Tips for the user. TVs. Measuring contrast in black and white images

Translation from CNET

Do you want to know why this important parameter Images like contrast grows endlessly and remains incomprehensible to many?

Understanding what contrast is and how to evaluate it will help you choose best tv, based on your capabilities. But it's a little more complicated than it might seem.

Essentially, contrast is the difference between the brightest and darkest images the TV can produce. More precise formula:

white level / black level = contrast.

If a TV can produce 45 foot-lamberts of brightness with a white screen, but only 0.01 foot-lamberts with a black screen, it has a contrast ratio of 4,500:1. Unfortunately, in reality everything becomes more complicated.

There are several ways to measure contrast. For example, a manufacturer can measure the maximum light output of one pixel on a certain pixel that is not available in normal mode, maximum, then measure the light output from the same pixel in the complete absence of a signal. This situation is hardly possible during normal viewing of films and television programs, but without a generally accepted standard for measurement, such trifles do not concern TV manufacturers.

Moreover, the contrast ratios in lately have grown to such extreme values ​​that sometimes there is literally no way to measure them. Why did this situation arise? The marketing department states the numerical values ​​with which it can sell the product. In the end, specialist developers have to cheat and, lo and behold, the TV gets the necessary contrast. The only way to find out the real capabilities of TVs is to read reviews, but even they are not always accurate, as we will see later.

Contrast ratio: good and bad

Since you are reading this article from a display that has its own contrast ratio, it is not possible to give you real examples, what good and bad contrast looks like, so you'll have to look for other ways.

You can check how well your monitor is configured by reading special articles. And below are two images, on the left with good contrast, on the right with poor contrast.

The left image is more accurate, with good contrast. On the right, the contrast is worse, the black level is higher.

It's quite easy to notice that the picture on the left is more correct. The image on the right has more high level black, and if you choose from two TVs placed next to each other, the choice is clear.

Contrast: natural and dynamic

There are two types of contrast. Most often they are called natural (native) and dynamic. Natural contrast is the one that display technology allows you to present without tricks. For an LCD display, this capability is determined by the liquid crystal panel. In the case of DLP technology, everything is determined by one or three DMD chips.

Imagine the above images on your TV screen. Natural contrast is determined by comparing the darkest part of an image with the brightest elements in the same scene. Let's call this “internal contrast of the plot,” although someone may have better definitions in this regard?

This contrast is different from that which is attributed to most TVs today and which is called dynamic. Dynamic contrast is an extended term to describe a technology that allows for infinitely higher readings than natural contrast. When playing a movie or TV program, the TV adjusts the overall light output according to the nature of the scene being displayed. You may have manually adjusted the backlight of the LCD display, but the TV does the same automatically, analyzing the video being played in real time.

This gray scale is an example of the relative brightness of a display. With the backlight set to maximum, the LCD display is at its brightest, but has bad level black color. If the backlight level is set to minimum, there will be a decent black level, but the overall light output will become insufficient.

Automatic backlight adjustment (similar to the projector's aperture adjustment) is performed by a video signal control circuit and allows real-time adjustment of the overall luminous flux depending on what is on the screen at that time. Example image using dynamic contrast:

In a dark scene, the TV dims the backlight (or covers the projector's aperture), so the image becomes darker. At the same time, bright areas on the screen are lost, which also darken.

In a bright scene, the TV increases the overall light output, but as you can see from the gray scale, the increased brightness comes at the cost of losing decent black levels.

Light scenes become brighter, and dark ones become blacker. This is very good, and does increase the visible contrast of the display, but not as much as the stated contrast parameters would suggest. A TV with a contrast ratio of 5,000,000:1 is fantastic. I would really like to see it, it’s a pity that it doesn’t exist. An HDTV with a high dynamic contrast ratio may look better than a TV that doesn't, but it won't perform as well as a display with a high natural contrast ratio.

Yes, the LED backlighting of the LCD panel can be turned off, creating true black color, but this is not always possible during the playback of a real movie. The display, with its high natural contrast, will show bright white text against a deep black background. A display with high dynamic contrast can have the same dark background, but the text will not be as bright.

Comparatively, it looks like this:

The image on the left simulates a display with high natural contrast. The one on the right has reduced natural contrast, but enhanced dynamic contrast. The right image can represent low level black, but does this by reducing the overall luminous flux. The high natural contrast of the display (left) allows you to reproduce maximum blacks and, at the same time, bright whites without technical tricks.

As you can see, a display with high natural contrast is what you need. The glare of street lamps is clearly visible against the inky black sky. During the day the sky is bright, but the black jacket is quite dark. It is more like a picture on CRT TV screens, more like a movie in a cinema, more consistent with a real image.

Today, the technology with the highest natural contrast is used in LCOS home projectors. Currently, JVC projectors use a proprietary version of this technology (D-ILA). These projectors have the highest measured natural contrast ratio. The version from Sony (SXRD) is quite behind, but takes second place. Plasma HDTVs can be considered third in terms of natural contrast, although some DLP projectors are close to them.

LCD TVs have made great progress over the past decade, but still lag behind other technologies in a number of ways. Fortunately, best manufacturers know this and have come up with several ways to imitate the high natural contrast characteristic of other technologies.

The best way for LCD TV How to achieve high contrast within a scene is to use local dimming. In this case, the display backlight is an array of light-emitting diodes (LED sources), the brightness of each of them can change depending on what is shown on the screen. Although this is not done at the pixel level, and the LEDs are not controlled individually but by zones, the overall effect is generally very good.

Unfortunately, today most manufacturers have abandoned the use of full LED backlight, which is the only type of good local dimming. Such TVs are more expensive to produce.

Most LCD LED displays today use edge lighting, in which backlight LEDs are placed on the top and bottom of the LCD panel. Several companies have developed methods for darkening specific areas of the screen and for such backlighting ( Edge LED), but the effect is not as good as using a full LED array behind the screen. Although many edge-lit TVs look great.

Measurement and all other problems

However, you naturally have long had a question: How can you find out which TV in the store has the best contrast? Good question. However, increased lighting brightness distorts the real capabilities of the TV. In addition, some models may have glossy finish screen, while others are anti-glare, which also makes comparison difficult. As already mentioned, all manufacturers give inflated contrast ratios, which have little to do with the real capabilities of TVs. Therefore, you should not put much faith in the specifications.

So all that remains is to read the reviews. Unfortunately, only a few sites take contrast measurements during testing. And the obtained indicators can sometimes vary greatly, which is understandable; moreover, there is no single standard for measuring contrast. Someone might rate the contrast ratio as 20,000:1, while another tester might only get 1,000:1.

Much depends on what is being measured. You can take a black field (0 IRE brightness) from a tuning disk or signal generator, and then a white field (100 IRE) from the same sources. In this case, we get a decent overall contrast, but it is not very relevant when real viewing video (which is never completely dark or completely white). In addition, numerous video processing systems begin to work on real material, which affect the brightness of individual areas of the image.

A good addition would be to measure contrast using the ANSI method. In this case, eight black and white fields are displayed on the screen in a checkerboard pattern. All measurements are averaged. This gives a good idea of ​​the display's capabilities, and is much more relevant to actual video. A particular problem is that the brightness of the white boxes can affect the measurements of the black squares. Thus, to do everything correctly you will have to spend a lot of time.

Conclusion

IN in this case there is no clear answer. This is exactly the conclusion that can be drawn from everything written above. The best you can hope for today in order to get a general idea of ​​the HDTV models you like is fairly accurate measurements given in TV reviews from competent sites. And the knowledge gleaned from this article will also be very useful in order to imagine what the potential capabilities of the TV are. in the best possible way will reveal itself under viewing conditions in your home.

As with many TV selection guides, we can say the following. Much depends on the conditions in which you plan to watch TV. If you are a movie buff and watch TV in a dark room or at night, increased contrast plasma will look very cinematic.

If you watch TV more often during the daytime, nothing can compare with LED LCD in terms of brightness. Somewhere between them are LCD TVs with local or zone dimming LED backlighting. They can present better "internal plot contrast" than conventional LCD displays, but with a sufficient margin in terms of light output.

No matter what kind of TV you have in your home, it is very important to set it up correctly, as the initial factory settings may not fully unlock the TV's full potential.

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Do you want to adjust the brightness screen ?

Windows 10: Click the Start button, select Settings, and then select System. In the Brightness and Color section, set the Change Brightness slider to adjust the brightness. More information see the article changing screen brightness

Windows 8: Click Windows keys+ C. Select Settings, and then Change PC settings. Select Computer and Devices > Display. Turn on automatic setting screen brightness. For more information, see Adjust brightness and contrast

Learn more about adjusting pictures in Office applications.

This video demonstrates several ways to customize your design.

(While the video is playing, you can click the resize arrow in the lower right corner of the frame to make the image larger.)

Duration: 1:35

Note: even if you have made corrections. This feature is not available in Word or Excel.

Is color scheme Office too bright? Do you need more contrast in Office applications? For more information, see Change the Office theme (office 2016 and 2013).

You can change the brightness, contrast, or sharpness of an image using the correction tools.

Clockwise from top left: original pattern, pattern with increased smoothness, increased contrast, and increased brightness.

Tip: If you use PowerPoint, you can still keep the original version of the picture even if you change the brightness of the picture. This feature is not available in Word or Excel.

When it comes to measuring certain image parameters, an unpleasant subtlety immediately arises. Humans and computers perceive images differently. A person isolates objects from noise, can see something in low light, and a computer understands a picture as a set of coordinates with corresponding brightnesses. And when a person and a computer are asked about some distinctive features of an image, they will immediately disagree. We need to somehow make sure that the conclusions they draw are similar.
Let's look at the methods that are used to analyze contrast in black and white images, and we’ll try to choose something more or less objective.

Contrast is determined by the ratio of the difference in brightness of observation object 1 and background 2 to one of these brightnesses. The range of displayed values ​​is from 0 to 1.
In fact, it shows nothing. Let's move on.

There is one subtlety with calculating contrast. There are two objects nearby, one has brightness 10, the other 20, according to the first and third methods we get 0.5, in the second - 0.3. Brightnesses are 100 and 200, using the first and third methods we get the same 0.5, using the second - again 0.3, but with brightnesses of 10 and 20 you may not see the difference.

Now let's see this in action:

There are three images in a row - normal, with an equalized histogram, and ideal. The selected areas were analyzed in .bmp format, brightness range 0 - 255.

The contrast of a normal image is K = 0.67.
- contrast of the equalized image K = 0.88.
- contrast of an ideal image K = 1.

This is the story, thank you for your attention!

One of the most important characteristics of a TV when choosing is the contrast value of the image on the TV screen. If you choose a TV based on picture quality, be sure to pay attention to the contrast value of different models.

By definition, contrast is equal to the ratio of brightness in the most bright point screen to the brightness of the point where the darkest image is. In other words, we divide the white level by the black level and get contrast. Only the values ​​of these levels can only be obtained through a special test of the TV using specialized instruments. That's why to a simple user you have to trust either the manufacturers or various reviews on sites where TVs are tested. Who to trust more and how to check the contrast, and we’ll talk further.

We said that the contrast is one of the most important characteristics TV. Therefore, manufacturers try to maximize this value to improve sales. The manufacturer can measure the brightness of a pixel in the laboratory when applying a signal that has never been real conditions not used. Then measure the brightness of this pixel in the absence of a signal, which is impossible during normal viewing. After this, the contrast value is calculated. And the values ​​measured under such conditions are included in the product passport. Because of this, we see today that the contrast values ​​of many TVs are simply off the charts. All this is possible because there are no mandatory rules in the world for measuring the contrast of displays.

Separate static (natural) and dynamic contrast. Natural contrast depends only on the capabilities of the display, while dynamic contrast is obtained as a result of the use of additional technologies.

Static contrast is measured by the brightness of points in one scene (brightest and darkest). When measuring dynamic contrast, technologies are used to increase it. The TV itself, when playing a video, adjusts the contrast depending on the scene, which at the moment shown on the screen. That is, the backlight in the LCD matrix is ​​adjusted. When showing a bright scene, the luminous flux from the backlight increases. And when the scene changes to dark (night, dark room, etc.), then the backlight begins to reduce its luminous flux. It turns out that on bright scenes, due to the increase in light from the backlight, the black level is poor, and on dark scenes, the black level is good, but the luminous flux will decrease. This is hard for us to notice because in bright scenes even backlit black appears completely black. And in dark scenes, the brightness of light objects seems sufficient. This is a feature of human vision.

This backlight control scheme increases contrast, although not as much as the manufacturers claim. And indeed, many TVs with dynamic contrast have superior image quality to devices that do not have such an adjustment scheme.

But still, displays with high natural contrast will be valued higher. This can be demonstrated by displaying a picture of white text on a black background. On a screen with high static contrast, the text will indeed be white and the background will be black. But a display with high dynamic contrast, if it shows a black background, then the letters will already be gray. Therefore, when playing regular video on a screen with increased natural contrast, the picture will be as close as possible to the real image. For example, there will be bright street lights against the evening sky. And against the backdrop of daylight bright sky a black car will really be black. This is the image we see in cinemas.

The image was as real as possible, in terms of contrast, on the screens CRT TVs. But with the advent of the HDTV era, these television receivers gave up their place in the market to other devices. Today, high natural contrast values ​​are achieved using LCOS home projectors. The first place among these devices is occupied by JVC devices with their version of D-ILA. Next we can mention Sony with SXRD technology. In third place you can already put plasma TVs.

LCD TV manufacturers have introduced several technologies in recent years to achieve the level of contrast that is possible in other models. The best results in increasing contrast are achieved by using LED backlighting with local dimming. In this case, it is impossible to adjust the backlight of each pixel and each LED is not controlled individually, but the result is still good. But manufacturers have abandoned the most effective type of backlight, when LEDs are located over the entire screen area. Such production turned out to be expensive. Today, so-called side lighting is mainly used. Here the LEDs are located at the top and bottom. Local dimming schemes have also been developed for side lighting. TVs with such backlighting show enough good results by contrast value.

When choosing a TV in a store, it is difficult to evaluate the quality of the display contrast. External bright lighting interferes; screens can have different coatings: anti-glare or glossy. The passport does not always contain the true contrast value, because manufacturers measure it in laboratories and by applying special signals to the screen. Even after reading several reviews on the Internet, it is not always clear what the real contrast value is. After all, everyone measures it in their own way.

There are several techniques for measuring contrast. First, a black field is fed to the input and the brightness is measured, and then a white field is applied and the brightness is measured. The result is good contrast, but during actual viewing there will never be a completely white or completely black picture. At the same time, when a regular video signal is shown on the TV, video processing is turned on, which also makes its own changes. More accurate readings are given by the ANSI test, when a chessboard with white and black fields is shown on the screen. This is more consistent with the normal image. But in this case, the white fields will affect the measurement of the brightness value of the black fields. So one correct method There is no contrast measurement.

So the recommendations for choosing a TV with good contrast remain the same. If you'll mostly be watching movies in a darkened room, then plasma is your best bet. In a well-lit room, an LCD TV with LED backlight due to its high brightness. Between these models you can put an LCD TV if there is a reserve in light output. And you need to remember the main thing, any TV needs correct setting. Adjust the brightness and contrast of the device correctly to obtain the highest quality image.

Additionally:

Everyone knows very well that cameras are not perfect and do not always accurately match the color (light) in the photo. It happens that the flash does not have time to charge and we see an almost black square of Malevich, sometimes it works too hard and we see white square by an unknown artist with red dots in the middle (the eyes of a hamster), and it happens that we try not to depend on the flash, we try to shoot without it, but the photo turns out to be a yellowish-brown cloudy shade. All this can be easily cured using Photoshop(of course, within reasonable limits! Of course, a completely black or completely overexposed frame cannot be restored).

Let's start with the non-brief ones first and then brief definitions to understand what we are changing.

What dictionaries tell us on this topic:

Brightness is the light characteristic of bodies. The ratio of the intensity of light emitted by a surface to the area of ​​its projection on a plane perpendicular to the axis of observation.

Contrast - distinguishability of the object of observation from the surrounding background (monochromatic radiation); color contrast is a type of optical contrast associated with the difference in color shades.

Saturation - in physical terms, color saturation is determined by the nature of the distribution of radiation in the visible light spectrum.

Hmm... Indigestible terms... I’ll try to formulate it more simply and related to this topic:

Brightness - quantity white in your photo. The higher you set the brightness, the brighter the frame becomes.

Contrast is the difference between different colors located next to each other. The higher the contrast, the more sharply we see the transition from one color to another (sometimes contrast works as sharpening).

Saturation - how rich and bright a particular color looks. You can enlarge it within immodest limits - then the photo even begins to “hurt” the eye.

You can, of course, describe each of these characteristics point by point, but this would be wrong. It is correct to change all three frame settings in a comprehensive manner. How? Now let's figure it out...

Let’s take this dark, low-contrast photo for consideration...

These commands are hidden in the “Image” menu, then “Correction”, then “Brightness / Contrast” and “Hue / Saturation”:

And

When we press the “Brightness / Contrast” button we see a window like this:

When choosing “Background Color / Saturation” this is what it looks like:

First, open “Brightness / Contrast” and calmly and peacefully move both sliders to the right to the required brightness and contrast values ​​(all this is done in a purely intuitive way and in each case in its own way!). You should not always set exactly the same values ​​as in this frame:

It seemed to me that first you need to set the brightness to +120 and the contrast to +30. But everyone notices that the colors are too bright and unnatural. It’s good that we know where the “Background Color / Saturation” menu is located, which will help us fix this:

It seems to me that if you reset the saturation value by 13 points it turns out quite well. That seems to be all, but I always play it safe and go into “Brightness / Contrast” one more time, checking if something else needs to be changed to achieve the most beautiful result and realistic photo.