Under normal conditions, sunlight passing through a window and falling on your LCD TV screen is unlikely to cause any lasting damage. Direct sunlight can cause problems however. Excessive heat build-up can physically damage the screen and prolonged exposure to unfiltered ultraviolet rays in sunlight can cause the chemicals in the LCD to deteriorate. This will not usually cause damage that is immediately noticeable but it can shorten the useful life of your television.
"LCD" stands for "liquid crystal display." An LCD screen is comprised of thousands of tiny pixels containing liquid crystals arranged in cells between two transparent layers. Light is shone from behind the screen via cold cathode fluorescent lamps (CCFLs) or light emitting diodes (LEDs). In a colour LCD screen, each pixel contains three liquid crystal cells. Each cell is fronted by a red, green or blue filter and the way the light passes through the liquid crystals and filters creates the colours we see on the screen.
Extreme or prolonged heat, including that caused by direct sunlight, can cause damage to an LCD screen. The screen's plastic frame or enclosure can become warped and "hotspots" on the screen can cause the liquid crystal cells to break down, affecting the picture you see. Businesses that use an LCD screen outside, for entertainment or advertising purposes, will often fit an LCD enclosure. This should be waterproof to protect from rain damage and can also be fitted with an airflow system to cool potential hotspots caused by direct sunlight.
Ultraviolet (UV) rays can cause the chains of molecules in liquid crystals to break down. They will no longer become completely opaque or completely transparent when they should. In effect, light will "bleed through" when it should be blocked off or will be partially filtered when it should be allowed to pass freely. This can lead to a loss of contrast, brightness and definition. The effect is cumulative and can occur if an LCD screen is exposed to direct sunlight over a prolonged period. Modern glass windows filter out most UV rays so, potential heat damage aside, sunlight falling on a TV through a window will not be particularly harmful.
Positioning your TV
There are various factors, including aesthetic considerations, that might affect the best place to site your TV. It should also be positioned out of direct sunlight and away from other potential heat sources such as radiators and fires. Sunlight passing through window glass is not usually as harmful as direct sunlight but it can still sometimes lead to heat damage, as well as potentially causing glare that can make the screen difficult to see from certain angles.