The contrast ratio

In the field of displays, especially where video projectors are concerned, there is one figure that we users tend to value very highly: the contrast ratio. With the clear aim of achieving the best possible picture reproduction, the ratio between the whitest white and the deepest black often tips the balance toward one model or another. It is still, for example, the Trojan horse of LCD devices, the saving grace of DLPs and the reason for the continued success of the almost forgotten CRTs. Is it really that important?

The truth is that it is, since it is a value that gives us a great deal of useful information, more than we might at first suppose. So much so that many manufacturers have realized this and are capable of disguising measurements and data simply in the hope that users will be impressed by the advertised figure, choose their product and head to the checkout. Once the equipment is installed, they realize their mistake: the real contrast ratio, the one achieved with the equipment properly set up, is well below what was advertised. Not good.

In the following lines we will talk a little about the contrast ratio and the importance of this value in today's display systems.

We should all know by now that our ears do not have a linear "listening response". For example, at low volume levels, we find it very difficult to hear frequencies in the region of 150-200 Hz, while a 1 kHz tone at the same volume level may be perfectly identifiable. The same happens with our eyes. What in audio we hear from lowest to highest (frequency), in video are the colors. What in audio means more or less volume, in video is brightness. Despite this, the aim of a video image is the same as that of an audio sequence: to reproduce reality as faithfully as possible in a "synthetic" environment. Most of the time, a video camera (or the telecine transfer of a film negative) captures the light of the image, converts it into an electrical or digital signal, which is processed, stored and transported. Then this objective data travels the reverse path until it is finally displayed. Throughout this process there are many correction stages specific to each of the technologies involved. For example, the CCDs (Charge-Coupled Devices) in video cameras are not able to "capture" all colors and intensities with absolute perfection either. The same happens with display systems. We have seen it, even today, with LCD televisions: they often deliver a red that differs noticeably from the real red. Both when capturing images and during the final processing of the image for display, a series of color corrections are applied to achieve the theoretical ideal that should reproduce any image perfectly.

Since each technology (LCD, CRT, DLP...) has its own different "correction pattern" (gamma), this seemingly simple process has a certain complexity. Moreover, we come from an era dominated exclusively by CRTs, so for some time the goal has been for modern display systems to converge visually toward the standard to which CRTs accustomed us. How many times have we wanted to compare our plasma or LCD TV or our projector with the colors of the old CRT television? It is curious to note that the CRT is one of the devices that requires some of the most radical color correction. But what is the relationship between the rendering of colors and contrast? The colors we see on a screen are nothing more than colored "gray dots". In fact, the leap to color television at the time was based on this premise: a black-and-white image was still "drawn", while information about the color of each of the pixels displayed was provided at the same time. If we ignore color for a moment, we will understand that what matters now is achieving an excellent grayscale. But we have already seen that, very often, even if a good black and a good white are achieved, it will be difficult for us to be 100% sure that the 50% gray is actually mid-gray. Perhaps it is above (or below) the ideal. It all depends on the gamma.



BLACK AND WHITE



The contrast ratio measures the ability to reproduce the range between the highest and lowest video levels, that is, between white and black. These two concepts may seem easy to understand (black and white) but sometimes we can be in for a surprise. For example, even though we know that black is the absolute absence of brightness and that, therefore, on a hypothetical scale defined in volts, it should only be the signal that has no voltage (0 V), in many video standards this voltage would lie between 0 and 10 V. Even small amounts of voltage are still black.

The same sometimes happens with whites. Look at the following image: is the central dot white?

As with black, current video standards do not define white solely as the point at which the voltage level is at its maximum. They still allow a "margin" which, in the example we have given, would not be at 100%, but around 96-98%. At 100% we would have peak white. If in the previous image I replace the black background with a "pure" white one, we get the following:

Comparison is always necessary to see the differences between different displays, especially bearing in mind that in the world of cinema much of the detail lies precisely in the dark areas of the image. The background of these pages is a slight gradient from very pale gray to white which, I am convinced, many would not notice if they did not have as a reference the white box preceding these lines. If a display is unable to distinguish between the gray background of this article and the maximum white we can show in that box, it means that all the information lying between those two values will not be reproduced (for example, all the detail in a cloudy sky).



MEASURING CONTRAST



We are used to reading contrast ratios in the order of 5,000:1, 10,000:1, etc. What does this mean? How is it achieved? These are the measurements manufacturers choose, since they give the "highest" values. It is more impressive to say that my television has a contrast ratio of 5,000:1 than to say that it is 200:1 ANSI. The first value is obtained using the On/Off technique. This measurement is made in a completely dark environment (the walls are black, so they do not reflect any light) and with no ambient light. A white image is projected and the result is measured. Then the same is done with black. The contrast ratio is precisely the ratio between the white and the black.

The most demanding video enthusiasts prefer to know the ANSI contrast ratio of their devices, mainly because it is a more realistic and practical measurement. This method uses a video pattern similar to a chessboard; in other words, the white and black areas are measured in the same image. The difference from the On/Off system is that ANSI takes into account the effect of reflected light when measuring black. This method, which takes into account the ambient conditions of any room, shows that sometimes, by lowering the brightness of the whites, the real contrast ratio increases. A curious paradox. This is because, by reducing the brightness of the white, we prevent the black from being affected by the residual light emitted by the white, increasing the contrast ratio in practice. In other words, the contrast ratio depends not only on the device itself, but on the light pollution in the room and, in the case of projectors, on the type of screen used. With very bright projectors, for example, it is often advisable to use screens with negative gain or gray screens: the results can improve considerably. Conventional televisions, especially bearing in mind that they are rarely used in the dark, also depend on the amount of ambient light they receive, so even the type of protective screen they use becomes important (whether it has a greater or lesser mirror effect, whether using a polarizing filter shifts ambient light toward blue, etc.).

The human eye is capable of resolving contrast ratios infinitely higher than those currently offered by the latest projectors. Some calculations speak of ratios of 1,000,000,000:1 or more, although there are nuances. Although it is true that our eye is capable of resolving the range between a blinding white and the darkest black (which would give a contrast ratio like the one mentioned), it is also true that day-to-day life points to something else. We are able, for example, to enjoy the snow on a sunny day at 12 noon, but we would not think of reading a newspaper in those conditions. Meteorologists define "night" as the point at which it is difficult (or impossible) to read a newspaper outdoors without artificial light. What is the human "working" contrast ratio? Much lower than the extreme value mentioned above.

Human daily life, moreover, involves slow changes in lighting; we manage to adapt to any change, although slowly. As the sun rises higher in the sky (on a clear day), the human eye progressively adapts to the changes in light; and conversely, toward total darkness on a moonless night. But it does not react quickly and effectively to sudden changes in light (a flash, or leaving a dark room for a brightly lit space); a certain adaptation time is needed and, in an emergency, our eyelids act "automatically". A movie can "compress" a person's day into just a few minutes, so sudden changes in lighting are a constant. Not to mention cinematic effects, explosions, etc. If we make abrupt changes between white and black with very high contrast ratios, rather than enjoying ourselves we will suffer pain and fatigue, so I doubt we will ever reach very high ratios. We don't need them. Some studies say that the human eye is capable of "seeing" an On/Off contrast ratio of 1,000:1 simultaneously without problems; in other words, any current projector or display achieves an optimal contrast ratio when changes are sudden and dynamic and we don't want to "tire" the eye. On the other hand, it is true that in certain movies, where the transition from white to black is gradual, we may demand a higher contrast ratio since, with no sudden changes, our eye can adapt more easily. In this case, we would be talking about a ratio higher than 500,000-700,000:1, an almost impossible goal today, both in displaying and in capturing such contrast.

From the above we can deduce that the contrast ratio required may vary depending on the type of content we are going to watch. We know that it is not a very critical value, for example, for someone who wants a conventional television to enjoy TV at home. In most cases they will be watching a degraded video signal due to the poor transmission quality we suffer in this country, where brightness is given priority over dynamics. They will watch television in a living room that is probably very brightly lit, so the black level will not matter much to them, but a high contrast ratio will.

Projector users, on the other hand, are also keen home cinema fans who love watching movies. Movies tend to work with low light levels (grays and blacks), so black matters, but if the contrast ratio is lower in exchange, that is no great problem either.

The secret is "balance", and within this frame of reference absolute black is no longer so important. Measurement is still necessary, but what matters, as always, is how things work in practice. Philips, for example, whose commitment to LCD is well known, has found a very effective solution to the "poor" black level of LCD panels in general. Its Ambilight technology is not just a system that tricks our eyes into making a 42" set seem "emotionally" larger; it also enables its LCDs to deliver a good black level. Since there is almost always "some light" (even with the room darkened) floating around the space, the "perceived" black, even if it is higher than ideal black, is dark enough to be convincing.

In practical terms, a high contrast ratio means a darker black, even when the black measurement does not indicate as much. The perception of black in this case derives from that of an even brighter white. Yes, it is true that in a shot containing only black the viewer will notice that it is not a deep black, but when there is some white in the scene (a face, a candle, "something" moderately bright), the perception of black will improve considerably.

When we set up displays, we realize that one of the most complicated adjustments is, curiously, brightness. It is responsible for the equipment's good performance in the blacks. Curiously, this control defines which voltage level will be the minimum, even for obtaining black. High brightness will turn black into gray; very low brightness will turn gray into black. The contrast setting, on the other hand, will be responsible for achieving a better (or worse) white, and the interaction between the two, brightness and contrast, is such that, as DVE insists, both controls need to be adjusted again and again in search of that perfect "balance". The goal is to achieve a good white and a good black. Both.