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LED backlight design for large displays

03 Mar

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.

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.

 Making the most of your LED backlight

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.

Display brightness is affected by the transmission ratio of a TFT display. A small proportion of each pixel is obscured by the thin film transistor controlling it. Technologies such as low temperature polysilicon (LTPS) reduce the size of this transistor.

Whilst the performance figures can provide a guide to drawing up a shortlist of potential display options, the best advice for applications where good visibility is required under demanding conditions, and power consumption is also an issue, is to mock up the application on a number of display alternatives and measure the backlight power in each case. Factors to be considered are the ambient light level in the environment in which the display is to be used, and the likely viewing angle. Sometimes varying the colour in which key information is presented can also have an impact on display performance.

 OLED – the no backlight alternative

The driving of an LCD itself consumes very little power, and the power consumption of the display system is almost all down to the backlight – which always illuminates the whole display area unless it can be switched off.

OLED by contrast is an emissive technology. Each pixel emits its own light – so when it is off, it produces no light and consumes almost no power. Unlike backlit LCD displays therefore, OLEDs produce a true black, and their contrast ratio is much higher, typically 10,000:1 compared with 400:1 for a conventional TFT display. They are also brighter, partly because they don’t require the pair of polarizers which filters out half of the light emitted by the backlight in an LCD display.

Aesthetically OLED technology wins hands down over LCD, with much improved brightness and contrast. The response time of an OLED display is typically 50μs versus 25ms of LCD, meaning full motion-video is faster and grayscale rendition is far superior.

Despite its higher cost and shorter lifetime, OLED is being used in a growing number of cool consumer products, including the Sony X-series Walkman, the Nokia N85 and the Microsoft Zune HD. The technology is now also becoming available on the industrial market, and OLED display options in a range of sizes (0.79in to 7.0in) and resolutions (64×48 to 480×272) are offered, supported by development and evaluation kits.

OLED power management

The power consumption of an OLED display is not a fixed value but varies depending on the image being displayed. In typical video and image display applications, it can be as low as 25% of the theoretical ‘maximum’ power that would be consumed if all pixels were fully illuminated.

For applications where power is a major concern, image design can contribute to reducing consumption. For example, displaying an image in negative mode (white text on a dark background) can be much more efficient than in positive mode (dark text on a white background), since you need to switch on only around one-tenth of the pixels.

Pixel brightness also impacts power consumption. The relationship isn’t quite linear, but this is a good first order approximation. Power can be saved not only by reducing the brightness of all pixels when ambient light levels permit, but also by context-sensitive brightness management – for example by dimming menu areas that aren’t available. 

Reducing brightness can also help extend the life of the display. Where power really is a challenge, changing the colour of frequently displayed menu items could also be considered, as the red and green pixels are more efficient than the blue. A research team from British Columbia has been able to design “energy aware” colour sets that gave energy savings of around 40% compared to a standard colour palette.

Refresh rate also has an impact on power consumption due to the capacitative characteristics of OLED pixels. A very high frame frequency increases power consumption by increasing the number of charging cycles. It can also cause the display to dim, as the pixels don’t have time to charge fully during each refresh cycle. Although the contrast setting can be increased to compensate, this further increases power consumption. The refresh rate should be set as low as possible without causing visible image ‘flickering’. A suitable nominal value is 75Hz though it is sometimes possible to get away with as little as 60Hz.

Since OLEDs are an emissive technology, very slow degradation of pixels occurs with continuous use. In addition to using stand-by and time-out modes to reduce ageing, it is worth considering a screen saver. The number of pixels used in the screen saver and their brightness should be managed as above.

 

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