Static or dynamic contrast?

How to get out of the jungle of exaggerating technical specifications

Reutlingen, (PresseBox) - Very often in technical specifications of displays like LCDs, DLP cubes or projection systems it can be recognized that some manufacturers show off very high values for contrast. But here we have to distinguish between static and dynamic contrast and what it really represents, how does this affect rear-projection cubes?

The "static contrast" describes the contrast ratio of the luminance of the brightest color (white) to that of the darkest color (black) at a fix display setting, i.e. without any modification of the parameters during the measurements but merely a switch of the video content from full white to complete black. Due to the characteristics of the technology, projection systems based on a single DLP chip come with a "static contrast" of 1,000:1 to 2,000:1.

In contrast to the static contrast ratio there is the "dynamic contrast". Due to its measuring methods it results in much higher values than the "static contrast" and is therefore preferred by manufacturer of consumer LCDs.

The measurement of the dynamic contrast is based on modification and individual settings for each colour, the brightest (white) and the darkest (black). I.e. for white all display parameters are set to maximum whilst they are reduced to minimum for black.

Significance for videowalls Transferred to LED-based systems, that means that you turn the LEDs to full peak for white and dim them down as much as possible for black.

The result is a very high contrast ratio that fits perfectly in any brochure. Manufactures of consumer LCD utilize this principle by applying some electronics that measures and defines the average brightness of each provides frame and adjust the brightness of the LED or backlight accordingly. This feature works pretty well if you simply watch an action movie from DVD (and if you don't have a too close look) but can lead to a "pumping" image because of the steadily changing brightness. But it is a totally different story when it comes to video wall applications where a picture is displayed over multiple screens.

It isn't hard to imagine what happens. Each image has brighter and darker areas. If you split this picture into portions because you're going to feed each into a separate displays (video wall), each portion has a different average brightness. Now, if each display adjusts the brightness accordingly to its individual average brightness level without taking the other LCDs/portions into consideration, the result is everything but homogenous.

For TV/Broadcast studios it's even worse because such adjustment means don't feature colour fidelity.

Infinity contrast If you switch the LED / backlight off for the measurement if black, you reach a contrast ratio of ?:1.

Brightness The measurement of luminous flux (luminous flux unit= ANSI LUMEN) doesn't exist for rear projection cubes but only for the integrated projectors. In general LED engines used in rear-projection cubes come with 600-800 ANSI Lumen.

The brightness of a rear projection cube is stated in luminance (luminance unit cd/m²). This value depends on the one hand of course on the luminous intensity of the used projectors but on the other hand also on the size and kind of the projection screen.

Given an equal luminous flux of the projector, the luminance of the cube decreases the more the size of the projections screen increases.

Furthermore a screen which provides good contrast and good viewing angles provide less luminance than a screen which was manufactured for having a brighter image of the same size, because the screen made to have a good viewing angle scatters the light in a wider angle.

Therefore for rear projection cubes the luminous flux of the projector doesn't mean much and is hence of no real relevance. The achieved luminance on the screen is of much more significance.

ANSI Contrast

There are vast numbers of different features that shall enhance the contrast ratio of a display/projector and hence many methods to describe this ratio.

E.g. some projectors have dynamic irises that open and close depending on the average light level in a given scene. Some have dynamically changing lumen output of the lamps. Some projectors have color wheels with white segments that can be turned off when black is displayed.

These operational differences mean that the contrast ratings vary widely and thus don't tell you anything about how one model will compare to another.

Beside the internal features of the projectors, there is another important influencing factor for the contrast ratio - the ambient light and this regardless whether you have a front or a rear projection.

Therefore, contrast ratios shall always be measured under ambient conditions that are similar to the conditions on-site. I.e. artificial/ arbitrary ambient conditions will result in unreliable values and hence won't match the situation that can be expected on-site.

Full-on/Full-off contrast Especially for multi channel projection, a dynamic contrast adjustment is rather disturbing than helpful because it results in an inhomogeneous image over the entire projection area due to the individual adjustment of each projector which is based on its individual content.

Therefore only the full-on / full-off contrast is really applicable for static adjustments of projectors.

The following setup is advisable:
- Switch off all dynamic adjustments at projector
- Simulate ambient conditions expected for operation on-site as detailed as possible (ambient light, lighting etc.)
- Measure brightness of a full white screen from a certain distance
- Do the same with a full black screen (keeping distance and measuring device)
- Divide the measured data

The resulting value defines the maximum static contrast for this projection system where a value between 1,000 and 2,000 is ideal.

ANSI Contrast Another way of describing the contrast is the ANSI contrast. The ANSI contrast defines the contrast within a displayed scene. The ANSI contrast reflects best the real contrast of a system in operation.

For the measurement of the ANSI contrast, a chessboard pattern with 16 equally sized areas (8 white and 8 black) is displayed. Then the brightness of each white and black area is measured. During the measurement all parameters remains unchanged.

The ANSI contrast describes the ratio of the average brightness of the white areas to the average brightness of the black areas. This value should be in the range from 300:1 to 500:1.

Like at the other contrast ratios, the ANSI contrast as well is strongly influenced by the ambient light conditions.

At the end the best way to see if a product is suitable for a certain application or to check if one or the other is better is not to read the technical specifications but to see it live or to see both systems side by side.

eyevis Gesellschaft für Projektions- und Großbildtechnik mbH

eyevis from Reutlingen in Germany is specialized in the development and production of high-quality large screen systems and guarantees a continuous further development in high-tech, quality and complete specific customer display solutions. Besides the rear projection units, based on the DLP(TM) Technology from Texas Instruments and controllers to manage multi-projection walls, eyevis is also developing intelligent software solutions to achieve a complete, unique and perfect large screen system. The name eyevis stands for quality, innovation, flexible and redundant systems and solutions in the field of large screen visualisation. For every application, whether control room or communications, conference or congress area, foyer, show room, information system, exhibition, convention, event or training centre, eyevis offers a solution tailored to your individual requirements with the best image quality. The product range also includes high-resolution TFT LCD monitors and special solutions for simulation and virtual reality applications.

eyevis' ambitious principle has always been the permanent further development of its product range which aspires to include the most modern technologies in perfect quality. The result of this "state-of-the-art" philosophy is the permanent development of products which constitute cutting-edge technology in their field of application. This makes eyevis one of very few manufacturers who are able to provide complete systems - high-end quality "Made in Germany ".

Thanks to its many years of experience and technological advantage eyevis is the ideal partner for the realisation of complete and customised solutions. Continuous investments in research and development guarantee a head start in the field of large screen technology.

eyevis has a worldwide network of offices, subsidiaries and Service & Sales partners that have trained engineers for consultancy, installation and service for eyevis systems in their country or region. Today, after over 15 years of experience and presence in the fields of large screen visualisation eyevis has become one of the leading producers of large screen technology. eyevis systems can be found in numerous control rooms and other applications all over the world.

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