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2.3 The digital camera 75

where again, (X n ,Y n ,Z n ) is the measured white point. Figure 2.32i–k show the L*a*b*
representation for a sample color image.

Color cameras

While the preceding discussion tells us how we can uniquely describe the perceived tri-
stimulus description of any color (spectral distribution), it does not tell us how RGB still
and video cameras actually work. Do they just measure the amount of light at the nominal
wavelengths of red (700.0nm), green (546.1nm), and blue (435.8nm)? Do color monitors just
emit exactly these wavelengths and, if so, how can they emit negative red light to reproduce
colors in the cyan range?
In fact, the design of RGB video cameras has historically been based around the availabil-
ity of colored phosphors that go into television sets. When standard-definition color television
was invented (NTSC), a mapping was defined between the RGB values that would drive the
three color guns in the cathode ray tube (CRT) and the XYZ values that unambiguously de-
fine perceived color (this standard was called ITU-R BT.601). With the advent of HDTV and
newer monitors, a new standard called ITU-R BT.709 was created, which specifies the XYZ
values of each of the color primaries,

X 0.412453 0.357580 0.180423 R 709
⎡ ⎤ ⎡ ⎤ ⎡ ⎤
Y = 0.212671 0.715160 0.072169 . (2.108)
⎣ ⎦ ⎣ ⎦ ⎣ G 709 ⎦
Z 0.019334 0.119193 0.950227 B 709
In practice, each color camera integrates light according to the spectral response function
of its red, green, and blue sensors,

R = L(λ)S R (λ)dλ,

G = L(λ)S G (λ)dλ, (2.109)

B = L(λ)S B (λ)dλ,

where L(λ) is the incoming spectrum of light at a given pixel and {S R (λ),S G (λ),S B (λ)}
are the red, green, and blue spectral sensitivities of the corresponding sensors.
Can we tell what spectral sensitivities the cameras actually have? Unless the camera
manufacturer provides us with this data or we observe the response of the camera to a whole
spectrum of monochromatic lights, these sensitivities are not specified by a standard such as
BT.709. Instead, all that matters is that the tri-stimulus values for a given color produce the
specified RGB values. The manufacturer is free to use sensors with sensitivities that do not
match the standard XYZ definitions, so long as they can later be converted (through a linear
transform) to the standard colors.
Similarly, while TV and computer monitors are supposed to produce RGB values as spec-
ified by Equation (2.108), there is no reason that they cannot use digital logic to transform the
incoming RGB values into different signals to drive each of the color channels. Properly cal-
ibrated monitors make this information available to software applications that perform color
management, so that colors in real life, on the screen, and on the printer all match as closely
as possible.