The Contrast Ratio Game

Big numbers sell better, and manufactures know this very well. It is a favorite game - playing with numbers - and this applies to all areas in the manufacturing industry.

In the electronics world, we see this number battle everywhere - from the use of interpolated image resolutions instead of true optical values in image scanner and digital cameras, to the latest hot specs in HDTVs - 240Hz in LCDs and the 600Hz sub-field drive technology in plasma displays. It is as if manufactures do their best to come with the biggest number possible to deliver the message that they have the best product. But...

The truth is that unless you know what you are really talking about, you may easily end up misinterpreting facts.

The same applies to contrast ratio. It was Samsung that originally started boasting of unprecedented high contrast ratio ratings when it came out with the first flat-panel TVs with 4000:1 contrast rating. By 2007, figures for contrast ratio reached 15,000:1 and last year we have seen the first HDTVs with 50,000:1 for CCFL based LCDs and 1,000,000:1 for LED TVs and plasma HDTVs.

Now we are talking about multi-mega contrast ratios. It is as if there is no limit to how much these numbers can continue to grow. However...

This contrast ratio issue is more complex than one may ever realize. Apart from the fact that TV makers do hardly specific whether they are referring to static or dynamic contrast - some HDTV manufactures are taking consumers for a ride by quoting excessive high figures for contrast ratio using unspecified test methodologies.

Unfortunately, this number game has become quite common with some TV makers; these seem more interested at inflating the end figures to help sell their product over the competition than anything else.

Is your Black really Black?

What is Contrast: Defining and Measuring Contrast Ratio

Before continuing with our discussion on this subject, it is important to first have an understanding of what is contrast.

Contrast is the ratio between the white and black parts in an image. The larger the contrast ratio of a display device, the greater is the difference between the brightest whites and the darkest blacks a video display or projector can show. A contrast rating of say 300:1 would imply that the luminance for black level is 300 times darker than the white.

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 time.

Instead, dynamic contrast refers to the maximum ratio between the deepest blacks and the brightest whites that a display can show on its screen but not at the same time. A processor inside the display analyzes the picture content and adjusts the light levels on-the-fly to render deeper blacks and brighter whites. When done properly, dynamic contrast can help give the impression of higher contrast ratios, but done poorly, it can crush black to the point of losing shadow detail.

As expected, dynamic contrast ratios results in much larger numbers and is often the one most quoted by TV makers.

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Note: Unfortunately, some definitions found on the web for contrast ratio would imply that an increased contrast ratio yields better subtle detail and increases the color palette. This is NOT true. An imaging device ability to display subtle detail arises out of its ability to display various levels or intensities of gray - often referred to as shades of gradation - from complete black to complete white i.e. it depends on its grayscale capabilities rather than out of a higher contrast ratio.

And with grayscale there is also the issue of grayscale uniformity. This represents a device ability to maintain all shades of gray as close as possible the D65 (or 6,500K) standard across the entire range to preserve color fidelity at all luminance (brightness) levels. Many TVs fail here and instead tend to shift towards red in the darker areas and blue in the much lighter parts of an image.

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There are various measuring methodologies that can be used to determine the contrast performance of a display device. The two methods mostly used by the projection and video display industry are the Full On/Off, and the ANSI Contrast ratio measurement:

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 goes without saying that when comparing the contrast ratings of home theater projectors and other video display devices, it is essential to make sure 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.

The unfortunate thing here is that there is nothing mandatory as to the methodology used by manufactures to measure contrast performance of their projectors or display devices. Even worse, manufacturers are not obliged to at least 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.

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Don't be impressed with BIG Contrast Numbers!

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Making the Display: Contrast Ratio Facts  ...and numbers

There is no doubt that contrast is an important attribute in a display or video projector performance. It is contrast that helps make an image 'pop-up' in that it also affects our perception of color as well as image brightness. In fact, a minimum level of contrast is necessary for the eye to perceive an image as 'bright'.

The truth is that contrast cannot be taken on its own. A high contrast ratio 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 ratio implies a device has greater ability to display black as 'black' instead of dark gray. This also means that a home theater projector with a contrast ratio rating of 10,000:1 is more capable of showing dark subject matter than one with a 1,000:1 contrast ratio. At the same time, it is important to realize that the difference in performance would only be apparent provided the projection is taking place in a completely darkened room i.e. one in which is there no light other than that generated by the video projector.

Equally important here is the fact that the eye would not detect a 10 times improvement in contrast performance - rather, the perceived difference in image performance between these two devices is marginal and detectable only if a room is in total darkness.

The complete absence of stray light in a room is a critical issue when it comes to contrast performance. In everyday life, this is an extremely rare situation unless viewing is taking place under a controlled environment, such as that of a dedicated home theater with black painted walls.

There is a very simple reason for this: Black can only be as black as the projection screen surface is in the ambient light present in the room.  Hence, if the room is not pitch dark, the screen surface will reflect some light - thus turning black into dark gray, and therefore reducing image contrast ratio.

To a certain extent, this applies also to direct-view systems such as plasmas and LCD TVs, in which case black is often nothing more than very dark gray, even though plasma and LED TVs (using local dimming technology), can manage to approach true black better. Direct-view systems however have the advantage that their contrast performance is significantly less affected by the presences of light in the room - mainly thanks to the use optical coatings on the display screen surface that help block external light from entering the display panel.

Presence of light: To better understand the impact of the presence of light in a room on the contrast ratio performance of a projected image, it is sufficient to realize that with the light emitted by just one candle in a room - that's just one LUX - there would not be any difference between a 1000:1 and a 5000 or even a 10,000:1 contrast ratio!

Assume that instead of pitch darkness, you have say 30 LUX of light in a room. This corresponds to just less than the typical level of natural light you would get in a room during the day without direct sunlight.

With just 30 LUX, contrast ratio figures above 100:1 would turn out to be simply academic even in the case of video projectors with relatively high brightness ratings (2000/2500 Lumens and above). Why?

Just consider a 'low' 100:1 contrast ratio under 30 LUX of ambient light. Your projector would have to generate at least 3000 LUX for the brightest areas to maintain this minimal contrast. But to maintain this level of brightness over say a 100-inch diagonal 16:9 unity gain screen (30 square ft), you need a your projector with at least 8000 ANSI-Lumens output. In other words, your 2500 ANSI Lumens projector will never maintain its quoted 5000 or 10,000:1 contrast ratio even in a dimly lit room. (For more information on the issue of projector screens and video projector output, please refer to our article: Understanding projector screens.

It is therefore clear that unless you watch your moves in a completely darkened environment, it would be useless to stress too much and pay more for a device with a higher contrast ratio!

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 that 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: Interesting here is how professional digital cinema projectors perform. Present-day digital cinema projection equipment supports a typical contrast ratio of 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.

A point of clarification: We are not discussing image resolution. The reference to the performance of digital cinema projectors applies only to contrast ratio. When it comes to image resolution, traditional 35mm and in particular 70mm movie film systems, support a far greater image detail than that supported by professional digital cinema equipment today.

Grayscale: One cannot talk about displays without mentioning something on the grayscale performance of a display or video projector. As indicated earlier on, grayscale represents a device ability to represent various luminance levels. The truth is that while contrast is an important attribute, yet contrast alone does not provide useful picture information - only the various levels of gray does.

It is the grayscale performance of an imaging device that is the single most important attribute to consider. A larger number of gray scales or shades of gradation would imply a device greater ability to display subtle detail in dark or bright scenes. It is also this element that enables a home theater projector or a video display to create a wide color palette. Without shades of gray, contrast alone is of no use in delivering image detail.

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Your eyes are the limiting factor!

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The Human Eye: an issue on its own!

The eye is not an absolute sensor. Unlike electronic sensors used in digital imaging systems - which are all absolute in their response, the eye has a dynamic response to contrast - also referred to as dynamic range. This in itself alters the whole equation in that what is really important is not some unrealistically high absolute ratio rating, but rather, how the end image is perceived by the eye. Why?

The eye is an extremely complex sensor with far greater response range than any film or digital imaging device. It is able to function both in bright sunlight as well as in very faint light during the night - that's a range of over 10 million to one! In photographic terms, that's about 23 1/2 stops.

At the same time, it is not possible for the eye to see the faint light from a star in the sky during the day. From a photographic perspective, this is like saying that a camera can operate over an extended range, but then the full operating range is achieved by adjusting the film ISO speed, the camera aperture setting, and even the exposure time.

It is the same with the eye; at any given instant, the eye can possibly see over a range of 400 to 800:1 in contrast detection. Here, there is a whole debate about this with some saying this is 100:1, others say 1000:1, and some even mention 10,000:1. As soon as the eye moves (saccades), it re-adjusts its exposure both chemically and by adjusting the iris. Over time, it is possible for the eye to resolve a contrast ratio range of between 1,000,000 and 10,000,000:1, but...

The eyes' ability to detect contrast depends on the scene brightness, with the contrast sensitivity of the eye decreasing to about 8% of its maximum at low light levels; the eye sensitivity also decreases with lower contrast subjects.

To complicate matters, the eye Contrast Sensitivity Function (CSF) is not linear - in as much as it decreases with a decrease in brightness, there is a brightness level above which the eye contrast sensitivity falls once again. It is estimated that the sensitivity of the eye is some 600 times less in bright sunlight.

At low light levels, the eye is able to integrate image information over a period of some 15 seconds to be able to see dimly lit subjects (it is like when you increase the camera exposure time to record subject information during low light level conditions).

However, moving from one brightness range to another would take time for the eye to adapt.  A 'dark-adapted' eye would be able to see a faint star in the sky, but this dark adoption process by the eye takes 30 minutes or so to complete - provided you haven't been exposed to bright sunlight during the day, otherwise, it may take even up to one hour for full adaptation to take place. Looking at a bright subject by a dark adapted eye would simply kill your night vision

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This implies:  The first 30 minutes or so during a movie are not the best in terms of contrast sensitivity. What's more, looking at a bright movie scene would not help when the next dark scene comes around!

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And as if this is not enough, there are other factors as well that affect our contrast perception. For example, the eye would perceive a higher contrast when looking at static images than it would if it were to look at moving pictures even if these were to have similar contrast ratio to that of the static scene.

There is also the issue of the eye contrast sensitivity with spatial frequency.  Technically speaking, spatial frequency is the number of cycles subtended at the eye per degree of vision.

For an online demo explaining the variation of the eye contrast sensitivity with spatial resolution, check here: 'USD Internet Psychology Laboratory'.

And what about the loss of contrast sensitivity with age? The amount of light reaching the eye photoreceptors diminishes as we grow older; this in turn affects the eyes' response to brightness and contrast perception.

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So what's the whole discussion here?

Do not get carried away with big contrast numbers. The eye dynamic response is the limiting factor; at any one point in time, you would not be able to perceive a contrast ratio of more than at most 1,000:1 - and this applies irrespective of the display technology.

Similarly, important is the fact that the presence of even a minimal amount of ambient light would make a home theater projector with a high contrast ratio rating behaves the same as one with a much lower rating. Direct-view systems are less affected in this respect though their blacks would still suffer under a bright light environment.

Therefore, opt for a high contrast projector or display device only as long as you can view the image in a light controlled room. Despite what might seem to be a huge difference in absolute terms, you will have to view the image in a completely darkened room to be able to perceive the resultant minimal difference in picture performance between say a 100,000:1 and a 1,000:1 contrast ratio rated device. Paying extra just to enjoy a higher contrast ratio rating is just waste of money.

Conclusion: Numbers are there to impress and help the marketing people sell the product. But quoted peak contrast ratings do not really give you any information on the video projector or display device ability to render images with lifelike gray scales and colors. Rather, the only information that contrast ratio can deliver is how much brighter the 'whites' can be than the 'blacks'!