Home Theater Projectors and Electronic Displays
The Contrast Ratio Game

Contrast Ratio: The latest craze in the video industry is to play with contrast performance numbers.  6,000:1 for video projectors is becoming quite common. For LCD TVs, some manufactures are talking about dynamic contrast ratios of 10,000:1. And with plasma and LCD displays ...5,000:1 ...10,000:1? No, a more! 15,000:1 is the standard in the game. And these numbers are still growing.

Surely, we all love numbers - the bigger the better - BUT do these BIG contrast performance figures really mean something in the end?

The Latest Trend: Playing with Contrast Performance Figures 

Big numbers sell better, and manufactures know this very well. It applies to all areas in the manufacturing industry.

We have seen this number battle in the scanner and digital camera world when some manufactures got the insane idea of quoting the interpolated image resolution rather than the true optical resolution of the device. Surely a scanner with a 9600DPI or 19200DPI would seem better than one with 1200DPI  ...unless you know what you are really talking about.

The same applies to contrast performance. As we will soon see in this article, this is an even more complex situation where some manufactures are taking customers for a ride by quoting unrealistic high figures for contrast resulting from unspecified testing methodologies that aim more at inflating the end figure for contrast performance than any thing else.

How Black is your 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 black level is 300 times darker than the white.

---

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 of gray i.e. its gray-scale performance rather than out of a higher contrast ratio.

---

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:

• Full On/Off Contrast measures 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 measurement most favored by manufactures as it yields a larger number for the contrast ratio - typically 25% more - than ANSI Contrast measurements for the same display device. What's more, some manufactures of DLP projection displays and projectors, would carry out this contrast ratio measurement with the 'white segment' of the color wheel turned on. This would increases the measured figures for the white, 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.

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, nor are manufacturers 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.

Don't be impressed with BIG Contrast Numbers!

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'. But it 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.

Most home theater projectors are rated at between 1000 and 5000:1 contrast ratio, while some of the latest flat panel displays have a quoted contrast ratio of anything between 10,000 and 15,000:1. And a recent announcement by Samsung went even further ...a lot further. Samsung is about to release its high-end LCD HDTVs for 2007, the LN-T81F, with a claimed 100,000:1 contrast ratio rating.

Gray-scaling performance apart, 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 1500:1 is more capable of showing dark subject matter than one with a contrast ratio of 500:1. 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 3 times improvement in contrast performance - rather, the perceived difference in image performance between these two devices is marginal and detectable only if the 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 one is making use of 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.

Similarly, this applies also to direct-view systems such as plasmas and LCD TVs - most often, black is nothing more than dark gray!

With this much light in the room, there is no difference between 500:1 and 10000:1 Contrast Ratio!

Presence of light: To better understand the impact of the presence of light in a room on the contrast ratio performance of an imaging device, 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 500:1 and a 5000 or even a 10,000:1 contrast ratio!

Increase the level of light in the room to just 30 LUX - that's equivalent to a dimly lit room - and contrast ratio figures above 50:1  would turn out to be simply academic even in the case of video projectors with relatively high brightness rating (2000/2500 Lumens and above).

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.

Gray-Scaling: One cannot talk about displays without mentioning something on the gray-scales performance of a display or video projector. The truth is that while contrast is an important attribute, yet contrast alone does not provide useful picture information - only grayscales do.

It is the gray-scaling performance of an imaging device that is the single most important attribute to consider. A larger number of gray scales 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.

Your eyes are the limiting factor!

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 sun light.

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

---

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!

---

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.

It can be thought of gaps between adjacent image parts that vary in contrast. There is a point above which increasing the spatial frequency i.e. decreasing the gap between adjacent image parts, will reduce the eyes sensitivity to contrast even though the contrast level of the image remains the same.

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 effects the eyes' response to brightness and contrast perception.

So what's the whole discussion here?

Do not get carried away with big contrast numbers - the eye dynamic response is the limiting factor.

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 behave the same as one with a much lower rating.

Therefore, opt for a high contrast projector or display (typically 2000:1), only as long as you can view the image in a light controlled room; you need to view the image in a completely darkened room to be able to perceive the marginal difference between a 2000:1 and a 500:1 contrast ratio. Otherwise paying extra for a high contrast ratio projector or monitor would be waste of money since in moderate ambient light conditions such as a living room, anything higher than 500:1 would not be perceivable by the eye.

Conclusion: Numbers are there to impress and help selling 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'!

More info will follow soon.   Remember to subscribe to our free monthly e-zine to remain abreast with the latest additions to our website.  Click here for details.

 

Last updated on:

23rd August 2007

...back to Projection TV Section Index

 Copyright © - Practical-Home-Theater-Guide.com  All rights reserved.