighting application, they produce light by totally different mechanisms. Tungsten filament based incandescent lamp produces light by incandescence where in the tungsten filament being an electrical resistor gets hot when a sizable electrical current is passed through it by a process called Joule heating.

The incandescent lamp generates lot of heat due to the glow of tungsten filament, which has to be dissipated effectively and evenly in all directions. Hence the filament with proper support-lead is enclosed in a spherical glass shell, a simple geometry.

On the other hand, fluorescent lamps (both conventional and lengthy type, also compact CFLs) produce light by a different process called fluorescence underlying the conversion of invisible ultra-violet radiation (UV) to visible light with the help of fluorescent compounds seen as white coating inside the lamp.

UV radiation is produced along the length of the fluorescent lamp through electrical discharge process in the mixture of mercury vapor and inert gas held at low pressure.

Since the conventional fluorescent lamp was too lengthy, way-back in 1970s researchers in Philips Lighting, the pioneer in luminescent lighting worked out various models and methods to make miniaturization in lamp dimensions, geometry eventually paving way for the birth of CFLs. The length of electrical discharges column, hence lamp length also diameter are very crucial in determining the light out from the CFL.

Since then lighting engineers have worked-out several models such as helical coil, double-bent helix and so on taking into consideration the required light output, powerrating etc. In the case of fluorescence no heating is involved, hence no severe restrictions in the geometry or design of the lamp.