|
Luminance and Contrast
The luminance ("brightness") and contrast characteristics are very important to the overall performance of a display. If the image appears too dim or too washed out, not only will it not be enjoyable to watch, but over time it can also be fatiguing and stressful to the eyes. Here we discuss the related display performance evaluations and measurements of luminance and contrast.
[Note that there are several links to WalVisions test patterns here. When one of these patterns is selected, it will open in a new window; to then view the pattern in nearly full screen, maximize the window
by using the F11 (full screen) key.]
Luminance: The display luminance is a measure of what the eye subjectively perceives as "brightness". But what the eye sees as "bright" depends upon a number of factors, primarily the viewing environment and what the person has been viewing recently. An example is the LED clock (or radio) digital display that is used in some automobiles - if you activate the nighttime settings by turning your headlights on during the day, these displays might appear to be quite dim and difficult to see due to the relatively bright surroundings, while at night these displays will appear to be quite bright. The luminance output level for the display hasn't changed, just the viewing environment. Thus while one can give guidelines, care must be taken in stating just what luminance is necessary for a home theater - it depends strongly on the home theater viewing environment.
Actual luminance measurements are made in foot Lamberts (ft-L), or nits (candelas per square meter). Here we use the ft-L system; to convert to nits simply take the ft-L reading and multiply by 3.426. For self contained displays, these measurements are usually made with a detector that is placed directly on the viewing surface. For front projection systems, these measurements are made with a photometric "spot meter", which is pointed at the display and a circle in the viewfinder is centered within the area to be measured. In movie theaters typical luminance levels are about 15 ft-L.
Total luminance output (lumens) can be measured in two distinctly different ways. The first method is to determine the full light output capability of the display. This measurement uses a full white field for the test pattern, and then averages the luminance levels at nine positions, each at the center of an imaginary 3x3 grid. The
ANSI Lumen test pattern (WV-36) is a pattern that shows these locations. Note that the display brightness and contrast adjustments should be properly adjusted before the measurements, and that's the reason for the small rectangles at the center of the image - the rectangles display 0, 5%, 10%, 90% and 95% video levels. While these ft-L measurements correspond directly to what we see, and thus are the most important, they can be also used to work backwards to get the lumens rating for the display, as long as the screen gain (for projectors) is known. The lumens rating is the average luminance level times screen area (in square feet) divided by the screen gain. For example, if a 20 ft-L average is measured on a 96" x 54", 1.3 gain screen, the lumen rating is (20)*(96/12)*(54/12)/1.3, or 554 lumens.
The second method of measuring
total luminance output is based on the peak luminance level. Here a
Window test pattern (WV-38)
can be used, and with this particular pattern only the center 15% of the display is white, with the rest of the screen being black. The luminance is measured at the center of the window, and then can be directly compared to the full light output measurements. For bulb type projectors, these measurements should be essentially the same. But for plasmas, direct view CRTs and CRT projectors, however, the measurements may be different
(higher) by up to a factor of about five! This is because these displays actually generate the light as needed, and they have limitations on both how much total power is available and how much power can be used before reliability problems arise. Thus these displays all have brightness limiting circuits, which electronically reduce the power levels so they don't exceed preset levels. For most typical video programming, these brightness limiting circuits won't be active and the "peak luminance" measurement will accurately characterize the display's luminance, but when large areas of white are present, the circuits may turn on and the display brightness might be reduced. This effect may be viewable in the
Growing Fields test pattern (WV-16) or the
Dynamic Brightness test pattern (WV-17). Both these pattern cycle the overall brightness/power level, and for the perfect display the luminance levels of the unchanging parts will be constant even as there are changes in other parts of the display.
Full Field Contrast: The full field contrast measurement is simply the ratio of the full white field to full black field luminance measurements. The eye works well over a wide range of luminance levels, and a display needs to be able to display both brighter and darker scenes realistically - such as is necessary when switching from an daytime outdoor scene to a night scene. The full field contrast is a measure of the display's ability to provide these overall scene brightness changes. The higher the ratio the better, and more that 1000:1 is desired, which is about the level of a typical movie theater.
Dynamic Contrast: The dynamic contrast ratio,
sometimes referred to as the ANSI contrast ratio, is a measure of contrast within a single image. In this case, some of the light intended to go into the brighter image parts may be scattered and illuminate the black parts of the image. For projection displays, the dynamic contrast rating is virtually always less than the full field contrast rating. The standard measurement is to use a
4x4 Checkerboard test pattern (WV-37), and the rating is determined by dividing the average luminance level of the white squares by the average luminance level of the black squares. Note that this is simply one specific measurement of contrast ratio that can be used to compare displays, and it represents a fairly extreme image consisting of one half fully black and one half fully white.
How much dynamic contrast ratio is necessary will depend largely on the scene being viewed. For adjacent dark-light areas the eye can only discern about a ratio of 100:1. For a demonstration
of both display
performance and the eye's ability to see contrast, check the
Animated 4x4 Checkerboard test pattern (WV-12) and the
Animated 2x2 Checkerboard test pattern (WV-13). You can notice both how the dark areas are washed out by the white areas, and also how the eye has difficulty picking up dark detail that is adjacent to relatively bright areas.
Dynamic Contrast Loss Due to Environment: The home theater viewing environment can have a significant effect on the system dynamic contrast performance. Contrast at the screen is lost as light from the display illuminates the room (walls, ceiling, floor, furnishings and people!), and some of the reflected light returns to the
illuminate the screen to reduce the dynamic contrast ratio. The
Animated 2x2 Checkerboard test pattern (WV-13) can be used to check the room contribution to image washout, which can be a significant problem for front projection systems. First make sure you are viewing in full screen, so the black rectangle in the lower left is at the edge of the display. For front projection displays you can then use a book or magazine to block the light going from the projector to that lower left corner - the illumination that then occurs
in the shaded area during the bright cycle is due to reflected light from the room. For self contained displays, the check is basically the reverse - you need to try to shade the room light from getting to the lower left corner, which is more difficult because it's coming from everywhere, and then note just how much the room light washes out the display in the non-shaded section.
|
