 Figure 1A - A projector's native color point is defined by X and Y coordinates. Those coordinates are mapped to a color point using this CIE1931 spectral chart. (Click image for a larger view.) |
Most display applications require imaging performance that meets specific color criteria. Whether you're working on a permanent installation or a staged event, generally you must make projector calibrations to achieve the required color criteria.
Color adjustment capabilities are found in various forms in different projection devices. Knowing about your projector's capabilities and how to use them will help ensure you achieve ideal color performance for every application.
What is native color?
“Native” color (i.e., color temperature) is the color point, or white point, of the projector without any user adjustments applied. The native color point of a projector is defined by a set of X and Y coordinates, as measured by an illuminance/color meter. Those coordinates are mapped to a specific color point using a CIE1931 spectral chart. (See Figures 1A and 1B.)
Before attempting to make any color adjustments, it is important to know the native color point of a projector to be sure that it fits the needs of your application, and that the projector will require minimum adjustment.
Depending on the application and the native color point of the display, minimum projector adjustment sometimes is not realistic. Projectors that have a native color point notably different from your application's target color point will require significant adjustment, which in turn, will affect other performance aspects of the image.
In the case of LCD, DILA, or DLP projection systems, the native color point can be influenced by the spectral output of the lamp, reflector, optics, spectral filters, and the image modulator. The ability to adjust color and grayscale depends on the electronics of the projector. Ideally, the electronics should not have any effect on the native color point of the display, with all adjustments set at factory default levels.
Because of the nature of color adjustments in projectors, any deviation from the native white point to any other white point will result in reduction of light output. This is because changing the white point requires one or more of the red, green, or blue primaries to be reduced. The reduction of the primaries decreases light output. The amount of light reduction can be nominal or significant, depending on the amount of adjustment required to achieve the desired color point.
The reduction in light output also reduces the projector's total dynamic range (contrast ratio), as well as color gamut. Conversely, when a projector is operating at its native color temperature, it is performing at the maximum lumen, contrast, and color gamut possible. Thus, it is best to select a projector with a native white point that closely matches the requirements of your application.
Determining the white point
Now that we know the definition of native color point, we can apply it to the color performance criteria required for specific applications.
While a film presentation may be specified to achieve a 5400-degree Kelvin (SMPTE196) color temperature, a broadcast facility may target 3200 degrees K for studio specifications. When viewing graphics, 9300 degrees K may be defined as the target for the material.
 Figure 1B - Video content is usually specified to display at 6500 degrees Kelvin, as shown in this color chart. (Click image for a larger view.) |
Video content is usually specified to display at 6500 degrees K. There are also scenarios when the color point is defined from an artistic point of view to provide effect or to yield a certain “look.”
Thus, color temperature requirements can be as varied as applications. The simplest solution is to talk to the customer, producer, cinematographer, or graphic artist to gain a firsthand understanding of the color performance needed for their particular application.
Dialing in a target color
On most projectors you can adjust the red, green, and blue primaries to create the proper mix of R, G, and B light to attain a range of target white point values. As mentioned previously, if you choose a projector with a native color point as close as possible to your application target, a minimal amount of adjustment will be needed. This will preserve other vital image quality parameters, such as light output, contrast ratio, and color gamut.
To adjust the projector, you will need a colorimeter to find your desired color point. To make adjustments easier, it's a good idea to become familiar with the adjustment menus in the projector. For example, if a single-increment increase of the red gain scale causes a change of .004 in the X value, simple math concludes that a .032 variance from the color target along the X-axis will require the scale to be incremented eight steps.
A good rule of thumb is to make all adjustments after the projector has warmed up. Warm-up times usually vary between projector models, so it may be wise to wait 20 minutes before starting to calibrate the projector's colorimetry. This warm-up time should allow any lamp type or projector model to run up to its full operating temperature.
For displays that use a lamp as the illumination source, the type of lamp will play a major role in defining the projector's native color point. As an example, Xenon arc lamps usually perform in the 6100- to 6500-degree Kelvin range. By comparison, metal halide lamps tend to exhibit native color in the 7000- to 8000-degree Kelvin range.
It's important to remember that the colorimetry requirements of an application not only involve the target color temperature, but also the color gamut, or spectral field of colors. Xenon lamps (see Figure 2) have a flatter and broader spectral output in the visible light region than other lamps (and thus have been chosen as the lamps for digital cinema).
 Figure 2 - Lamp types play a major role in defining a projector's native color point. For example, xenon lamps have a flatter and broader spectral output in the visible light region than other lamps. (Click image for a larger view.) |
Metal halide lamps (known as UHP, HTI, HID, and so on) require smaller and less expensive power sources, and are very efficient in raw lumens/watt performance. They also offer long lamp life, which lowers operational cost of ownership. Unfortunately, metal halide lamps are known to have deficiencies in reproduction of visible red light frequencies, and also exhibit exaggerated yellows. (See Figure 3.)
Depending on the projection technology and lamp type, periodic re-calibration of color may be needed to correct for the lamp's aging and/or optical system components. It's hard to give a standard length of time for when to make readjustments since there are so many potential system variables. Therefore, it's best to evaluate the color frequently until the system's aging characteristics are understood.
It's also important to consider the effect of the projection surface on the targeted color point. In cases where the screen surface is not neutral, noticeable color shift may appear across the entire image, or distorted colorimetry may be a problem for viewers who are positioned off-center to the projector/screen axis.
Choosing a projector
It's not likely that the projector you've picked will perfectly match an application target white point, but clearly some projectors are better suited for specific applications than others. While most manufacturers include information on the native color point of their displays, many still do not. To ensure the best color performance, never use a projector without knowing its native white point.
In situations where no projector is closer than another to the required color temperature, the goal should be selecting the one with the most flexible and accurate color calibration. The parameters to look for are total range, granularity, and linearity. Range is the extent to which adjustments can be made, granularity is the fineness of steps in the range, and linearity is how well the adjustments track across the range.
 Figure 3 - Metal halide lamps are very efficient in raw lumens/watt performance and have long lamp life. Unfortunately, they are also known to have deficiencies in visible red light frequencies. (Click image for a larger view.) |
A good way to benchmark the performance characteristic of a projector is to measure the its output with an illuminance/incident color meter. This type of meter reads the light output of the projector, as opposed to the light reflecting off of the screen surface. To determine the color performance of the image as reflected by the screen, a luminance/color spot meter is used. Since viewers ultimately see the image as reflected by the screen, making final adjustments with a spot meter ensures that, from the viewer's perspective, the image has the correct colorimetry.
The most important thing to remember about native color performance is that excellent results can almost always be achieved. It simply takes a good understanding of the user's requirements. With a bit of product research, some technical know-how, and if necessary, a little help from industry experts, color performance can be correctly calibrated for each and every application.
Keith Frey is applications engineering manager for Digital Projection Inc. Frey has more than 15 years of experience in the commercial A/V industry, working for both projector manufacturers and professional integration companies. He currently manages DPI's training program and is involved in product development, most recently on Digital Projection's new Mercury series of projection systems. Email him at [email protected].