Approximate Luminance Levels and Eye Characteristics    Note:  1 Foot Lambert = 3.426 Nits (cd/m^2)
Foot Lamberts	Object or Conditions	Eye Comment
300,000,000	Sun	.
30,000,000	.	Eye Damage
1,000,000	Bulb Filament	.
50,000	.	Eye Upper Limit      Photopic Vision
20,000	.
10,000	.
5,000	Snow, Clear Day
2,000	.
1,000	.
500	Full Moon's Surface
200
100	Brighter Conventional TV
50	Sky, Heavy Clouds
20	Paper, Good Reading
10	.
5	Dusk
2	Sunset, Cloudy
1	Paper, 1 Foot from Candle
0.5	.	Mesopic Vision
0.2	Sky, 15 min After Sunset
0.1	Snow, Deep Twilight
0.05	.
0.02	.
0.01	Snow in Full Moon
0.005	.
0.002	.
0.001	Paper, 32 Feet fm Candle	Scotopic Vision
0.000 5	.
0.000 2	.
0.000 1	Snow in Starlight
0.000 05	.
0.000 02	.
0.000 01	Snow in Overcast Night
0.000 005	.

This table shows the wide range of luminance levels that we find in nature, and how the eye responds to those levels.  The table "scale" is geometric, which is in the same format as the eye's sensitivity - the level of each step is about 1/2 the level of the step above.  This is similar to musical scale for which each doubling of frequency is an octave, and also similar to the way we hear - an 10 db increase in level represents 10 times the sound energy.  Note that the eye can respond over an amazing range of about a 100 billion to one!

The eye consists of "cones" and "rods". The cones (photopic vision) are used for higher luminance levels, and can detect good color and resolution. The rods (scotopic vision) are used for very low levels, but can't detect color well or very much detail, but are good at detecting motion. At intermediate luminance levels both rods and cones are active, and this is mesopic vision.

Fortunately the home theater doesn't have to reproduce the entire luminance range to make a realistic display. The highest luminance levels can be uncomfortable to watch, and the eye's pupil dynamically closes to block much of the light from the more luminous sources. Unless you are in a very well lighted environment, luminance levels of 50 to 100 ft-L (Foot Lamberts) will appear plenty "bright".

At the low end of the scale the scotopic vision takes over. Since this low level vision is colorless, low resolution and can require significant dark adaptation to be fully effective, it's not critical for normal viewing in the home theater. Thus we have defined an excellent range of luminance for a home theater to be from about .001 to 50 ft-L, assuming the theater has the advantage of a dark environment.

For most scenes with brighter areas throughout, however, the eye only makes out dark levels that are a few hundred times dimmer than the brightest areas. Thus if we accept about 20 ft-L as a very good "white level", than a dark level of about .1 will be adequate for most scenes, and a dark level of .01 would be good for dimmer scenes where the white level is about 2 ft-L. We thus define the very good range of luminance to be from .01 to 20 ft-L. For the third good range for luminance we have defined a range from .1 to 10 ft-L - this will look pretty good for most video with plenty of bright information, although for darker scenes there will be a clear washout effect masking the dimmer image details.

Now that we've defined the luminance levels that we need for home theater, let's investigate the options for achieving these white and dark levels in the home theater setting.

White Levels:  The white level is primarily determined by the projector or display device. While the ambient (room background) light can contribute to the white level, it should be only a minor amount.  For a self contained display, the luminance should be listed in the specifications.  For a front projector, there should be a specified light output rating, given in lumens.  To determine what the luminance level will be for a particular projector and a particular screen, divide the projector lumens rating by the screen area, in square feet, and then multiply by the screen gain.  For example, with a 800 lumen projector on a 96" x 54" (8 foot by 4.5 foot) 1.3 gain screen, the peak luminance level should be (800*1.3)/(8*4.5) = 28.9 ft-L. In practice the peak luminance will likely somewhat less since 1) the specified light output level will probably correspond to a setup that maximizes light output at the expense of quality, 2) that light output will only be available with a new projector lamp and 3) the screen gain may be overstated and not include some losses.  Realistically you might expect to obtain about 15 to 20 ft-L in this situation.

Black Levels:  The black level is the level of luminance when the input signal corresponds to black, but will only be at the true black level of the display if the Brightness control in the display or projector is set properly. For a direct view or projection CRT display, this black level can be essentially zero if placed in a darkened room, and in a lighted environment the dark level will be whatever room light reflects off the front of the CRT front glass or the projection screen.  For plasma and LCD direct view displays, there will be some black level washout due to the panel itself or the backlight, plus whatever room light reflects off the display.  For both rear projection and front projection displays, the black level will be the background light leakage from the projector, plus whatever room light reflects off the screen.

For self contained systems you can usually calculate the black level by simply dividing the luminance rating by the contrast rating.  For front projection displays, divide the specified light output rating by the specified full field (ON/OFF) contrast ratio, and then, as above for the white levels, divide by the screen size in square feet and multiply by the screen gain.  If in the above example (800 lumens onto a 1.3 gain, 8 foot by 4.5 foot screen) the specified contrast ratio is 3000:1, then the black level will be (800/3000)*1.3/(8*4.5) = .01 ft-L. This projector thus just meets our criteria for "very good" - before the viewing environment is factored in.

Of all these display types, the one clearly most affected by room light is the front projection system, since the room's ambient light can illuminate the screen to a significant level.  In all the other cases the basic display design is such that room light will mostly absorbed and not contribute significantly to the black level.  In addition, the front projection systems almost always are used to create larger images, so the white level will usually be less.  Thus to maintain a good contrast ratio the black level must be low as well.  This is why a controlled light environment is particularly important to front projection systems.

Another related factor that increases black level is the projected image itself.  If part of the screen is illuminated and other parts are black, the illuminated part will light up the room which in turn will light up the screen!  This is why high end home theater have relatively dark, and neutral colored, walls, ceiling, floor and furnishings.

Full Field Contrast Ratios:  There are two types of contrast ratios, and they both are important.  The first is the full field contrast ratio, which is the ratio of the white level in a completely white image to the black level in a completely black image. This is indicative of how dark the dark scenes can be compared to brighter scenes, and is the contrast ratio that manufacturers almost always specify.  As you can see in the above table the human eye is capable of seeing 100,000,000,000:1, but of course that is extreme and unnecessary. Our "excellent" home theater has a ratio of 50,000:1, while our "very good" system has a 2,000:1 ratio.

ANSI Contrast Ratio:  The second contrast ratio frequently referred to is the ANSI contrast ratio, which is the ratio of the average white level to average dark level within a single scene consisting of a checkerboard pattern with half the squares being fully white and the other squares being black.  For projection displays, this ratio will generally be significantly lower that the full field contrast ratio, and this is due to some of the light from the white squares scattering to the black squares.  But since the eye's ability to see detail in darker areas is limited by the presence of lighter areas, the ANSI contrast ratio doesn't need to be nearly as good as the full field contrast ratio. An ANSI contrast ratio of 100:1 is good, and 500:1 is excellent.

Shades of Gray - The "Gamma" Factor:  So far we have discussed white and black levels, and now we will go on to discuss those levels in between, the shades of gray that go from just above black to just below white.  Gamma is the term that is used to characterize the luminance of these levels - and it refers to just how rapidly the gray level rises as the video input signal rises.  When the video standards were created, the only display device was the CRT.  Since the light output level of CRTs is proportional to the input signal raised the "2.5" power, in the video standard the video signal is roughly compensated by the inverse function, thus the displayed level will best match the level as seen by the camera.  Note that the "2.5" is considered the display gamma, and the video compensation is actually for a gamma of "2.2" to give a more realistic image since display will typically be viewed in a darker environment.

What happens when the display gamma is set to be different than 2.5?  The effect frequently will not be obvious, but the intermediate gray levels will darken or lighten, and in most cases will make the image look different than what the camera saw, and what the director wanted you to see.  The effect will be similar to changing the scene illumination, like adding (or removing) lights to illuminate (or cause to darken) different parts of the image. An example is what you see when looking into a shaded forest area during the afternoon as the sun goes from high in the sky to lower in the sky and behind you. At first the tree trunks will be fully in the shade, and relatively dark, but as the sun goes down and shines into the forest, the tree trunks lighten.  To view the image correctly, the display gamma should be about 2.5, but should be a little less for some displays and installations that have higher black levels and/or are viewed in brighter areas.  For displays in relatively bright surroundings, a gamma of 2.0 to 2.2 may appear best.