Oscillate dark to light and back again

Again, no make or model, if I tried to answer me or anyone else would just be guessing, it sounds like LED backlight failure but could be something else, post the model and make and I'll be glad to give you some professional help

HI there do check all circuit boards in back cover for any blown fuses or other electronic components , If all good then you will have to remove LCD screen to test LED lights . switch TV on from standby mode and shine a LED torch onto screen with room completely dark if ICONS do display ,Then i wood recommend for you start off checking your power supply circuit board or driver board . A easy way of checking if LED lights are working again in a dark room with TV switched on look through back cover vent holes for any light shinning through small openings this would determine if some or all back LED lights are blown.

The TV's main culprits to be causing this problem are "dynamic contrast" which maybe darkening the backl light in dark scenes an example a night time or a dark scene in a movie. To make the blacks be deeper (as LCD/LED TV are rubbish at blacks) or brighten it on bright scenes to make the light be lighter and once again to make the blacks be deeper so the images pop out. Also "eco lighting" this will dim or brighten the image based on the amount of light in a room in order to be saving energy. Ether way the easy fix is to turn both of these features off. You will get rid of the problem and enjoy more subtler shades of colour that would otherwise be bleached (brightend) or crushed(darkend) out..

Two things to check. The T- Con board and the back light strips. The T-con board can cause dimming on parts of the screen. A bad section of back light LEDS can do it also.
Parts can be found here: TV Parts Universal TV Stands DLP Chips ShopJimmy

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
Some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement.

Double check your connections. General rule of thumb, if the problem changes after you are in the set, it is probably something you did.
As for your original problem, it sounds as if you either have a bad capacitor or the tcp onthe panel is going bad.

You need to give a more detailed description of the problem.
The LG 47LH90 is an LED backlit TV with 'local dimming' technology. This means that there are lots of white LEDs behind the screen and these can be controlled in groups to allow localised areas of the screen to be dimmed to produce better contrast between light and dark. It is very likely that when the room is dark, this local dimming is more noticeable. The actual individual LEDs cannot be controlled and so the area that is dimmed will generally be greater than the area that is required to be dark 'blooming effect'. In addition, LED backlit displays only maintain their contrast through a tight viewing angle. If you sit too farto the right or left, then light from non-dimmed LEDs will leak through and the screen will look lighter.
In short, there may not be any fault with your TV.

Go to this Website and type in your Model number they will List Parts/Prices for your TV, They also have CD Service Manual you can order!
Encompass Parts Distrubtion 1-800-638-3328

When you know Part number you can go to Ebay ,Look under Electronics ,Thats where I find alot of Parts Cheaper then what companys want for them.

You might be Looking at replacing Light Engine or the Dmd Moduel Order the CD it will help you out alot.If one side of screen starts turning Green and wrong flesh tones it is the Light Engine

A grinding noise in that model usaully means the light engine is defective or it could be the color wheel. But light engine is most suspect as there have been some issues with the light engine assemblies