Attaining the NTSC Television System Potential — Part 3: The Viewing Environment

We can’t begin to over emphasize that if high quality pictures are your goal good video is not available in a “plug-and-play” format. As much as convincing you otherwise is in the interest of mass market television, it just doesn’t work that way. One of the reasons most of you have invested in the LaserDisc and DVD mediums and some level of home theatre is the potential for superior picture quality. Getting that quality out of either of these formats requires more than just a good player and a properly calibrated display system. The environment of the display is as important as any of the electronic elements.

Proper environment for a display device is among the most missed parameters in setting up video.

If you’ve tried making the Brightness and Contrast adjustments, you already know that when a CRT based display device is calibrated to deliver the best possible picture, there isn’t much light available. The display device probably isn’t bright enough to compete with any ambient light hitting the surface of the screen. In obtaining the best possible contrast ratio, no ambient light or reflected light from the set itself can hit the face of the screen. That’s rule number one and it applies to all display devices.

If most sets won’t produce a good contrast ratio with ambient light falling on the surface of the picture, why not watch television in complete darkness? In the case of a large screen home theatre, where the picture occupies a large percentage of your field of view, complete isolation from ambient light is often necessary—a totally dark room. In other circumstances, where the picture occupies a smaller part of your field of view, some ambient light is required to make watching that picture comfortable.

Human Factors

The study of what is needed for ideal viewing conditions is a study of Human Factors. The requirements of the environment are based on the way we see light. The environment becomes a large part of the visual interface between the display device and our eyes. As an example, if there is light in the room, which is needed when watching most direct view monitors and many bright rear screen sets, the color of that light and environment become important. A quick review of some of these human factors is in order.

A person’s absolute perception of individual colors is not very good. The same holds true for hearing. Very few people have perfect pitch. On the other hand, most people are good at determining small differences in both sight and sound in an environment where the differences can be directly compared. Put two slightly different colors next to each other and most people will be able to see the differences between the two. If the two objects are separated by a larger distance than the field of view of an individual, it will be very difficult for the average person to distinguish the small differences in the objects.

Extending that beyond two objects to an object and its environment, a person’s perception of the color of an object is somewhat dependent on the environment of the object. A case in point, if I were to wear a cyan (blue-green) shirt with a green sport coat, the shirt would appear to be blue. If I were to change into a blue sport coat, the shirt would appear to be green. The color of the shirt hasn’t changed, the environment of the shirt changed; along with your perception of the shirt’s color. What does all of this have to do with watching television? Viewed from the proper distance, which will be covered later, a television picture makes up about 20 percent of a person’s field of view. The other 80 percent of the optical input, the environment of the monitor, can have a significant influence on the perception of the color coming from the 20 percent, the monitor.

Conclusion? The 80 percent of the field of view has to be tightly controlled if the 20 percent is going to convey the desired information. What’s involved, how much control is necessary? We’ve already mentioned the idea of eliminating all light in the 80 percent of the field of view. There are two difficulties with this approach. The perception of individual colors by the viewer is going to be totally dependent on the rest of the picture content. More important is the problem of eye strain that will result from a totally dark environment.

Studies of viewers watching a 19-inch monitor at 30 foot-Lamberts peak white light output in an otherwise totally dark environment show fatigue occurring within 30 minutes.

If a small amount of ambient light is placed behind the set, biasing the irises of your eyes closed down a bit, you’ll have less difficulty dealing with the changes of light in the program. The fatigue factor jumps from less than 30 minutes to more than 8 hours with this minor change.

In an 80 percent black environment, the iris of the eye is wide open, the eye is trying to collect any light it can get. Along comes a bright picture from a particular scene in the program and there is instant overload of the optical nerve. The constant change in brightness from the picture can cause significant eye strain. Moving closer to the monitor, trying to duplicate the theatre experience, doesn’t work because of the course line structure of the television system and the higher light output from the CRT.

In a motion picture theatre, the image on the screen can represent a much larger percentage of the field of view. In terms of picture height, most movie viewers sit much closer to the screen than television viewers should sit from the monitor. That’s all right because the resolution of a projected motion picture image is much better than a television image. Another important point is that the light output from the motion picture screen is not anywhere near as bright as from a television set. Most theatres still use two bladed shutters.

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, and better able to handle the very bright picture transitions while still having 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 by much research into human factors. This light introduces an environment around the monitor, which will influence the color perception of the picture coming from the monitor. The conditions of this bias light become significant.

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 can shine directly on the display or the viewer or be reflected back to the viewer by the display device. Ideally, that means that the light should be behind the display aimed in the opposite direction of the viewer. The color perceived by the viewer of the light and environment in combination is the next consideration.

Staying with the point 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 entire 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 percent of the peak white level on the viewing device. How much less? Most viewers in the professional world settle in at around 5 percent.

Be prepared! The initial reaction of most interior decorators not familiar with the requirements of television is going to be “off the wall.” These people have unknowingly managed to destroy proper perception of color in the process of creating a “warm, friendly” environment. They must be made to understand that there are times when the viewer needs to properly see information the system is trying to present. The warm, friendly colors they are used to using in other rooms are not very warm and friendly to proper color perception.

Keep in mind that, with a little creativity, gray does not equate to “drab.” In reality, the background grays should have shades, relief, texture, and/or design. There is a great deal of room for creativity within two basic parameters. The maximum level of light coming from any point in the back ground should be no more than 10 percent of the peak white level of the display device, as 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.

Viewing Distance

When watching a video display, it is desirable to see the entire picture as a picture rather than seeing elements that make up the picture. If the observer sits close to the display, the horizontal line structure making up the display becomes as much a part of the perceived information as the picture. Some display devices, such as the consumer version of the Trinitron®, have their own structure that can become an observed part of the picture if the viewer is to close to the display.

In the 525 line display system, most “trained observers” have to be eight times the picture height dimension away from the monitor before the line structure is no longer a factor in the picture. If there were more lines making up the picture, the observer could be closer to the display without seeing the line structure. In a motion picture theatre, the observer needs to be far enough away from the screen so as not to notice the film grain. In some special film formats, that dimension, in terms of the picture height, is so small that the projected image extends beyond the field of view of the observer. The observer’s entire optical environment is filled with the motion picture image. In short, the size of the structure making up the picture, relative to the entire picture size, is the principle factor in how far away from the image the observer must be placed.

There are several human factors reasons that a dimension less than eight times the picture height can be considered for 525 line television images. In real life this dimension is not practical. People that are not trained to look for the horizontal line structure, or are good at ignoring it, can sit closer and still see the picture instead of the elements of the picture. In a working environment, such as a video post-production house, the minimum distance between the monitor and viewer should be four times the picture height.

In a standard viewing environment, such as a quality control area, the minimum distance should be five times the picture height. The minimum distance of five times the picture height also holds true for viewing a CRT display at home, although eight times is better.

Viewing distance from a large front screen projection device will probably have to be less than five picture heights. Many projector manufacturers recommend a minimum viewing distance of 1.5 to 2 times the picture height. That can be made to work if a high quality video processor is driving the projector, minimizing the appearance of the line structure.

Viewing Angle

The proper viewing angle is within 15 degrees on either side of a center line coming out from a CRT-based display device. The total wedge shown in the “Ideal Viewing Environment” section of Chapter 5 in A Video Standard is 30 degrees. Light fall-off and color changes can become significant at angles greater than 15 degrees from center. These numbers can change depending on the type of CRT. Flat surface CRTs are usually better than curved CRTs as an example. The curve in a CRT will often distort the geometry of the picture as you go off axis in addition to changing luminance intensity and color.

The proper viewing angle can change radically when looking at projection systems. Here, the quality of the screen becomes a large factor. This is particularly noticeable in many rear projection television systems where the gain of the screen is greater than one. Rear screen and curved front screen projection systems have screen gains as high as 15. In their case, any movement away from center, in any direction, will cause a noticeable shift in both light output and color. Here the acceptable viewing angle could be less than 1 degree off axis. In a properly set-up home theatre, the viewing angle should be large enough to accommodate the audience. That will most likely mean that a flat CRT based viewing device will be needed or a low gain screen in the case of projection systems.

Space Behind the Monitor

The principle reason for placing a monitor out away from the wall is to help reduce eye fatigue. When viewing the monitor in its free standing environment, only the monitor is in focus. This keeps your attention focused on the picture, which is the primary objective, and provides another plane of focus when you want to pull your eyes away from the set. Even in an ideal viewing environment, eye strain will occur. Having a second plane of focus will quickly relax the eyes, yet keep them in a properly calibrated color environment. You won’t lose your color perception capability in the process of relaxing for a moment: Having the monitor designed in the wall is actually an advantage. It makes the focus transition much easier. The free standing position of the TV set has another advantage. Speakers are usually brought out to the plane of the monitor. In doing so, most speakers will sound better.

Usually the depth of a smaller monitor, in the order of 19-inches, will place it 1.5 to 2 times the picture height away from the wall. The extra distance required to get to the minimum of 2.5 times the picture height away from the wall is accomplished by making room for the wires to be connected to the set. As direct view sets reach consumer size, 27- to 35- inches, obtaining the minimum 2.5 times the picture height distance becomes much more than allowing room to make connections behind the set. It involves making room for you to walk behind the set.

When setting up a system around a large rear screen projection set, there often isn’t enough space to pull the set more than a few inches away from the wall. Enough space should be provided to get a light behind the set, aimed at the wall.

Just accounting for fatigue and color perception alone, it should be clear that you’ll have to control the amount of light, its location, and its color. In an ideal world, the environment of the display device would be set up before moving the TV set or large screen projection system into the room. Unfortunately, few room designers outside the world of home theatre under stand the need for tightly controlling the environment of a display device. Worse yet, most rooms used for watching video serve multiple functions, often at the same time. That can be made to work if you’re just casually watching TV. If your goal is to get involved in a program, as you would in a movie theatre, the house lights must come down.

Elements in the Environment

Our perception of luminance and color are dependent on the environment of the object being viewed. We’ve illustrated this in Video Essentials. You’ll find that area of the program just after the montage of images. In the LaserDisc, it’s on Side 2, in Chapter 15, Index 8 or at Frame 45,103. In the DVD, you can find it as Item 8 in the Title 15 menu or go directly to it in Title 19, Chapter 3.

The demonstration opens with an example of luminance perception. Two gray fields of the same level are surrounded by different backgrounds. They appear as if they are different levels of gray because of the background. The two fields of gray are then joined to prove that they are actually the same. It may take you a while to see that as you’ve already convinced yourself that they are different. You may want to pause the program at the point where they are brought together. That sequence is followed by changes in color. We start out with both sides of the picture at gray. We change the left side of the picture, making it slightly plus red, taking it back to gray, then making it slightly minus red. The right side of the picture remains fixed, although it also appears to change in color. These two examples are designed to clearly illustrate that our perception of an object is dependent on the environment of the object. It is designed to help you understand why the environment of the display is so important to what you see from the display. A quick view of that is found in the “Ideal Viewing Environment” section of the program, Chapter 1, Index point 3, Frame 6,808 in the LaserDisc or Title 1, item 3 on the menu or Chapter 5 for direct access.

In the home environment, neutral gray plus elements that are close to gray, if desired, are usually good enough for the field of view in which the monitor is located. The choice of other colors that may be used with gray should be limited to what Munsell calls “Nearly Neutral.” Munsell 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 background elements are being provided here to help get the creative thought process going. They are not intended as anything more than examples.

Paint the wall behind the monitor any desired shade of matte gray, anything from a deep gray to white. Hang gray vertical blinds on the wall. They can be gray fabric or gray plastic. The gray in the blinds may provide a contrast to the gray on the wall. The gray fabric may contain elements of nearly neutral colors. The vertical blinds might be covering a window or sliding glass door or made to look as if they were. The light behind the set, which will be detailed in a moment, 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 level control of the ambient light may be in the same order of magnitude as electronic control.

The picture monitor does not have to be placed against a flat wall. 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. Some very creative things can be done with lighting in a corner.

Choice of Lighting

The correct color of light is known as “Daylight” in the lighting industry. There are several colors that have the name Daylight attached to them. They range in color from about 5400° to 6500° Kelvin. In consumer applications, any of them will work well enough, but D6500 or 6500° Kelvin is the best choice. It is the same color of gray as is set on a correctly adjusted TV set. In the program production world, it is important to adhere to the D6500 parameter as closely as possible.

We’ve just discussed a range of colors that might be acceptable in the consumer world. Why would it be any less stringent than in the professional world? If the object of the room setup is achieving an ideal viewing environment, there is no compromise, but most of us aren’t required to make critical judgments about the color quality. Something close to correct won’t alter our perception enough to make a noticeable difference.

D6500 lamps are available in all sorts of shapes and sizes, but often require a bit of searching to find them. You might start with stores that specialize in fluorescent lighting. Ask for brand names such as Duro-Test, Lumichrome, and Ottlite. We suspect there are at least several others, but these are the ones that surfaced in our search. Prices for these bulbs have ranged from $10 to $30.

“Soft white” fluorescent lamps, which are much easier to find, can be filtered to about 5500° using Rosco’s 3202 filter material. Lee and GAM also have filters that will do the same thing. These filters are common in the motion picture and television industry. They should be avail able from lighting supply places that work with these two industries. The filter material is inexpensive.

Filtering “soft white” lamps is presented as an expedient option as it took us a long time to find a store that stocked D6500 lamps. Much of our business in this area is done via mail order. The soft white lamps are also 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 usually about right after being filtered. That light level would otherwise be high unless the environment is a deep gray. Depending on the light absorbing quality of the surface behind the set, a 2 to 5 watt fluorescent bulb is all that is normally necessary in a single monitor application. If needed, neutral density filters can be used to bring the light level down further when using the correct color of lamp with a higher light output.

Color and CRT of Lighting

You may also want to be concerned with the Color Rendering Index (CRT) of the lighting used. CRT is a measure of how continuous and consistent the light spectrum of the bulb is over its entire visible light spectrum, violet through green to red. Said another way, the color rendering index is a measure of how well pigments will be rendered recognizable to human beings. The overall color of light is only a small part of CRT. A daylight fluorescent bulb could have a poor CRT while an incandescent bulb would have a high CRT. On the other hand, given equally high CRT values, say near a peak value of 100, color fidelity will be better if the color of light is somewhere in the daylight range of 5400° Kelvin.

In terms of light, the color white is obtained when all colors of light are equally added together. Fluorescent lamps produce colors of light that include the primary colors—red, green, and blue—plus other colors in the visible spectrum. In most cases, the light output of any particular color is more or less than others in the spectrum. While a fluorescent lamp may look white to you, it can be almost void of particular colors in the visible spectrum and have high amounts of others. It is seldom uniform in its light output across the entire spectrum. If the light source is lacking the particular color of a surface, the surface color won’t show up correctly.

CRT becomes important in the combi nation of lighting and pigment color resulting in a D6500 look. If the light source has a CRT below 80, the chances are good that a multi-pigment back ground will look different in color than when lit with a bulb having a CRT near 100. In critical environment conditions it is important to use the ambient light source when evaluating the color of background material. You need to find a combination that will equal D6500 in appearance.

Fluorescent lamp colors are given names. In several cases there is consistency in color of light among manufacturers for a given color name and sometimes even consistency in CRT. Knowing the approximate color of the bulb, proper filter material can be chosen to correct the bulb color to the desired 6500° Kelvin, if you are having trouble finding D6500 bulbs directly. Some of the names, with colors and CRT5, are provided here. Two incandescent lamps have been included for reference.

The name “Warm,” as used to describe color of light, is usually below 3500° Kelvin while “Cool” colors of light are usually much above 3500° Kelvin.

The CRT has a scale of 0 to 100 and is the qualifier of the entire spectrum content for a given color of “white.” The index number specifies the degree which pigments will be rendered recognizable. Any “color” of white can have a maximum CRT of 100.

Use of incandescent bulbs is less than ideal as a neutral color light source. While their light output is easily controlled and the CRT is high, the color of the 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 determining factor of color.

Control of Ambient Lighting

In many situations, it will be desirable to have control over the light output from the bias or back ground lights. Fluorescent lamps can be dimmed to about 25 percent of their full light output without flicker or change in color. That makes them ideal for use in highly controlled ambient light applications.

Controlling the light output of a fluorescent lamp is not quite as easy as putting a dimmer on an incandescent bulb. The fluorescent lamp quickly starts to flicker when dimmed from a standard fixture. There are special ballasts for fluorescent lamps that will allow a very large range of control of light output without flicker. A professional righting supply center or contractor may have to be found in order to get information on dimmable ballasts. That application won’t reach the consumer market until energy conservation becomes a bigger issue. Dimmable ballasts for fluorescent fixtures do exist and are in common use in many video production and post-production facilities. At the moment, they are still expensive. Our sample units were well over $100 each.

Fluorescent lamps with a high CRT, that produce the proper color of light, without being filtered, are most easily available in four foot or 18-inch lengths. They require a large housing that will hold a dimmable ballast. The four foot bulbs produce much more light than is normally needed in a home environment. There are a variety of fixtures available that can accurately control the area being lit. As lighting fixtures go, these are among the more expensive, in the order of several hundred dollars including the dimmable ballast. 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 work properly.

Getting Accustom to the New Look

A note of caution to the viewer in 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 people, and seeing much more red from others. Within one or two weeks there should be a general consensus of perception among all the viewers, the pictures look a lot better and far more consistent. During this transition period in particular, share ideas with other viewers about the changes being noticed. Use the CIE diagrams in the disc(s) to help analyze the changes everyone is seeing. You’ll learn a great deal about color in the process of making these changes.

General Expectations and Compromises

Placement of the monitor and viewer within the room is well defined in the “Ideal Viewing Environment” animation in Chapter 5 of A Video Standard. The information in the LaserDisc, taken from the SMPTE Recommended Practice document for Monitor Environment, evolved out of the human factors research mentioned earlier. 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. A different look at the same topic can be found in Chapter 1 of Video Essentials.

Seemingly, major changes to these rules will be made for front projection video systems and well setup rear screen sets. Knowing the reasons behind the rules of the ideal viewing environment will help make these apparent discrepancies easy to under stand. Viewing distance and light output from the display device are the keys to the changes. In high quality, line doubled front or rear screen applications, the picture makes up a large portion of your field of view. You are no longer subjected to the small light spot in the middle of a dark environment. The light output from a large screen projection system is also fairly low, say in the order of 10 foot-Lamberts or less. This light level is usually not high enough to cause problems with fatigue.

When putting together large rear screen viewing systems, without line doubling, provisions may have to be made for a bias light on the wall behind the system. This could also be true if light levels from any front or rear screen system are above 15 foot-Lamberts. Once again, the size of the picture, as a percentage of your field of view becomes important.

While D6500 lighting is ideal, almost anything in the daylight range is good enough for anything but critical viewing applications. Keeping the intensity below 10 percent is critical.

There are times when other lighting in the room will be necessary. The color of that lighting should be nearly the same as used behind the set. Its intensity should be just enough to accomplish the task of reading or seeing controls. The directivity of the lighting should be away from the display screen. Down lighting on a surface angled away from the display is a good approach.

If there is room for compromise in the color of light in consumer applications, is there also room for differences in the pigment? Well, maybe a little. When discussing the color of the surface, anything that is defined as Nearly Neutral will work just fine in non-critical applications. Certain combinations of daylight lighting and nearly neutral surface colors can skew the environment too far away from the aim point. You will need to be careful in making compromises.

Extended Application of the Ideal Viewing Environment

What we’ve covered so far, amounts to an introduction to the role the environment plays in picture quality in television. In reality this information applies to any display format. As we’ve seen, some of the numbers will change based on picture detail, amount of light, and color of gray used in the display device. Properly setting the gray scale and viewing environment of a computer monitor is just as important as it is with watching video. If this type of information were more widely known, our television system would appear far better than it does today. Getting the best picture quality from your investment in any video format requires more than spending money and pressing PLAY.

Any future television system will be subjected to the same problems we are currently encountering with our “Never Twice The Same Color” system if we don’t pay attention to the environment of the picture.

[Adapted from Widescreen Review--Imaging Science Theatre 2000 (1998)]