Bias Lighting Fundamentals

In an effort to get around the problem of the eye's iris being wide open, ambient light is added to the environment. The eye's iris is biased by the ambient light. It is closed down a little, from the original wide open position, and better able to handle the very bright picture transitions. If the bias light is set properly, the eye still has enough dynamic range to allow the person to see into the dark areas of the picture. The need for a "bias" light has been clearly established as a result of several years worth of human factors research. Introducing this light introduces an environment to the monitor, which will influence the color perception of the picture coming from the monitor. The conditions of this "bias" light become very important.

What are the major considerations of introducing this light? There are some obvious points. The light needs to be in the environment of the display device. It shouldn't be positioned where it canshine directly on the display or the viewer or be reflected back to the viewer by the display. Ideally, that means that the light should be behind the display aimed in the opposite direction of the viewer. The conditions perceived by the viewer of the light/environment combination are very important.

Knowing that a person's perception of color is dependent on the environment, is it possible to specify a single environment that will assist in properly determining the color spectrum capability of the television system? Yes, just as the "color" of gray on the monitor itself will effect all the other colors, it turns out that a neutral surround, gray; which contains all colors, is the right choice for the environment. Ideally, the gray in the environment has to match the properly calibrated gray of the monitor.

 What about intensity of the light? The SMPTE Recommended Practice document says the level should be less than 10% of the peak white level on the viewing device. How much less? Most viewers in the professional world settle in at around 5%.

Be prepared! The initial reaction of most interior decorators is going to be "off the wall" when this information is first presented. In the past they have managed to destroy proper perception of color in the process of creating a "warm, friendly" environment. There are times when the viewer would like to see the information the system is trying to present. The warm, friendly colors in the environment are not warm and friendly to proper color perception.

With a little creativity, gray does not equate to "drab". The background grays should have shades, relief, texture, and/or design. There is a great deal of room for individual expression within two basic parameters; the maximum level of light coming from any point in the background should be no more than 10% of the peak white level of the display device, (measured in the plane of the display device), and the background color should be neutral, the same color of gray as the gray scale of the properly calibrated monitor.

 4. Elements in the Environment

In the home environment, mostly neutral gray, plus elements that are close to gray, if desired, are usually acceptable for the field of view in which the monitor is located. The choice of colors that may be used with gray should be limited to what Munsell calls "Nearly Neutral".

Munsell5 is an organization that defines colors of pigments. They provide samples to the art, fabric, and paint industries as references.

The use of fabric and/or texture is encouraged. Several examples of elements in the environment are provided to help with ideas. They are intended only as examples.

Paint the wall behind the monitor any desired shade of matt gray, anything from a deep gray to white. Hang gray vertical blinds on the wall. The blinds can be gray fabric or gray plastic. The blinds may or may not provide a contrast to the gray on the wall. Gray fabric may contain elements of the "Nearly Neutral" colors mentioned. The vertical blinds might be covering a window or sliding glass door or made to look as if they were. The bias light (which is detailed later) is aimed at the blinds. The amount of light reflected back into the room is controlled by rotating the blinds open or closed. The cost of this mechanical control of the ambient light level may be in the same order as an electronic control. It will help if the wattage of the light is chosen so that the maximum reflected light does not exceed 10% of the monitor peak white level.

The picture monitor does not have to be placed out away from a flat wall, as shown in the animation. If it fits better out away from the corner of a room, and the sound quality is not compromised, there is no problem in using that configuration. Very creative things can be done with lighting in a corner of a room.

5. Choice of Lighting

As for lighting, "soft white" fluorescent lamps can be filtered to about the right color using Rosco's 3202 filter material. Rosco6 filters are common in the motion picture and television industry. They, or their equivalent, should be available from lighting supply places that work with these two industries. The filter material is inexpensive.

Filtering "soft white" lamps is presented as a cost effective option. These lamps are available in many sizes and shapes. The lamp/fixture combination can be small and easy to fit behind the monitor in addition to being inexpensive. The light output of a 15 watt lamp is often about right, after being filtered, for the 10% of peak white specification.

Filtering other commonly available colors of fluorescent lamps to a neutral gray is also possible. Combinations of existing filters may have to be used to obtain the desired gray.

6. Color and CRI of Lighting

Color Rendering Index (CRI) can be another important factor in choosing one particular light source over another.

Background: In terms of light, the color of "white" is obtained when all colors of light are added together. Most room lighting sources produce colors of light that include the primary colors; red, green, and blue, plus other colors in the visible spectrum. In most lamps, the amount of light output of a particular color is greater than other colors in the visible spectrum; a factor in determining the "color" of white the lamp produces.

CRI is a measure of how well a particular "color" of white will render an object familiar. CRI is scaled from 0 to 100. Any "color" of white along the black body curve of the CIE diagram can have a maximum CRI of 100. The CRI of a lamp becomes important when many pigment colors are added together to form the neutral gray in the background. How well these objects are rendered to human perception is important. The actual color of the background material, as seen by the viewer, will depend on the CRI of the light source. If the light source has a CRI below 80, chances are good that a multi-pigment background will look different in color than when lit with a bulb having a CRI near 100. In critical environment conditions it is important to use an ambient light source with a high CRI.

Fluorescent lamp colors are given names. There is some consistency in color of light among manufacturers for a given color name and sometimes even consistency in CRI. Knowing the approximate color of the bulb, proper filter material can be chosen to correct the bulb color to the desired "daylight" color. Examples of names, with colors referenced to the black body curve on the CIE diagram, and CRI's are listed below. The abbreviation "Inc." is used for incandescent lamps.


25 W. Inc. : 2700

Warm White : 3000 56 150 W. Inc. : 3100 98 to 100

Soft White : 3200 75 to 85

Cool White : 4200 68

D50 : 5000 90 to 100 Daylite : 6500 75 to 95

D65 : 6500 95 to 98

D75 : 7500 90 to 98

The lighting industry in general and individual manufacturers in particular have their own terminology. So called "Warm" colors of light, those at the red-orange end of the black body curve of the CIE diagram, are usually below 3500ø Kelvin. Anything above 3500ø Kelvin is said to be "Cool". The terms "Warm" and "Cool" refer to color rather than color temperature. Just another example of where "Warm" is cool and "Cool" is warm. The "scale" is biased by the color of incandescent lighting.

Filtering incandescent bulbs to a neutral gray color is less than ideal. While the light output is easily controlled and the CRI is high, the color of light changes dramatically as the light output is changed. The heat generated by these lamps is rough on the filters needed to make them the right color. The wattage of the lamp is also a factor in color of light produced by the bulb.

7. Control of the Amount of Light

There are two levels of control that must be considered. The first is the lighting in the entire room and the second is the light providing a background for the monitor. Ideally there should not be any room light falling on or reflected by the display device towards the viewer. In most home environments, where the viewing room serves functions beyond viewing, absolute control of ambient light may be difficult. The room lighting should allow implementation of proper viewing conditions when that is the principle activity in the room. The perceived level of picture black is dependent on ambient lighting conditions, meaning that a fixed black level on the set will require tightly controlled ambient conditions.

Having control over the monitor background lighting, up to the maximum reflected value of 10% of the peak white level of the monitor, is often a matter of convenience. Fluorescent lamps can be dimmed to about 25% of their full light output without flicker or change in color, if thecorrect ballast is used to control the lamps. That makes them ideal for use in highly controlled ambient light application. The special ballasts for fluorescent lamps will most likely be available from a professional lighting supply centers or lighting contractors. Their use is becoming common in many video production and post-production facilities.

Fluorescent lamps that produce the proper color of light, without being filtered, come in four foot lengths, as of the beginning of 1990. They require a large fixture with a dimmable ballast so that the light output can be accurately controlled. (These four foot bulbs produce much more light than is normally needed.) There are a verity of fixtures available that can accurately control the area being lit in addition to controlling the amount of light. As lighting fixtures go, these are among the more expensive. Even at that, they represent a small fraction of the total investment in audio and video equipment. They are like interconnecting cables, a necessary part of making the entire system function properly.

Many other options are available. Look for papers written for the television industry for additional help. Reference Recordings may publish separate application notes on this subject by the end of 1990.

8. Getting Accustomed to the New Look

A note of caution to the viewer about this new environment is in order. From experience in working with post-production facilities that have made dramatic changes in monitor gray scale and room environment all at once, it can take up to two weeks to get used to all of the changes. The picture will look completely different when the monitor gray scale is changed from blue to gray and the lights and background changed from orange to gray. Not everyone will see the change the same way. There will be comments about being able to see green for the first time from some, and seeing much more red from others. Within one or two weeks there should be a general consensus of perception among all the viewers. During this transition period in particular, share ideas with other viewers about the changes being noticed. Use the CIE diagram in the disc to help analyze the changes everyone is seeing. A great deal will probably be learned about the perception of color in this transition period.

9. General Exceptions and Compromises

Placement of the monitor and viewer within the room are well defined in the "Ideal Viewing Environment" animation. The information provided in the disc; taken from the SMPTE Recommended Practice document for Monitor Environment, evolved out of human factors research. Many of the dimensions are specified in terms of the height of the picture being watched. This way, the specifications can easily be applied to individual circumstances. Major exceptions to these rules will probably have to be made for front projection video systems. Knowing the reasons behind the rules of the ideal viewing environment will assist in making compromises, if or when they become necessary.

When putting together large screen viewing systems, either front projection or single piece rear projection sets, provisions should be made for a bias light behind the screen. The level of the light should be reduced to much less than 10% of the peak white level coming from the screen,

probably in the order of less than 5%. The bias light of the proper color will make viewing the big screen much more enjoyable.