Re: LCD Backlight question

Having done a simpe backlight repair once or twice to a couple of old
laptops, it is uually done using one or two thin fluorescent tubes
positioned at the side(s) of a sandwich usually made up of (from the front
of the viewable screen) a protective plastic front cover, the LCD screen
itself, a thin diffuser layer of plastic (think of a piece of tracing paper
in terms of appearance) then a piece of perspex or glass that has finely and
highly polished edges - this was about 5mm thick on the models I repaired,
and finally a very white *** of plastic. There was also a protective
plastic covering at the very back.

The process uses the internal reflective properties of glass/clear plastic
in a similar way to how light is channelled down a fibre optic cable. In
simple terms, the light from the tube at one side of the perspex is
reflected millions of times inside, and any light that escapes is reflected
off the white backing *** back into the perspex ***. This process
removes most of the uneveness by effectively scattering the particles we see
as light. The result of this is then blurred by the diffuser *** which
also serves to prevent what's under the LCD screen being clearly visible to
the user.

Some screens with less efficient light scattering properties can appear
darker or brighter in certain areas when viewed from certain angles. The
LCD part of the screen needs the backlight so that the user can see what's
being displayed. When a backlight dies, shining a flashlight from certain
angles at the front of the screen usually allows enough light to pass
through to the underlying reflective material to allow you to see whether
the LCD itself is working.

Repairing LCD backlights can be straightforward if the only problem is the
tube and as long as you're very respectful of the electrical properties of
the inverter circuit which can give a nasty shock even when the screen is
powered down due to capacitors remaining charged. Definitely not for
someone who doesn't know what they're doing. Unfortunately, LCD screen
problems are often down to a bad inverter, and these are often hard to
source. With the price of LCD technology tumbling at an alarming rate, it
probably isn't worth risking injury trying to repair them.

Nick


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