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A Guide to HDTVs

[Page 5] Brightness, Black Level & Contrast Ratio

A major determinant of the image quality of any display is its brightness, black level and contrast ratio. These related factors affect how much depth and detail the image on an HDTV screen appears to have. A display where blacks look more like greys, or where the image isn't particularly bright, or looks flat is not a particularly desirable one. Unfortunately choosing the right display is not just a case of picking one which is the brightest, or looks to have the darkest blacks in the showroom. For one thing, the typical TV store's display area is quite bright, which favors TVs which are brighter, but which may not have good black levels. It is difficult to distinguish a TV's true contrast ratio in a bright environment. There are also a range of tricks of the trade manufacturers use to enhance these aspects of any TV, at the cost of other areas of image quality. Let's understand the fundamentals first before getting onto those.

The Brightness of a display is measured by its Luminance, usually presented as a figure in either candela per square metre (cd/m2) or foot-lamberts (fL), where 1 fL= 3.426 cd/m2. You will see various reviews or technical specifications quoting the maximum and average brightness of a display, sometimes with very high values up to 500 cd/m2 or more shown. In practice a target value of between 80 - 120 cd/m2 is suitable for most displays in a normal viewing environment. While an LCD can typically provide greater maximum brightness than a plasma, both display technologies can usually achieve sufficient brightness to suit most people. Keep in mind however that when discussing the brightness of a TV, there are two other factors to consider: the amount of ambient light in your viewing environment, and the contrast ratio of your TV - we'll discuss these shortly.

One important difference between plasma and LCD is that while the liquid crystal in an LCD can twist to varying degrees to allow different amounts of light to filter through, and hence vary its brightness that way, plasma phosphors are either lit up brightly (on) or dark (off) at any one moment. Once lit up, they only stay lit for the merest fraction of a second. By using Pulse Width Modulation (PWM) to pulse the amount of current flowing through the cell, the phosphors are lit up hundreds of times a second to maintain brightness, and by varying the width of the pulse so that each phosphor stays on for slightly more or less time for each pulse, the level of brightness of the image can be varied.

Black Level is another oft-quoted but not-fully-understood metric which is a critical element of good image quality. The ability to create darker blacks allows a TV to have a higher contrast ratio, a term which will be explained shortly. The darker the blacks, the greater the appearance of depth and richness in the image shown. Black level is actually not particularly complex; it's just a measure of the level of brightness of a display when showing video black. It usually has as a very low luminance value, such as 0.004 fL (0.013 cd/m2), but rarely reaches 0.0 cd/m2/fL (true black) because most HDTVs can't achieve this.

To confirm this for yourself, show a black screen on a typical LCD or plasma TV in a pitch black room, and you will still see some light coming from the screen, as the photo above demonstrates. So why isn't black on an HDTV actually equivalent to zero luminance, which is the total absence of light?

On an LCD-based display, black is simulated by the twisted crystals of the panel being completely shut, along with a polarized layer behind the crystals, to prevent light from the backlight filtering through when not required. Yet precisely because the backlight is always on, and the structure of the crystal array is not perfect, some amount of light will leak through the crystals and be seen - this is discussed further in the Screen Uniformity section. More recently, with the advent of local dimming backlighting, some LCD-based displays can switch off portions of their backlight to produce close to true black in parts of the image which require total darkness. Unfortunately this method isn't perfect, as there may be haloing of light around any brighter parts of the image.

On a plasma display, each pixel can be independently switched off to remove its light output, and since there is no backlight, in theory a plasma can produce true black. The reality is that each plasma cell has to be consistently pre-charged so that it can respond quickly enough when light output is required, giving plasma its extremely fast response time. The side-effect of this pre-charging is that there is always some residual glow in the pixels, and thus true black is usually not possible on a plasma. On average though, plasmas provide much darker black levels than LCDs.

It should be noted that what some consider the king of black levels, the traditional CRT TV, does not necessarily achieve perfect black either. A CRT's black is darker than either plasma or LCD, primarily because a CRT can simply have its electron beam avoid lighting up particular portions of the screen. However when displaying any scene containing brighter elements, some stray light may affect the dark areas of the screen. In other words, when a CRT is showing an all black screen, black levels are pretty much true black, but when displaying a normal scene containing a mix of brighter and darker elements, black levels on a CRT are not true black, and may be similar to or even worse than a plasma screen.

Now that we understand the way HDTVs can display brightness and the lack of it on the screen, it's time to look at a metric which is supposed to show the range between these extremes on any TV. Contrast Ratio measures the difference in the luminance between the whitest image and the darkest image that a display can show. It's usually presented in a format such as 4,000:1 - this example would indicate that the whites on this display can be up to 4,000 times brighter than the blacks. The main benefit of a high contrast ratio is that in a scene containing both bright and dark elements, a TV can reproduce both elements correctly. That is, the dark areas will look suitably dark, while the bright areas will remain bright. Displays with poor contrast ratios will give more of a "washed out" image due to less of a difference between dark and bright areas.

Unfortunately manufacturers quickly became aware that consumers were paying attention to contrast ratio figures, and since there is no standard enforced as to how to consistently measure it, contrast ratio figures have now been elevated into ridiculously high numbers, such as 5,000,000:1 or 9,000,000:1. This can be achieved for example by taking measurements from a pixel when it is completely switched off, then comparing it to the pixel when it is lit to the maximum possible level of brightness, then contrasting the two numbers, even though this in no way represents real-world contrast ratios in normal scenes consisting of both bright and dark images. As we see in the Motion Handling section, a similar approach is taken to Response Time measurements. It makes the contrast ratio numbers you see in technical specifications virtually meaningless.

In order to achieve darker blacks and whiter whites, which in part justify these unrealistic figures, a technique known as Dynamic Contrast Ratio is now frequently used in HDTVs. The way it works is that the display constantly alters the brightness of the entire image, reducing screen brightness for scenes which are predominantly dark, and increasing the overall brightness for scenes which are mostly bright. The true measure of contrast ratio, also known as Static or Native Contrast Ratio, should provide the difference between the darkest and brightest luminance possible in the same scene - and most displays don't have native contrast ratio capabilities anywhere near the dynamic contrast ratio numbers. A dynamic contrast ratio has several unwanted side-effects, including greyer blacks in bright scenes, and washed out whites in dark scenes. Additionally, depending on how it's implemented, the constant shift in overall panel brightness may become noticeable to the viewer, resulting in what's often described as Floating Blacks, Fluctuating Brightness or Fluctuating Gamma.

If the TV has any option to disable dynamic contrast then you can turn it off, however some TVs do not have any such option available. Furthermore, for plasma owners, something known as the Automatic Brightness Limiter (ABL) can't be turned off. This is a protective feature built into plasmas to control power consumption, since on a plasma brightness is directly related to the amount of power consumed. In scenes with a high Average Picture Level (APL) - that is, scenes which have a high proportion of bright elements - the ABL will reduce the overall brightness output of the plasma panel to stabilize power consumption; conversely in scenes which have a lower proportion of brighter elements (low APL), the overall brightness of the scene is allowed to be higher. For example a full white screen is not going to have as much luminance as a small window of white on a dark background, precisely due to ABL.

From this discussion we can gather an important fact: most HDTVs, whether LCD or plasma, now use some type of dynamic contrast ratio. This can have annoying side-effects, and can render contrast ratio figures meaningless. So how does someone make sense of all of this? The answer is to take into account your viewing environment, combined with the measured black level and maximum luminance of a display as typically given in reviews. These factors will be sufficient to make a determination, as explained below.

Even the best of us can't see differences in the range of brightness at any one time beyond a notional 1,000:1 contrast ratio. Our eyes work by having a form of built-in dynamic contrast ratio, whereby depending on the ambient lighting of our surroundings, we can detect a lesser or wider range of differences up to the 1,000:1 ratio. As our surroundings become darker or brighter, our iris adjusts to allow more or less light in, and this affects our perception and general sensitivity to brightness and darkness at any point in time. For example, in a pitch black room, if someone shines a weak torch in your eye it can effectively blind you; in a bright sunlit room, the same torch would have much less effect. Similarly, in a pitch black room, you will notice some light coming from a black screen on even the best HDTV, while even a small amount of ambient lighting in the room can make that same screen look completely black.

Taking advantage of this property of our eyes, if most of your viewing is done in a brighter environment, such as a sunlit room, then a display capable of higher levels of brightness, such as an LCD-based TV, is advisable. The black levels on such a display may not necessarily be great, but your perception of black levels will also be reduced in a bright environment, making it less of an issue. Plasma TVs suffer more than LCDs from having their image "washed out" when subjected to bright ambient lighting. Many plasmas and some LCDs come with special coatings on the screen, known as Anti-Reflective (AR) Filters, designed specifically to counter reflections and glare and thus help preserve a good image under bright light. While the AR filter helps, in practice it still doesn't fully prevent plasmas from suffering more than LCDs in a bright environment.

In darker environments, our perception of light becomes heightened, and thus blacks can look more like greys if the TV doesn't have a good black level. For this reason, plasma is more appropriate for those who do most of their viewing in a darker environment given its superior black level. Alternatively, if you have an older plasma or an LCD TV with relatively poor black levels, or indeed any TV viewed in near darkness, you can install what is known as Bias Lighting - soft ambient lights situated behind the TV which greatly improve perceived contrast on the TV.

For those who have mixed viewing habits in both dark and bright environments, an LED-LCD with a full-array local dimming backlight is a reasonable compromise, capable of both good blacks at night and higher brightness levels during the day.