Strictly interpreted, denotes the light level at which a video signal representing picture black is reproduced on your TV screen. You probably already know that in terms of light output from a TV set black areas of the picture should be represented by an absence of light. Something that is black or below black in the video signal shouldn't produce any visible light from the display. VE T9-11. Some TV sets actually use Black Level as a control name. It is a far better description of the function of that control than the most commonly found name for it, which is Brightness. The multiple use of the term black level in the real world comes in the number of variables that affect the way you see light. Among them is the set's ability to produce an absence of light. How dark is it in the "off" state? VE T1 C5. Black level also becomes a rather large problem when implementing light valve technology. It is difficult to shut light completely off in a small area of the picture. Light output, from a standard direct view TV is obtained by lighting up phosphors. If you want to produce something representing black, just don't turn on the phosphors in that area of the picture. Simple enough, right? Well no, it isn't actually that easy. If ambient light is hitting the surface of the set, the level of light required for a dark area of the picture will have to compete with that ambient light. The appearance of the "off state" of the phosphors is of course dependent on ambient light. If you were around in the early days of black and white TV you may remember that TV sets had a medium cream color when the set was off. The only way you could get a good black out of a BW set was to keep all ambient light off the surface of the set. Black matrix picture tubes have been designed to assist with these problems in color TV's. Even black and white sets now have tinted glass fronts in order to reduce the effect of ambient light. This increases the apparent contrast of the picture. These changes in the surface condition of the set increase the apparent contrast when confronted with ambient light. Ambient light rejection is a great idea for most of us, but it comes at a price. If you can attenuate light hitting the set, you'll probably also attenuate light (picture) information coming from the set. The most obvious example of the price you pay comes in the many sets that put a dark piece of glass in front of the picture tube. This is essentially the same technique used in current BW sets except that the tinted glass in an integral part of the BW picture tube. You can't get rid of it. Picking on a set that we've measured, the piece of glass in front of the set attenuated ambient light by 60% as it passed through the glass on its way to the picture tube. The black matrix in the picture tube itself took the remainder of the incoming ambient light down by an estimated 80% over what we believe an early BW set would have done. This leaves about 8% of the light to be attenuated again by another pass back through the glass to the viewer. In other words, ignoring the reflections of the polished glass surface, more than 95% of the ambient light hitting the set is absorbed by the set. The cost? At least 60% of the light output capability of the picture tube. The attenuation of the glass was the same in both directions. The picture tube had to be driven well into blooming in order to get a usable amount of light in a well-controlled environment. The quality of the picture was dramatically improved by removing the glass on the front of the set.

Obtaining a good black level from a front screen projection system is even more difficult. Most screens are designed to reflect light back to the viewer, all light including ambient light. There are screens on the market that will attenuate ambient light that is coming in from side angles, but, as with the same thing in a TV set, it comes at a price of picture quality.
Tight control over ambient light is the best way to obtain a good black level from most display devices. Much of the rest of the story about black level is spread out over several other terms. VE T1 C5.