TV Contrast Ratio Explained - Part 1
Playing with Numbers
...is there a limit to this manufacturers' number game?
It has always been industry's favorite game - playing with numbers, and TV contrast rating is one of the preferred names, with the latest craze in the video industry being the new mega contrast ratings. Dynamic contrast ratings of 3,000,000:1 have become common with LED and plasma TVs. CCFL LCDs have reached 150,000:1 while home theater projectors are at 60,000:1. And the numbers are still growing...
Surely, we all love numbers - normally the bigger the better - but do these BIG contrast performance figures really mean something in the end?
In the first part of this article, we discuss the basics of contrast ratio to have a better understanding of contrast rating and how this impacts picture quality. In the second part, we discuss the eye response to variations in image brightness and how this affects our ability to detect image contrast in a TV picture.
BIG NUMBERS sell better, and manufactures know well. This number battle is everywhere, from the use of interpolated resolutions in digital image scanners and cameras, to the latest 480Hz refresh rates in LCD and LED TVs and the 600Hz sub-field drive in plasma displays.
Manufactures do their best to come with big numbers to deliver the message their product is the best! However numbers on their own do not mean anything; unless you know what you are talking about, one may end up misinterpreting facts. The same applies to contrast ratio.
2004 brought the first TVs with a 4,000:1 contrast rating. By 2007, figures for contrast ratio reached 15,000:1; in 2008, we saw the first 1,000,000:1 ratings for plasma and LED TVs.
Now we speak of multi mega contrast ratings. It is as if there is no limit to how much these numbers can continue to grow. Yet this contrast issue is more than just numbers. Apart from the fact that most TV makers often fail to differentiate between static and dynamic contrast - some TV manufactures are taking consumers for a ride by quoting ultra high ratings for contrast ratio using unspecified test methodologies. Their only purpose is to help sell their products over that of the competition.
We are not saying that the latest mega-contrast ratings associate with some HDTVs are incorrect - most probably they are correct if measured in a totally dark room environment.
What is actually being measured by these display manufactures?
The 'dark room' setting is not the typical viewing environment, so how does ambient light affect the resultant contrast ratio of a display device?
What is the implication on perceived contrast by the eye - is this as much as manufacturers are trying to imply with their mega numbers?
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Is your Black really Black?
Contrast is the ratio between the white and black parts in an image. A contrast rating of 300:1 means that the luminance or brightness level of the white areas in the image is 300 times brighter than that of the blacks. The larger the contrast ratio, the greater the difference between the brightest whites and the darkest blacks a display device can produce.
Contrast is an important image quality attribute that affects our ability to perceive both image brightness and image detail. Without a sufficient level of contrast, images appear flat with the blacks and white taking a gray shade while finer image detail would be lost.
Static or Dynamic? Static contrast refers to a display device native contrast ratio. It is the more important of the two despite being smaller as it represents the 'true' contrast i.e. the darkest blacks and the brightest whites that a display can simultaneously support at any one time.
Instead, dynamic contrast refers to the maximum ratio between the deepest blacks a display can show and the brightest whites it can handle over its entire operational range but not at the same time. A display device will never be able to handle these two extremes simultaneously.
Dynamic contrast uses a processor inside the display to analyze the average, overall picture brightness and adjusts the backlight level on-the-fly. This helps render deeper blacks in predominantly dark scenes and brighter whites in mostly bright picture content. When done properly, dynamic contrast can give the impression of higher contrast ratios and solid black levels; done poorly, it can crush black to the point of losing shadow detail.
Yet dynamic contrast may also alter the tone response characterizes, or gamma setting of the display and the video signal on-the-fly to produce what most consider a more attractive image. But altering these characteristics means that the presented image may not necessarily be a true and faithful representation of the original video signal. This may not be an issue with video games and most TV content, but it is with material of artistic nature.
As expected, the dynamic contrast rating results in much larger numbers and is often the contrast ratio most quoted by TV makers.
Note: Some definitions for contrast imply that an increased contrast ratio yields better subtle shadow detail and increased color palette. This is NOT true. These definitions fail to differentiate between an imaging device ability to display finer image detail for which a sufficient level of contrast is necessary for the eye to perceive the finer detail, and subtle shadow and color palette.
A device ability to display subtle shadow arises out of its ability to display various levels or intensities of gray - referred to as shades of gradation - from complete black to complete white. This depends on its grayscale capabilities rather than on a higher contrast ratio.
There is also the issue of grayscale uniformity - the ability to maintain all shades of gray close to the D65 (or 6,500K) standard across the entire brightness range to preserve color fidelity at all brightness or luminance levels. Many TVs fail here - shifting towards red in the darker areas and blue in the lighter parts of the image.
Measuring Contrast Ratio
The two methods mostly used by the projection and video display industry to determine contrast performance of a device are the Full On/Off, and the ANSI Contrast measurement:
Full On/Off Contrast is the ratio of the light output of an all white image (full on) and the light output of an all black (full off) image. This is the most favored measurement by manufactures as it yields a larger number for the contrast ratio - 25% to 100% more - than ANSI Contrast measurements for the same display device. Some manufactures of DLP-based displays would even carry this contrast ratio measurement with the 'white segment' of the color wheel turned; this increases the measured figures for the white luminance, hence inflating further the end result.
ANSI Contrast is measured with a pattern of 16 alternating black and white rectangles - also referred to as the 'checkerboard' test pattern. The average light output from the white rectangles is divided by the average light output of the black rectangles to determine the ANSI contrast ratio.
Checker-box image courtesy of ausmedia.com.au
ANSI Contrast represents a more fair way to test contrast ratio as the presence of black and white at the same time is closer to the real world environment rather than the all black or all white image used during the full ON/OFF contrast measurement. This renders the average reading obtained from the ANSI Contrast more realistic.
It is thus clear that when comparing the contrast ratings of home theater projectors and video display devices, it is essential to ensure you are comparing the same type of contrast. Full On/Off contrast will always be a larger number than ANSI contrast for the same projector or monitor.
Unfortunately, there is nothing mandatory as to the methodology used by manufactures to measure contrast performance. Even worse, manufacturers are not obliged to specify the method used to arrive at the quoted figures.
This renders a straightforward comparison of contrast performance figures between different imaging devices - especially from different manufactures - almost meaningless.
Contrast is an important attribute in a display or video projector performance and helps make an image 'pop-up' as it affects our perception of color differences apart from image brightness. A minimum level of contrast is necessary for the eye to perceive an image as 'bright'. Studies show that for a binary black and white image, a minimum contrast ratio of 2 is necessary for comfortable viewing, while a contrast ratio of between 5:1 and 7:1 is necessary for the eye to distinguish color differences.
(available from amazon)
Yet, today's video projectors and flat-panel TVs come with more than just a minimum level of contrast. Most affordable premium home theater video projectors come with a rated contrast ratio in excess of 50,000:1, while the latest flagship model from Panasonic, the PT-AE8000 featured here, has a rated on-off contrast ratio rating of 500,000:1!
Mind you, the PT-AE8000 does not come cheap but it is a superb home theater projector capable of 100-inch plus bright image 2D and 3D projections at a price that is well within that of premium 60-inch LED TVs. For those who would like to know more on this projector, please refer to our review article here.
Even more impressive are the mega contrast ratings for the latest plasma and LED TVs. During 2010, we saw the first 5,000,000:1 CR plasma and 9,000,000:1 CR LED TVs! However, since 2011, we started seeing a new trend among TV makers, namely that of simply stating that their HDTV has a mega contrast ratio. In fact, the term 'mega-contrast' has become one of the latest buzz words in the HDTV industry.
A few TV makers like LG still go on quoting a figure for their TV contrast ratio; a case in point is the popular LG 55-inch 55LM6200 3D Smart TV, which comes with a quoted dynamic contrast ratio of 6,000,000:1.
While it is true that these video projectors and HDTVs have been rated as among the best we have seen so far from the giants in the industry, yet contrast cannot be taken on its own. A high contrast rating can be misleading at best if used incorrectly; and in the promotional media, it can even turn out to be a lie.
A higher contrast rating implies a device has greater ability to display black with a deeper shade of black instead of just dark gray with respect to its brightest whites. A home theater projector with a contrast ratio rating of 8,000:1 is more capable of showing a dark subject than one with an 800:1 contrast ratio. But the difference in performance between the two would only become apparent if projection takes place in a completely dark room, one in which is there no light other than that of the projected image.
Equally important is that the eye would not detect a 10 times improvement in contrast performance between these two projectors. Rather, the perceived difference in image performance between these two devices would be just marginal and detectable only if the room is in total darkness.
Marginal, because the eye contrast sensitivity is not linear. While a difference in a contrast ratio of between 10:1 and 20:1 will be definitely distinguishable and a difference between 100:1 and 200:1 as clearly visible, the eye would see the difference between say 400:1 and 800:1 as a minor change. And anything above 1000:1 may not be visible at all.
And it is detectable only in the total absences of ambient light because as we will see further in this article, complete absence of stray light in a room is critical when it comes to contrast performance. This is a rather rare situation in everyday life unless viewing takes place under a controlled environment, such as that of a dedicated home theater with black painted walls.
This absence of light in contrast performance is important as the brightness level of what the eye perceives as black in comparison to the brightest parts of the image, can only be as low as the light reflected by the darkest parts of the image. This applies irrespective of whether the image is being displayed on a projection screen or a direct-view display such as that of an LCD or plasma TV.
Any ambient light will be reflected by the projection screen surface, or the glass surface and the internal display structure in the case of a direct-view display, thus turning black into what may appear as some shade of gray. This lowers image contrast considerably.
Direct-view systems however have the advantage that their contrast performance is significantly less affected by the presences of light in the room. This is mainly thanks to the optical coatings on the display screen surface that help block external light from entering the display panel, and the use of a black surface structure inside the display between adjacent pixels to block more of the ambient light from entering the display.
Presence of Ambient Light and Perceived Contrast
With this much light in the room, there is no difference between 900:1 and 10,000:1 Contrast Ratio!
Presence of light: To better understand the impact of the presence of light in a room on the perceived contrast, consider the light emitted by just one candle in a room one meter away from a display surface whose black level stands at 0.1 cd/m2 in total darkness. This is sufficient to render the image from say a 10,000:1 rated contrast ratio video projector the same as one projected by a 900:1 CR rated projector when the latter is displayed in a totally dark room!
Increase the light in the room to approximately 30 LUX; this corresponds to the light emitted by a small incandescent lamp. Contrast figures above 100:1 would now turn out to be simply academic even in the case of video projectors with say a relatively high 2500 ANSI Lumens brightness rating. Why?
To maintain a 'low' 200:1 contrast ratio in less than 30 LUX of ambient light, your projector have to generate more than 6000 LUX for the brightest areas. But to maintain this level of brightness over say a 100-inch diagonal 16:9 unity gain screen (30 square ft), you need a projector with at least 16,000 ANSI-Lumens output. Your 2500 ANSI Lumens projector will never maintain a 200:1 contrast even in a dimly lit room, least imagine its quoted 10,000:1 CR rating under the presence of higher levels of ambient light. (For more information on projector screens and video projector output, please refer to our article: Understanding projector screens.)
Let's continue playing with numbers! To better understand the whole issue of the presence of ambient light and its effects on contrast performance, consider a contrast rating of say 2,500:1 and a maximum luminance level of 250 cd/m2 for the brightest part of the image. Then the darkest part of the image will have a luminance of just 0.1 cd/m2 in the complete absence of ambient light.
Let's introduce the presence of a dim light source in the room and assume that just 5 cd/m2 of this light is being reflected by the screen surface.
Contrast ratio would than change as follows:
= White Luminance/Black Luminance
= (250 + 5)/(0.1 + 5) or 255/5.1 = 50:1
This means that our contrast ratio has now changed from the original 2,500:1 to just 50:1!
And it only takes a very dim light source for a display to reflect as much as 5 cd/m2 of light. A shiny screen surfaces such as that of a CRT may easily reflect as much as 80% of the light falling on the screen.
It is thus clear that unless you watch your moves in complete darkness, it would be useless to stress too much and pay more for a device with a very high contrast ratio rating!
Important Factors worth considering
Contrast and Brightness: As already stated, contrast does not stand on its own! A higher image brightness level would yield a better perceived contrast ratio for a fixed level of ambient light. This means a projector with a lower contrast ratio but with higher rated lumens output may under certain light conditions, perform better than one with a higher contrast ratio but lower brightness level.
Cinema: It is interesting how present-day digital cinema projection equipment performs with respect to contrast ratio. Typical contrast ratio is 1000:1; however, this varies according to make and model. For example, one of the world's most popular digital cinema projectors, the Christie CP2000 2K 3-chip DMD DLP Cinema™, despite its 35 trillion colors, has a contrast ratio rating of just 500:1 ANSI, 2000:1 full field.
Note: The reference here to the performance of digital cinema projectors applies to contrast ratio. When it comes to image resolution, traditional 35mm and in particular 70mm movie film systems support far greater image detail than that supported by professional digital cinema equipment today.
Grayscale: While contrast is an important attribute, contrast alone does not provide useful picture information - only the various levels of gray does. It is the grayscale performance of an imaging device - its ability to represent various levels of luminance levels - that is the single most important attribute to consider. Larger number of gray scales or shades of gradation implies a device greater ability to display subtle detail in dark or bright scenes, while creating a wide color palette. Without shades of gray, contrast alone is of no use in delivering image information.
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