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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