LED Backlighting:
LEDs are becoming increasingly common as backlighting elements. They are already the standard in industrial panels up to 10.4 inches and are an option in panels larger than 15 inches. The advantages are considerable. LEDs use direct current, making them easier to manage; they have no inverter, thus improving efficiency; and their power consumption varies almost linearly with brightness, simplifying power control. As a result, most small and medium-sized displays come equipped with them. iSuppli Corporation forecasts that this trend will continue to grow and extend to larger displays. For example, they estimate that 39% of LCD televisions will have LED backlighting in 2013, compared to 3% in 2009. The analyst points out that this trend has been accelerated by the decreasing price of LEDs compared to cold cathode fluorescent lamps due to the higher performance index of LEDs, as well as an oversupply in the LED market.

Despite its advantages and growing popularity, LED backlighting also has some drawbacks. A white LED isn't truly white. It's actually a blue LED with a built-in yellow phosphor to give the impression of white light, and its spectral curve has gaps in the red and green parts of the spectrum. To achieve the best color balance, high-quality LCD displays are equipped with RGB backlights.
Most displays of any size will require more than one LED to achieve an acceptable level of brightness, and acceptable uniformity is difficult to achieve, especially as the LEDs age. Furthermore, power efficiency can be a challenge. Although LED displays are more power-efficient than cold cathode fluorescent lamps (CCFLs), this isn't always the case, as some implementations use the same or even more power than their CCFL counterparts. LEDs are continually improving in terms of light output, efficiency, and lifespan. LED brightness has increased to the point where fewer LEDs are potentially needed per display. Manufacturers have turned to microelectromechanical systems (MEMS) and other light-guide technologies to spread illumination evenly over a large area, providing maximum brightness and uniformity.
Other sophisticated techniques include full-frame LED backlighting, in which an LCD panel is divided into up to 240 segments, and the brightness of the LED backlight can be varied locally to produce deeper blacks in dark areas of the screen while simultaneously reducing power consumption. Ultra-thin displays can be created using edge-frame backlighting.

Getting the Most Out of Your LED Backlight:
LCD displays have highly variable performance, and some displays can achieve the necessary level of readability with more help from the backlight than others. The key parameters to analyze are brightness, contrast, and viewing angle. Brightness is a relatively well-standardized parameter and is indicated in candelas per square meter in a darkened room with all white pixels and the backlight at maximum brightness. The contrast ratio is not as easy to compare because there are several ways to interpret this measurement, but generally speaking, it is the luminance index from the brightest color (white) to the darkest (black) that the system is capable of producing. The viewing angle is even more subjective.
The brightness of the display is affected by the transmission index of a TFT display. A small proportion of each pixel is darkened by the thin-film transistor that controls it. Technologies such as polysilicon or low-temperature polycrystalline silicon (LTPS) reduce the size of this transistor.
While performance figures can help in outlining a list of potential display options, the best approach for applications requiring good visibility in harsh environments, where power consumption is also a concern, is to simulate the application with several display alternatives and measure the backlight power in each case. Factors to consider include ambient light levels in the environment where the display will be used and the likely viewing angle. Sometimes, variations in the color of key information can impact display performance.

OLED: The Backlight-Free Alternative
. LCDs themselves consume very little power. In fact, the power consumption of display systems is almost entirely limited to the backlight, which illuminates the entire display unless it can be turned off.
In contrast, OLED technology is emissive. Each pixel emits its own light, so when it is turned off, it produces no light and consumes almost no power. Unlike backlit LCD displays, OLEDs produce true blacks, resulting in a much higher contrast ratio, typically 10,000:1 compared to the 400:1 ratio of conventional TFT displays. They are also much brighter, partly because they don't need a pair of polarizers to filter half the light emitted by a backlit LCD display.
From an aesthetic point of view, OLED technology surpasses LCD, offering improved brightness and contrast levels. The response time of an OLED display is typically 50 ms compared to 25 ms for LCD, meaning that full-motion video is faster and grayscale representation is far superior.
Despite its higher cost and shorter lifespan, OLED technology is being used more and more frequently in consumer products, including Sony's Walkman X series, the Nokia N85, and the Microsoft Zune HD. The technology is also available in the industrial market, and OLED display options are already available in sizes ranging from 0.79 inches to 7.0 inches and resolutions from 64 x 48 to 480 x 272, supported by development and evaluation kits.

OLED Power Control:
The power consumption of an OLED display is not a fixed value but varies depending on the image being displayed. In an image and video display application, it can be as little as 25% of the maximum theoretical power that would be consumed if all pixels were illuminated simultaneously. For applications where power is a significant concern, image design can help reduce consumption. For example, displaying an image in negative mode (white text on a black background) can be much more efficient than in positive mode (black text on a white background), as it only requires lighting one-tenth of the pixels. 

Pixel brightness also has a significant impact on power consumption. The relationship isn't always linear, but it's certainly a good first approximation. Power can be saved not only by reducing the brightness of all pixels when ambient light levels allow, but also through context-sensitive brightness control, for example, by dimming unavailable menu areas. Reducing brightness also helps extend the display's lifespan. While power consumption is the biggest challenge, changing the colors of frequently displayed menu items is something to consider, as red and green pixels are more energy-efficient than blue ones. A research team at the University of British Columbia has designed an "energy-conscious" color configuration that saves up to 40% compared to the standard color palette.

The display's refresh rate also impacts power consumption due to the capacitive nature of OLED pixels. A very high refresh rate increases power consumption because it requires more charging cycles. It can also cause the display to dim, as the pixels don't have enough time to fully charge with each refresh cycle. Although the contrast setting can be increased to compensate, this further impacts power consumption. The refresh rate should be as low as possible without causing flickering in the visible image. A suitable nominal value is 75 Hz, although sometimes it's even possible to go as low as 60 Hz.
Since OLEDs are an emissive technology, continuous use results in very slow pixel degradation. In addition to using sleep and power-off modes to prevent aging, investing in a screensaver can be worthwhile. The number of pixels used and the brightness of the screensaver should be controlled as explained previously.

Conclusion:
In an increasingly portable world, the demands for reduced power consumption, weight, and cost, along with the need to incorporate multimedia functions and add attractive aesthetic elements, apply even to the most commonplace test and measurement systems. The mobile phone market is paving the way for new technologies that can reconcile these conflicting needs. LED backlighting was first introduced in mobile phones and is now almost universal in small and medium-sized displays in industrial environments. OLED technology is undergoing virtually the same evolution.

Author: Mike Caddy, Product Manager (Displays), RS

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