White Balance - Smartek UCC User Manual

User Manual - twentynine

8.2.4 White Balance

White Balancing is the process of removing unwanted color casts in digital images, which derived from one
important characteristic of visible light - the color temperature. The color temperature is defined by the
radiation emitted by a glowing "black body" and is measured in Kelvin (K). Since an image sensor converts
light to electrical voltage which then underruns multiple processing steps until a digital image is saved or
displayed on the screen, the color temperature of the light is visible on the digital image in form of color
casts appearing to the human eye.
Figure 100 illustrates the color temperature spectrum of visible light in the range from 1000K to 15000K.
Figure 100: Color temperature spectrum in range 1000K to 15000K
Digital images which are captured in lower color temperature conditions (candle light, tungsten light) tends
to be reddish or reddish orange. The higher the color temperature (overcast sky) the more blue light will
outweigh, the digital image appears more bluish.
To fully describe color casts in digital images a tint adjustment is also required. While the color temperature
determines the warmth or coolness of an image, the tint defines the balance between magenta and green
color casts.
Figure 101 shows two images of a color checker chart. The image in the left shows the original values of
the patches while the color checker on the right is captured by a camera at day light condition. If we look at
the last row of the color chart on the left image, the Gray color fields tend to be green.
Figure 101: Comparison original color (left) and camera image (right) of a ColorChecker chart
Human eyes can automatically correct this effect, for a camera to compensate this effect automatic
white balancing is needed to accurately balance color. The white balancing feature implemented in the
ImageProcAPI adjusts the weighting for each color channel using digital gains in order to remove the
unwanted color casts.
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