Sadly it is not always a question of cleaning the plastic ...
If it is indeed and LCD ...then the lights you refer to are cold cathode fluorescents ..which contain mercury ...
This vapourises and current is passed ...much like a standard strip light fluorescent tube in your kitchen or workplace ..
Over time the ends blacken as the ccls age ...
You can see this every day on such lights ...assuming they aren't covered ..
Unlike a tube in the kitchen which is easily changed ...they are _much_ more difficult to change inside a TV screen ..
They are only a few mm thick ...
very brittle and easily broken ...
..but worse ...the manufacturer in his infinate wisdom ...and desire to get you to scrap and buy new ...rarely makes these available for purchase ..
Some do ...but they aren't cheap ..
to give you an idea of cost :-
2pcs CCFL LCD BACKLIGHT REPLACEMENT WHITE LAMP FOR SAMSUNG R20 X22
A large screen TV can have around 10 such lamps in it ...
Usually copies are the way to go ...but these are not easy to change ...
The clips colding them heat harden and go brittle ...thus easily broken ....
Never a good idea to change 1 or 2 ..best to replace the whole set in one go if possible ...
Yes it is a good idea to crean the plastics whilst you have the screen apart ....but this job can take many hours and care is needed removing the screen matrix as this is mm thin and easily cracks ..
Best way is to slide it flat onto a board at disassembly ...
On reassembly ...not one single bit of grit must enter between the diffuser filters and the screen
..and these diffusers MUST go back in the right order...
At best you will see this when the screen illuminates ...
At worse the screen bridged over a hard bit of grit will crack ..
There are also COFs chip on film devices which must be treated very gently ..the film is easily torn ...the chips easily broken ..
Basically this job should be done on a very large table in a clean room ....or you seriously run the risk of creating a giant paperweight..
not for the faint hearted ....and the larger the screen ...the more care needed..
Check out this vid ..to see what I'm talking about ..
rgds
VS