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Grayscale Controls (White Balance) Print
Written by The ISF Forum Library Community   

Applies To:  All Display Types

Grayscale Controls are required to allow adjustment of the levels of red, green and blue light that collectively produce the shades of gray that range from video black to reference white. Grayscale is also sometimes referred to as Color Temperature or White Balance, and while these three terms are not completely interchangeable, they all relate to how a television produces the "color of white" from a mixture of the red, green and blue primary color components.

Grayscale fidelity is a critical factor in determining the picture quality of a display, because most of the resolution or detail in a color video image is made up of grayscale information -- black (the absence of light), white, and all the shades of gray that exist between video black and the reference white level.

But before talking in any detail about the controls required to properly adjust grayscale, we need to delve into grayscale a bit deeper.

Grayscale in More Detail

Grayscale may seem irrelevant when you view a color image in all its chromatic glory, but its importance can be readily illustrated by lowering the TV's color control to the minimum level. What remains when the color signal is muted is the grayscale image, an picture that contains virtually all of the detail in the original color image but with no apparent color.

But is that grayscale image really devoid of color?

The answer is no -- in fact, all video displays create grayscale by combining red, green and blue light. And while there are millions of potential combinations that will produce what appears to be white or gray images, there's only one agreed, standard way to accomplish this. The correct color of white has been determined by the Society of Motion Picture and Television Engineers (SMPTE), and that standard is referred to as D6500. All films and television programming produced around the world adhere to the D6500 standard throughout the production process.

In effect, grayscale is the foundation upon which a full-color image is created; as a result, if a display's grayscale foundation is skewed toward blue, or red or green (or some combination of those colors), then the color error in the grayscale image will carry through when color is overlaid onto that grayscale foundation. And those grayscale errors will not be correctable through adjustment of the color or tint controls provided in the user menus for the display.

D6500 versus 6500K

It's a fairly common misunderstanding that 6500 Kelvin color temperature and D6500 grayscale are one and the same thing. In fact, they are very different.

When discussing the human perception of color, the standard reference is the CIE Chromaticity Diagram, created by the International Commission on Illumination in 1931. CIE 1931 captures in graphical form all of the colors visible to the human eye.

On the CIE 1931 chart, there is a 6500 Kelvin color temperature line that runs near an area labeled "purple" or "reddish purple," up through "the black body curve" and on to an area of CIE 1931 labeled "yellowish green." All of the points along that 6500K line are equal in terms of the balance of red and blue light, but as you move from the bottom of the chart (near purple or reddish purple) toward the top of the chart (yellowish green) you are adding progressively more green light into the mix.

D6500, on the other hand, is a single point on this 6500K line, and it's located just above an area of the 1931 chart known as the black body curve.

When it comes to grayscale fidelity, the distinction between D6500 and 6500 Kelvin color temperature can be absolutely massive or relatively insignificant. However, without knowing exactly where a particular shade of gray falls on or near the 6500K CIE line, you can't make a definitive determination of grayscale accuracy.

A more simplistic, but hopefully helpful, way of explaining the difference: 6500 Kelvin is to D6500, what France is to the location of the Mona Lisa painting in the Louvre museum in Paris. While both Marseilles and Calais are cities within the boundaries of France, neither is a particularly good vantage point for appreciating the beauty of Michaelangelo's masterpiece. In the same way, just because a display is aligned to 6500K doesn't mean it will produce the correct color of gray -- a display must be at or very near D6500 to produce correct grayscale.

Controls Required for Grayscale Calibration

To calibrate a display to D6500 requires, at minimum, the ability to adjust two of the three primary colors (red, green and blue) that combine to create grayscale. Depending on the display, there may be three color controls for the dark end of the grayscale range (typically referred to as “cutoffs” or “bias”) and three for the white end (“drives” or “gains”) -- but controls for at least two colors at each end of the range is the minimum needed to properly align grayscale tracking.

These “cutoff” and “drive” controls are iteratively adjusted until the best possible white balance is achieved, in relation to the D6500 target point, from video black to peak white.

It should be noted that it's virtually impossible to achieve absolute D6500 accuracy from near-black to peak white across the entire grayscale range; however, it is still vitally important for color fidelity for each point within the grayscale range to be as close as possible to D6500.

There are a variety of measuring tools available, such as Delta E, that are used to describe the degree of grayscale error that may occur with a display device.
  
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