How do i get the screen to light up more, its to dim?

could be a power saving feature check your manual

Normally, this is in part due to you not replacing it with what should've been an LCD. The reason why it's dim is because those diodes are not getting the required voltage to emit as it should. LCD's work under a different concept where crystals use the light modulating properties of liquid crystals. Liquid crystals do not emit light directly as LED's, because it stand for light emitting diodes.

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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.

If the screen light is really dim, its possible the lcd backlight has gone bad. Does it ever flashbright?

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If the screen light dim with battery power your laptop, well, this is how the laptop is designed for power saving, unfortunately.

The screen will be more bright when you connect with the AC power :-)

the problem is not with the inverter.
the dim light is due to the LCD Back light

Hello !

Since the screen is dim, the display's inverter board needs to be replaced.

If you were experiencing discoloration in the screen (pinkish lower light hues) before it failed, that would indicate a CCFL backlight issue. If the screen suddenly went dim on you rather than a slow degradation, the inverter would be indicated.

Hope this helps.

How long before it goes dim?

Does it start strong, then dims right away?

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

There should be either a button or a setting to adjust the lighting, try doing that.