Anyone interested in information displays, from home flat-panel TV users to professional broadcast markets, knows the terms brightness, luminance and contrast and knows that these should be as high as possible for the display to be any good. Naturally, the same brightness and contrast values are included in typical marketing materials and sometimes offer key differentiations between products. But, there are many more things that make the display what it is, and these can depend on the application and the ambient used.
Basic display specs
Brightness is a subjective quality, a perception of some object that humans express in terms of dark, dim, bright and so on. So, what are all these numbers in nit or cd/m2 that marketers use? Well, in a strict sense, this is called luminance. Luminance, unlike brightness, can be measured with objective instruments and, also unlike brightness, can be expressed in absolute numbers. Luminance is the technical term expressing how much light falls on how much area. This is a key item for rear-projection and flat-panel displays. It doesn't directly depend on the surface it falls on (whether it is white or black, flat or grainy). You can think of luminance in light density or density of light rays in the ambient. It is measured in lux (lx).
Brightness is not only subjective; it is also relative. It depends on the surroundings of the object we are looking at. For example, look at the two vertical rectangles in Figure 1. The rectangle on the left is brighter. But which one has a higher luminance? The one on the left? No. They are the same gray rectangles, copied and pasted on different places of the changing gray background. Even though the rectangles have the same luminance, the perception of their brightness is relative.
Keeping all other factors equal, such as the background color and ambient light, and changing only the luminance, there is a one-to-one correspondence between the measured luminance and what we perceive as brightness. A display will look brighter if we increase its luminance. Scientists have spent a great deal of time translating perceived brightness into numbers, and Figure 2 shows a satisfactory model that links luminance and perceived brightness.The tricky part is that the perceived brightness cannot be expressed in absolute units, but only relative to a reference. The reference is the brightest white object in the visual field, which the eye uses to establish a black-gray-white scale under the prevailing illumination.
When you have two displays with different luminance levels, the brighter display is the reference (for example, the 100cd/m2 box in Figure 3). The model in Figure 2 illustrates that if the other display has 50 percent less luminance than the reference to the human eye, it is only 24 percent less bright. Or, a display has to have more than 5X less luminance (18 percent) in order for us to perceive it as twice as dark.
The lumen is a unit that measures the total amount of light that comes out of a projector. This is used for projectors, not for flat-panel displays like LCDs and plasmas. However, in rear-projection displays, the lumen is not a suitable unit.
A front projector projects light (in the form of pixels that carry information) on a white matte screen. That light reflects and reaches our eyes. But so does ambient light, which is more or less homogeneous, and thus diminishes the contrast of the projected image. It is necessary for front projectors to be much brighter than any ambient light levels in the room, and lumens are a suitable measure for this “brightness.”
In rear-projection systems, however, the light is projected from the rear and is transmitted through a transparent screen toward the viewer. The whole lumen story is, therefore, a legacy of front projectors. In rear-projection technology, what matters is what you see on the screen, and this depends on the projection engine, as well as screen type, ambient light and so on. It is expressed in terms of onscreen contrast and luminance, not lumens.
Ambient light will fall on your display screen. It will reflect and mix with the light that is coming out of your display and carrying useful information. The different reflection mechanisms are shown in Figure 4. They are:
- Diffuse or Lambertian reflection
Every ray of the incident light reflects in all directions so there is no image of the light source. (See Figure 4a.)
- Specular reflection
A mirror-like reflection on the screen where you see the exact image of the light source. (See Figure 4b.)
- Haze reflection
This falls somewhere in between the other two. It's that fuzzy circle around the reflected image. (See Figure 4c.)
The type of reflection you have depends on your screen. The majority of screens are called diffusive and reflective.
The ratio between the brightest white and the darkest black that a display can produce in a completely dark room is called the full-field contrast ratio. Being defined this way, the contrast ratio is anywhere between one (with white and black being equal, meaning no contrast at all) and infinity (a very, very large number if the black is very low or the white very high, or both). Contrast refers to the ratio between the difference and the sum of the brightest white and darkest black.
Projector contrast measures the ratio of the white and black produced by the projector. But this measurement still has to be translated to on-screen contrast, which, as previously stated, depends on the screen type and on the inevitable ambient light.
In rear-projection displays, the light is projected on the screen from behind. In order to make the light coming from the projector parallel, a Fresnel lens is used. This parallel light then goes through a screen, which can be built in such a way that the light coming out of it radiates equally in all directions. (See Figures 5 and 6a.) Other kinds of screens can introduce some angle distribution of the transmitted light.
The screen gain is the ratio between the light that the screen transmits in the perpendicular direction (see Figure 6b) and the light transmitted in this direction if the screen were completely diffuse (shown in Figure 6a). By definition, a perfect diffuse screen has a gain of one.
Because of the angular distribution, at some point the luminance will become half the on-screen value. This angle is the half-gain angle. Values found in industry vary between 6 and 40 degrees. These values should be optimized depending on the application.
To perceive a display 50 percent darker, its actual luminance has to be 18 percent of the reference display luminance. Translating this to the angle dependence of luminance in projection displays, we can define a 1/5 gain viewing angle, at which the luminance is 5X (20 percent) lower than in the perpendicular direction, but the perceived brightness is only half of the brightness in the perpendicular direction. This angle is much wider than the half-gain angle and yet still meaningful with respect to screen visibility.
When talking about flat-panel displays, we meet the specification viewing angle. This is not to be confused with the half-gain angle specified for rear-projection displays previously stated. In LCD and plasma display specs, the viewing angle varies from 160 to 180 degrees. In these displays, luminance also has an angular distribution from the perpendicular viewing direction.
However, the viewing angle in flat-panel displays is defined as the angle span within which the dark room contrast ratio remains above 10:1. Remember, a contrast ratio of 10:1 is needed for good visibility (readability). So, this is really what the viewing angle spec says: Outside this angle span, the display readability is very poor. For example, if a display specifies a viewing angle of 176 degrees, that means that the contrast ratio falls below 10:1 when you look at the display from an angle larger than 176:2, or 88 degrees. This angle means you are looking almost parallel to the screen.
Display specs are not exactly what you might think they are. There are many other important issues to take into consideration when building a good information display. Because the perceived brightness and luminance are different items, increasing the gain of a screen offers more disadvantages than advantages.
Goran Stojmenovik is product manager and Jim Durant is market development manager for Barco's control rooms division.
VESA Flat Panel Display Measurements Standard Version 2.0, June 1, 2001
Fundamentals of Vision and Color Science, a short course given by Louis Silverstein for the Society of Informational Display (SID)
“Videowalls: The book of the big electronic image” by Robert Simpson, Focal Press, 1997
“Projection displays” by Edward H. Stupp and Matthew S. Brennesholtz, Wiley, 1999
“Liquid crystal displays: Addressing schemes and electro-optical effects” by Ernst Lueder, Wiley, 2001 (part of the SID series on display technology)