How LCD panels work ?

Liquid Crystal Displays (LCDs) have their advantages for both TVs and computer monitors. However, there are two things they haven’t done as well as CRT; high contrast and color depth/accuracy. This however is set to change with the introduction of LED back-lights. To see why requires a look at how ‘traditional’ LCDs work.

LCD panels use the variable polarization of electronically controlled liquid crystal cells to vary the amount of polarized light that will pass through the panel. In other words, an LCD panel is composed of a large number (a typical 1,600 by 1,200 computer monitor has 5,760,000 cells) of tiny, voltage controlled, light valves. The cells themselves are neutral density; they pass or block all frequencies of visible light more or less equally. To provide the sensation of variable color each pixel consists of three of these cells, with individual red, green and blue filters placed in front of each group of three cells. A wide range of colors can be produced by varying the relative amounts of light passing through each of the three cells making up a pixel. Ambient light can be reflected from a plate behind the cells, or a back-light can be used for a brighter, emissive, display. Most displays used these days have a back-light and until recently this would have been a Cold Cathode Fluorescent Lamp (CCFL)

One of the weaknesses of LCDs is low contrast. When the liquid crystal cells are fully open (in the off state with no voltage applied), they do not transmit all of the light from the back-light, for one thing the light passes through a polarizing filter, so at least half of the original light from the back-light is blocked. However, more significantly, when the cells are closed they do not block all of the light. The ratio of light passed, between the on and off state defines the displays contrast ratio (note – contrast is always a ratio, not an absolute measurement). In the older type of LCD panel the CCFL back-light remains on at a fixed brightness all the time (it may turn off during power saving) and has no effect on the contrast ratio, which is determined entirely by the on/off ratio of the liquid crystal panel (ignoring the effect of ambient light reflected from the internal structure of the LCD for the moment).

The Fluorescent lamp

Fluorescent lamps for general lighting were introduced commercially in 1938 and were first used in large quantities to light factories during the Second World War (commercial LEDs became available in 1968 and blue LEDs were not introduced until the late 1990’s). Fluorescent lamps are significantly more efficient than incandescent lamps and small fluorescent lamps (Compact Florescent Lamps with electronic ballasts – CFL) have been heavily marketed in recent years as ‘energy saving’ lamps. Conversion efficiency of electrical power to radiant power varies between 17 and 21 per cent compared to less than 5 per cent for the filament incandescent lamp. The luminous efficiency of CFL sources is typically 60 to 72 lumens per watt, compared to 8 to 17 lumens per watt for incandescent lamps.

In back-light applications, efficiency (regardless of the type of light source) is low because a lot of light is lost passing through the diffusers, polarizers and color filters, perhaps only 5 per cent of the light from the back-light reaches the viewer.

Very recently there has been something of a backlash against fluorescent lamps since they contain mercury, which is poisonous and tends to get released into the environment when old lamps are thrown away.

Fluorescent lamps work by ionizing a column of low pressure gas (mercury vapor) causing it to glow with ultra-violet light. The inside of the glass tube containing the gas is coated with a mixture of phosphors that react to the ultra-violet light by emitting light in the visible spectrum. Various mixtures of phosphors are used in the tubes to give an approximation of ‘white’ light with some variation. For example “Warm White” tubes have more red while “Cool White” tubes have more blue. The spectral output of a fluorescent lamp is surprisingly uneven.

The CCF lamps used for LCD back-lights use a high voltage to start the ionization of the gas while the tubes used for general lighting run at lower voltages and use a momentary heating coil. As far as the color range or gamut of LCDs is concerned, it is the uneven spectral response and the range of light frequencies emitted by the tube phosphors that limit performance. Early CCFLs used similar phosphors to the phosphor formulations used on the tubes used for general lighting. Recently CCFLs have appeared with modified phosphors to give a greater display gamut, for example Sony’s Wide Color Gamut - CCFL (WCG-CCFL) with a claimed gamut 30 per cent larger than a conventional CCFL, but it looks like these are going to be eclipsed by the new RGB LED back-lights, at least in part because using LED back-lights reduces the amount of mercury released into the environment.

By comparing the spectral output graphs of the two types of back-light it is easy to see that RGB LED produces higher peaks for red, green and blue and covers more of the visible spectrum. This results in a considerable improvement in gamut can be seen from looking at the gamut area plots.

RGB back-lights save the planet

RGB LED back-lights offer great advantages in terms of power saving and a longer service life than CCFL. According to Sharp, for example, their RGB displays consume 40 per cent less power and offer a back-light service life of 100,000 hours, about two thirds longer than regular LCD TVs. To put this in perspective – if the display is turned on for 5 hours a day, this is 20,000 days or over 57 years.

Back-light layout

The older panel designs with fluorescent back-lights normally use edge lighting where a CCFL is mounted near one edge of the display. Light from the lamp is guided and spread across the back of the display using a narrow wedge shaped light-guide. The light is further disbursed using a translucent diffusion panel and a prismatic panel that helps reflected light to bounce around. Some displays may use two CCFLs top and bottom, or even on large displays a number of tubes may be mounted in parallel behind the panel.

There are several ways to arrange and drive the LEDs in a LED back-light and these are known as 0D, 1D and 2D.

0D simply uses a number of LEDs and a diffuser system to evenly back-light the entire LCD panel, a direct replacement of the CCFL without any added functionality.

In a 1D back-light the LEDs are arranged in horizontal rows behind the panel and can be brightness modulated to achieve a crude degree of contrast enhancement.

Lastly, in a 2D back-light the LEDs can be arranged in groups (for an RGB back-light also in sub-groups of red, green and blue) that can be individually brightness modulated, an improvement in contrast over different areas of the display (as shown in the two photographs of an RGB back lit television display).

Color temperature

Previously the only way to adjust display color temperature was to adjust the individual gain of the red, green and blue color channels driving the display. RGB back-lights provide the opportunity to change the color temperature of the back-light itself. In other words, the shade of the ‘white’ light provided by the back-light can easily be varied, for example from a ‘warmer’ reddish white – 5,000K, to a ‘cooler’ blue-white – 9,300K.

Contrast improvement using LED back-lights

LCD panel contrast can be hugely improved by turning the back-light off. In this case the idea of representing contrast as a ratio gets a bit silly because the ratio of - some light - to - no light - is infinite. However this really only applies in a completely dark room. In practice there is often a high level of ambient light, which is reflected by the internal structure of the display, degrading contrast.

LED back-lights can be turned on and off very rapidly and they can also be brightness modulated over a wide range, neither of which can easily be done with CCFL back-lights. LED brightness modulation is usually performed by driving them with a square wave and modulating the duty ratio of the wave (pulse width modulation). This technique does introduce its own problems of RF radiation and the difficulty of actually transitioning to a true off state.

Of course, if a small high resolution panel could be made with a set of RGB LEDs at every pixel, the LCD panel would not be needed since the image could be reproduced entirely by brightness modulated LEDs. Large LED-only displays, where each group of three LEDs is small compared to the overall display size, do exist and are used for applications such as advertising hoardings. However, at the moment it isn’t practical to produce smaller high resolution displays in this way, so the LCD panel is retained to provide the required resolution and LED back-lights are used to enhance the contrast (and improve the color gamut). This is achieved by dividing the display up into quite low resolution segments, typically around 128 segments may be used. Within each of these segments, contrast may be enhanced by modulating the brightness of all the LEDs in that segment. This does mean there is some light spill or haloing in the areas where the image on the LCD changes abruptly from light to dark. Using more segments will result in smoother contrast changes across an image. At the moment, the marketing departments of most display manufacturers are keeping quiet about the exact details of their RGB backlit displays, Sony for example will not say how many LEDs they use or in how many segments. Sony do say they use 2 green with 1 red and 1 blue LED in their LED clusters. This compensates for the generally lower light output of green LEDs compared to red and blue.

With computer monitors that are used primarily to display business graphics it’s debatable that there is any advantage in using the area based back-light dimming that is being employed in the current crop of LED back-light TV’s for contrast enhancement. This really only comes in to its own for photorealistic images where there are large areas of light and shade.

Contrast measurement – the specification game

Marketing people love numbers they can stretch or misrepresent to make their products look better and for displays, contrast ratio is one of those numbers. It would seem that measuring contrast should be straight forward, as mentioned contrast is the ratio between peak white and black. However, as the ‘black’ level falls to lower and lower amounts of light it becomes more and more difficult to measure accurately. It is possible to claim huge contrast ratios simply by measuring black levels in a very dark room with the display or at least the back-light, literally turned off.

Measuring with a checker board pattern of black and white rather than with the entire screen black and white, allows for internal screen reflection and produces a more realistic result. As does measuring with a known amount of ambient light.

Typical contrast ratios for LCD panels are on the order of 200 to 400 to 1, although more recent developments in PVA (Patterned Vertical Alignment) and S-PVA (Super Patterned Vertical Alignment) panels provide claimed contrast ratios of over 3,000 to 1 (i.e. better black levels). The old CRTs provided good contrast because, when properly adjusted, black level means the display emits no light.

Color stability and temperature compensation

All color displays have suffered from color instability due to temperature change. With CRT and the CCFL back lit LCD panels the approach, certainly before color calibrating a display, was to leave the display switched on and displaying an image for between half an hour, to an hour, to allow it to ‘warm up’ and for the color to stabilize. There was no attempt in these designs to apply corrective feedback for color/temperature changes. The light output of LEDs does vary markedly with ambient temperature and in an RGB LED back-light this can result in color drift as the temperature characteristics of red, green and blue LEDs do differ.

White LED back-lights and comparative gamuts

Perhaps an obvious and relatively simple step in display design evolution is to take a conventional LCD panel and simply replace the Cold Cathode back-light with a white-light, LED back-light system. The advantage of doing this is that the electronics are simplified because the high voltage converter required by the CCFL isn’t needed and the environmental impact is at least modified because LEDs don’t contain mercury. White LED back-light displays often do not provide the best in wide gamut photo-realism. This is because the spectral output of the “white” light from the LEDs is not very smooth and does not provide sufficient energy over a wide range of frequencies. Most white LEDs are blue, or ultra-violet light LEDs, with the addition of a yellow phosphor in the lens cap, which converts some of the blue light into light at other frequencies, in the same way that a fluorescent tube phosphor converts the ultra-violet wavelengths into visible light. This is evident in the spectral plot (see – white LED spectral plot) where the blue shows as a sharp spike with a much lower broader yellow peak. Consequently the light from this type of ‘white’ LED is characteristically a hard blue-white light, due to the high blue peak. Just to confuse things, Samsung refer to the LED backlight technology they use as LED-BLU where BLU stands for Back Light Unit.

Typically, a blue plus yellow phosphor LED back-light will only manage a 71.6 per cent comparative gamut (relative to NTSC). Experiments have also been done using a blue LED with distinct red and green phosphors. When this LED design is combined with the typical LCD filter sets and color matching functions, a reasonably wide gamut of 91.9 per cent of the NTSC standard gamut is obtained.

In comparison manufacturers using back-light designs with discrete red, green and blue LEDs are claiming gamuts as large as 123 per cent compared to the NTSC standard.

Unfortunately, companies offering LED back-light displays use different designs and don’t want to reveal exactly how their products work, it may be difficult for the consumer to tell with certainty what the differences are between the various products available in order to make sensible choices. There is for example the significant difference between a ‘white’ LED back-light and an RGB back-light. A display may even use an RGB LED back-light and as a result offer a better color gamut, but not apply segmented brightness control to provide the improvement in contrast.