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.
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.
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.
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
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
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
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.