What is a backlight (CCFL) and how does it work?

Backlights in notebook LCD screens are referred to as cold cathode fluorescent lamps (CCFLs). They are long, thin sealed glass tubes with an electrode at each end filled with gas and a small amount of mercury. The inner wall of the tube is covered with phosphor layers. The lamp is powered when high voltage is applied between the two electrodes, which causes the invert gas to ionize and display ultraviolet light.

The voltage is supplied from the inverter. The inverter converts direct current (DC) to alternating current (AC).

The backlight is typically at the bottom edge of notebook LCD screens. On some notebooks, the light is brighter at the bottom edge; because this is where the backlight(s) rest. This is sometimes referred to as light leakage.

As you can see, a backlight is very small and long, similar width of a coffee straw.

Notebooks contain CCFLs referred to as edge lighting. Edge lighting uses one or two CCFLs to light up the screen. The notebooks also contain a thin light guide that distributes the light evenly across the display. If the notebook contains two CCFLs, it’s important the lamps are similar in age so that the light distribution and brightness is even.

The backlight also controls the brightness of the notebook screen. One often controls the brightness of the screen by holding down the function (Fn) key and hitting the up or down arrow keys. There are other ways to do this as well. The backlight is dimmed by pulse width modulation, which affects the current though the lamp(s).

Although there are other types of light sources for notebooks, CCFLs remain the most commonly used type. Other includes hot cathode florescent lamps, Xenon-based flat lamps and LED backlights. Two of these are mercury free, which are safer and easier to dispose of. Also, the CCFLs have a shorter life span than the other light sources.

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