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Understanding Black Level

The black reproduction of a monitor depends on many factors including:

External Reflections and Glare

The face of a monitor will reflect some amount of the light that strikes it. Some display technologies are better than others at reducing the amount of reflected light.

In a CRT, the phosphors themselves will reflect much of the light striking them. To reduce this, display manufacturers add light-absorbing glass in front of the phosphors. Light emitted from the phosphors will be absorbed once as it passes through this glass. Ambient light from the room will be absorbed twice as it passes through the glass, bounces off the phosphors, and passes through the glass again on the way out. By doing this, the ratio of wanted light (from the display) to unwanted light (from the room) goes up. This reduces the effect of glare on the monitor.

In a LCD, polarizing filters are required for the operation of the LCD and naturally act as light absorbers. Neutral density filters can also be added to increase the amount of glare reduction.

Unwanted light is also reflected at the interface between air and the foremost layer of the display. Surface coatings can be applied to reduce the amount of light reflected here, though most broadcast CRTs do not employ these technologies.

In any display, the surface of the display will still reflect a small percentage of the light striking the display. The smoothness of the display surface will determine what type of reflection occurs. A glossy surface will reflect light much like a mirror, resulting in mostly specular reflections. This allows the display to be rotated and moved to angle away objectionable light sources. With a matte surface, incident light will be reflected in all directions so this is not possible. If there is light hitting the display surface, there will be a small amount of glare in all directions that will raise the black level. Some people prefer matte displays as glossy displays can display discernible reflections of objects in the screen (e.g. overhead lighting, lightbulbs, etc.).

Some glossy display surfaces are intentionally created to be slightly rough and not perfectly smooth so that specular reflections will be directed at slightly different angles. Visually, this will blur out the reflections so that they are less objectionable. Many LCDs implement this type of surface.

In general, LCD broadcast displays are much less susceptible to glare than broadcast CRTs. In a brightly lit environment such as a trade show floor, LCDs will easily have better black levels than CRTs. Ideal lighting conditions are required for CRTs to achieve their best performance. Lighting in a color correction environment should be setup so that the area behind the monitor is lit and that the surround lighting does not land on the face of the monitor.

Further reading:

3M has a good Flash presentation on LCD surface coatings.

Inherent Black Level of the Monitor

Sequential contrast ratio is the ratio between the brightest white and the darkest black when displaying a full field of white and black. CRTs are capable of sequential contrast of well over 15,000:1, which is essentially overkill. Glare on the monitor and other effects will effectively limit the effective contrast ratio to a much lower number. LCDs on the other hand have historically had problems with achieving good contrast ratios. Current LCDs have contrast ratios of around 1,000:1. While they have improved greatly, they still do not produce perfect blacks and will have a 'glow in the dark' appearance in a dark room.

*Some consumer display manufacturers cheat their sequential control ratio measurements by adjusting the backlight intensity in their measurements. This is not a legitimate or useful way of measuring contrast ratio. Fortunately, manufacturers of professional broadcast monitors do not do this. At worse, they do not make this spec easy to find if their display has relatively poor contrast ratio compared to competing products.

When comparing contrast ratios, it may be useful to compare the difference between ratios in stops of light. This may help give the right perspective as a 1000:1 panel usually does not appear twice as good as a 500:1 panel. Similarly, a 15,000:1 panel does not appear 30 times better than the 500:1 panel. These ratios in stops would be ~10 stops for the 1000:1 panel, ~9 stops for the 500:1 panel, and ~14 stops for the 15,000:1 panel.

Flare and 'Halos'

In a CRT, black level is highly dependent on picture content. If the scene is bright, the CRT has a tendency to flare. When the electrons in the CRT hit a phosphor, light will be emitted in all directions. Some light will bounce back into the tube and be reflected out. The inside of the CRT is coated with light-absorbing material to minimize this scattering. Other emitted light will strike surrounding phosphors and then exit the monitor. And yet other light will be reflected off the glass-air interface of the monitor, strike other phosphors on the screen, and then exit the monitor. All these sources contribute to significant light scattering and limit the blacks that can be achieved with a CRT.

One measurement of flare is simultaneous contrast ratio. Although there are many different ways of measuring simultaneous contrast, the basic idea is to measure black when there is non-black content being displayed. One set of measurements show that a Sony PVM CRT achieves a simultaneous contrast ratio of a measly 75:1 compared to a consumer LCD with a simultaneous contrast ratio of 577:1. These measurements were done with a 9x9 checkboard pattern. This test pattern is somewhat unfair towards the CRT as the average luminance of the pattern is significantly higher than real-world images. Nonetheless, this worst-case scenario shows that the CRT is highly susceptible to flare.

In my opinion, flare is the dominant factor behind black level in CRTs. On typical scenes, LCDs and CRTs pretty much have the same contrast ratio. It is only on dark scenes where CRTs pull ahead. For color grading purposes, it is certainly important for the display to perform well in all situations including dark scenes. When it comes to measuring blacks, I believe it is essential to measure both sequential contrast and simultaneous contrast. There are some other display technologies (eCinema DPX, field emission) with sequential contrast comparable to the CRT, yet the blacks are better than the CRT as these technologies do not flare as much as the CRT.

Further Reading: Display Technology Shoot-Out - see the section on "Contrast Measurements".

Surround Effects

Our perception of blacks is also highly dependent on the content of the surround. A dark surround can make an image (e.g. the blacks on a LCD) appear brighter. With LCD displays, a dark surround will cause the display to have a "glow in the dark" look. In a brightly lit environment, the blacks on a LCD look perceptually blacker even though the black level is the same or marginally worse due to glare.

Note that surround effects are very weak (or non-existent?) if the display is showing a real world image. It seems that the visual cues inside the image will override the effect of the surround.

When evaluating broadcast monitors, watch out for surround effects as they can skew your judgement of blacks. Also watch out for the use of neutral density filters. They will lower the overall luminance of the display, effectively making the surround brighter in comparison to the display. This gives the illusion of darker blacks.

Further Reading: Color Correction Blog - Surround Effects.

Black Frame Insertion

Some LCDs implement a technique where the image displayed alternates between actual picture and a black frame. In this process, the backlight stays on for the entire duration. To turn the backlight off during the black frames would introduce flicker into the display and this would be too objectionable.

The use of black frame insertion (and black line insertion) typically halves the contrast ratio of a LCD. The amount of backlight leaking through the panel remains the same since the backlight is always on, while the luminance of the actual picture is halved due to the insertion of black frames.

A variation on black frame insertion involves arranging the backlighting in rows and to scan a black bar from top to bottom. This does not have the same effect on black level as black frame insertion. (I do not know the exact effect on black level as I have not seen information on this anywhere.)

Contrast ratio specs are typically for the display without black frame insertion enabled. However, the manufacturer may advertise black frame insertion as a feature while providing this higher contrast ratio figure.

For further reading, see Contemporary LCD Monitor Parameters.

Bottom Line

Between typical broadcast LCDs and CRTs, the CRT still produces better blacks in ideal lighting conditions. Blacks are roughly the same when displaying scenes that aren't dark and in lighting environments where there is some glare on the monitor. Comparisons between displays can be manipulated to change the impression of black level by controlling picture content, using ND filters, and by changing the lighting setup.

One exception to LCDs is the eCinema DPX, which has a (sequential) contrast ratio of over 15,000:1. I've seen it for myself and it does produce true black (!). It does not vary the backlight dynamically (which doesn't work) or employ other cheats to fake black. It also does not exhibit the same amount of flare as a CRT. It produces the best blacks of any display I've seen and carries a hefty price tag (in the ballpark of $38K).

Future technologies

Some display technologies on the horizon hold some promise in producing CRT-like blacks or better. I saw field emission display prototypes at NAB2008 and was impressed by the image as the black reproduction was better overall compared to a broadcast CRT (though the sequential contrast on a CRT looked better; I'm not sure why). Field emission technologies (a company that will produce field emission panels) has stated that it will buy a Pioneer PDP plant for $200 million to produce their panels. According to this Reuters article, the plant will be used for production of field emission displays and FED TVs starting with a 26" model.

On the LCD side, consumer displays are now employing modulated LED backlighting (also known as local dimming) to 'enhance' black level. Martin Euredjian of Ecinema Systems (a manufacturer of broadcast LCD displays) claims that they have experimented with this technology before and found it unsuitable for critical monitoring. He has posted an article on why modulated LED HDR displays won't work.

This site is maintained and run by Glenn Chan.