Sony has unveiled the latest in green technology - a TV that automatically switches off if no-one's watching it.

Sony_Bravia_V5_350 TVs in the Bravia V5 range are equipped with a small sensor underneath the display that keeps watch of the room to ensure someone is there. If all the viewers leave the room it cuts power to the LCD screen, only to switch it back on when someone returns. Sony says this results in a 50 percent reduction in energy consumption during the time the display is switched off.

Other circuits in the TV remain on and sound continues to be heard. However, if no one reappears in 30 minutes the set is switched to standby mode.

In general use, the V5 televisions consume less power than their predecessors. The 40 and 46in models draw 129W and 153W, respectively, compared to 217W and 263W used by the same size V1 sets, launched this time last year.

Sony is also adding a 'low-energy switch' that ensures power to the TV is cut completely. Without the switch, the power consumption of flat-panel LCDs is around 0.006W. Although this isn't much, it's still not zero.

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Sony has also cut power consumption on its J-series TVs. However, these are only sold in Japan.

TV makers like Sony are able to reduce power consumption by improving the design of the backlight and screen. In LCD TVs, a light panel typically remains continuously on while the set is in use, providing light that shines through the screen and allows the image on the LCD itself to be seen. This is one of the most power-hungry components in the TV set, but new designs allow the backlight power to be adjusted to suit the picture and more efficient engineering means less light is lost, and most can shine through.

Sony's focus on power consumption in these models is partly an answer to increasingly environmentally aware or cost-sensitive customers. Lower power TVs are both cheaper to run and result in a lower total carbon footprint because they draw less power.

The V5 will go on sale in America in the first half of 2008. Availability dates for the UK were not released. The 40in LCD is expected to cost $2,300 (£1,643) while the 46in version will be priced at $3.079 (£2,200).

See also: Sony unveils bendable OLED screen