in March 2003
Color Display Calibration
By Thomas Schulte, CET
Understanding how we perceive color, and how electronics
create color, helps produce consistent displays.
All color video display
devices share one critical calibration: color balance. This
is true whether the device displays standard NTSC signals,
standard computer-format signals, proprietary high resolution
signals or HDTV signals. To display the most accurate colors,
every video display’s color balance must be adjusted
to match the color balance of the signal source. This exact
balance of red, green and blue to make exactly the right
color of white and neutral grays in the picture (often called
White Balance) must be correct in both low brightness and
high brightness parts of the picture. To understand how
we see, specify and measure this color balance, let’s
review some basics of light and color.
Human Sight Characteristics
is electromagnetic energy within a narrow range of frequencies
that are higher frequency than microwaves, but lower frequency
than x-rays. If seen by itself, each different frequency,
or wavelength, of light energy is perceived by the human
eye/brain as a different, fully saturated, color. In a certain
respect, the eye/brain acts like a tuned, high-frequency
radio receiver (see Figure 1).
Three characteristics define the way the human eye/brain
sees light that is either radiated from or reflected off
- Brightness: The eye sees the total
amount of light energy radiated or reflected by an object,
in comparison to other light sources or lighted objects
in the field of view, as the brightness of the object.
- Saturation: A single frequency of light,
at any energy level, is seen as a fully saturated, vivid
color. The saturation of that color decreases as any other
light frequency, or any combination of light frequencies,
is added to the original light. If equal energy of all
visible light frequencies is added together, the result
is zero saturation, pure white light.
- Hue: For any combination of light energy
other than pure white, the dominant perceived frequency
of the combined light energy is known as the hue of the
specify different amounts and combinations of light energy,
we would like to measure light in a way that corresponds
closely to the way we see light. In the video-display industry,
two types of measurement units are used to measure light
and relate it to the human sight characteristics: luminance
• Luminance is a type of light measurement closely
related to the human sight characteristic of brightness.
The US measurement unit of luminance—foot-lambert—specifies
the number of lumens of light energy emitted from each square
foot of a light source, such as a video display, or reflected
off a solid object. (A footcandle, on the other hand, is
a measurement unit of illuminance, the light falling on
• Chromaticity is a type of light measurement related
to the human sight characteristics of hue and saturation.
This combined method of chromaticity measurement was developed
by the CIE (International Commission on Illumination) in
1931 and is used by the video industry for all display color
measurements. The CIE developed a chromaticity diagram that
graphically depicts the relationship between the hue and
saturation of light sources.
CIE Chromaticity Diagram (Figure 2) plots the pure spectral
colors (hues) around the curved border (380-780nm). The
results of mixing any of these fully saturated spectral
colors are shown at the base and interior of the diagram.
Any visible color can be specified by the x coordinate and
the y coordinate position of that color on the diagram.
CIE coordinate pair of x = 0.333, y = 0.333 (E) specifies
the white light produced by mixing equal light energy of
all wavelengths (zero saturation). The color of any point
immediately surrounding the equal energy white point would
also appear white, if seen by itself with no other color
would seem logical that equal-energy white (E) would be
used as the standard color of white for video display systems.
However, because a more bluish white appears brighter, the
CIE coordinate pair of x = 0.313, y = 0.329 (CIE standard
illuminant D65) is the white “color” that was
chosen as the standard white reference for all video and
computer display systems. This allows displays to appear
brighter without producing additional light energy output,
yet doesn’t shift the color of white enough to be
detrimental to color accuracy.
White From Red, Green, Blue
the CIE chromaticity diagram, if any three color points
are chosen, the area included by the connecting triangle
represents the range of colors that can be produced by mixing
the three chosen colors (Figure 3). The three points are
known as the primary colors. The connecting triangle encloses
the full range of colors (gamut) that can be created by
a display that produces mixtures of those colors of red,
green and blue light.
The Eye: A Tristimulus Device
human eye sees light through rod- and cone-type light receptors
(Figure 4). Red, green and blue cone-type receptors give
us color vision. The rod receptors give us black-and-white
vision, especially in small detail and low light.
of the red, green and blue cone receptors has a different
response to different colors (frequencies) of light. The
average response of the human eye receptors to light across
the visible spectrum is shown by the Standard Observer Response
graph, developed by the International Commission on Illumination
(CIE) (Figure 5).
call this tristimulus vision because there are three types
of receptors that individually send information to our brain
and allow us to perceive different colors for the different
mixtures of light energy within the visible spectrum.
Tristimulus Measuring Devices
color measurement devices are called colorimeters (Figure
6). This type of device works similar to the human eye.
Three filtered light sensors receive light from the source
to be measured. The filter for each light sensor allows
only a certain amount of each color of light to reach the
sensor. The response of each of the three filters is designed
to mimic the response of one of the types of cones in the
average human eye.
The measurement information from each of the three light
sensors is the same as the information from each of the
three types of cones in the eye, only in electronic-signal
form. This information allows us to compute a different
measurement result for the different mixtures of light energy
within the visible spectrum, in a way that duplicates the
response of the human eye/brain combination. To accurately
predict the response of the human eye to a combination of
light energy at different frequencies, a tristimulus color
measurement device must “see” light exactly
the same way the human eye sees light, as documented by
the CIE Standard Observer Response graph (Figure 5).
Display Technology Challenges
of today’s new display types—plasma, LCD, DLP,
LCOS, D-ILA, etc.—produces light energy with a spectral
power distribution (SPD) that is usually quite different
from the average spectrum produced by CRTs. This is a change
from working only with direct-view CRTs because all CRT
phosphor sets produced strong peaks of light and low levels
of light at pretty much the same color frequencies. Some
of the new display types produce strong peaks of light at
color frequencies where CRTs produce very little light.
Each of the new display types can still produce a standard
color of white by adjusting the relative balance of colors
in the red, green and blue portions of the spectrum (Figure
the new technology displays may produce strong peaks of
light at just about any color frequency, it is now critical
that a colorimeter’s optical filters must accurately
duplicate the CIE standard observer response at all color
frequencies, not just at the particular frequencies at which
direct-view CRTs produce high light output. Its three color
sensors must see light over the entire visible spectrum
with the identical amplitude response as the three color
sensors of our eye (Figure 8).
a colorimeter uses optical filters that are accurate to
the human-eye response at all frequencies of light, not
just high output CRT frequencies, the instrument will accurately
measure all displays of the past, present and future, no
matter what their SPD.
can be made to have accurate colors. Also, adjacent displays
can be made to produce the same accurate colors, whether
they are multiple projection displays in the same facility,
or the individual cubes of a video wall. Accurately measuring
and calibrating the color of each display is the key to
producing beautiful end results.
Thomas Schulte, an application
engineer at Sencore, Inc., for the past 16 years, has taught
and worked in the video industry for many years.