LED backlight design for large displays
LEDs use a DC supply, making them simpler to drive, there is no inverter, which improves efficiency, and their power consumption varies nearly linearly with brightness, simplifying power management. As a result the majority of small to medium displays are now fitted with them, writes Mike Caddy, displays product manager RS Components.
Despite their advantages and growing popularity, there are also drawbacks associated with LED backlights. A white LED isn’t truly white. It is actually a blue LED fitted with a yellow phosphor to give the impression of white light, and its spectral curve has gaps in the green and red parts of the spectrum. To achieve the very best colour balance, premium quality LCD displays are fitted with RGB backlights.
Most displays of any size will require more than one LED for an acceptable level of brightness, and good uniformity is harder to achieve, especially as the LEDs age.
Power efficiency can also be a challenge. Though LED displays are normally more power efficient than CCFL, this is not a given and some implementations use the same or more power than their CCFL counterparts.
LEDs are however continually improving in terms of light output, efficiency and lifetime. LED brightness has increased so that fewer devices are potentially required per display. Manufacturers have used MEMS and other light guide technologies to spread illumination evenly over a large area to give the maximum brightness and uniformity.
Other sophistications include full frame LED lighting, where the LCD panel is divided into up to 240 segments, and the brightness of the LED backlight can be varied locally, to produce a ‘blacker black’ in dark areas of the screen and simultaneously to reduce power. Ultra thin screens can be created by using edge lighting.
LCD displays vary greatly in their performance, and some displays can achieve a required level of ‘readability’ with less help from the backlight than others. The key parameters to look at are brightness, contrast ratio and viewing angle.
Brightness is a relatively well-standardised parameter, and is quoted in candelas per square metre in a darkened room with all pixels white at maximum backlight drive. Contrast ratio values are less easy to compare as there are a number of ways of interpreting this measurement, but it is fundamentally the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing.
Viewing angle is more subjective.
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