I. Color Constancy
White balance is based on the phenomenon of color constancy in human vision. Color constancy refers to the ability of the visual system to maintain a stable perception of object colors under illumination with different spectral distributions.

As shown in the Figure 1, when the color temperature of the light source changes from warm to cool, a white garment is still perceived as neutral white. This perceptual phenomenon is known as color constancy.

Figure 1 Demonstration of color constancy: A white garment is consistently perceived as neutral white under different light sources (tungsten light, natural daylight, and overcast shade).

II. Sensor Response Under Different Illuminants
Unlike human vision, camera sensors lack color constancy and only passively record physical light signals. For RGB sensors, the RGB values of white objects vary with the spectral power distribution of the illuminant.

Figure 2 Recorded RGB values of a white garment under tungsten light, natural daylight and overcast shade, illustrating the color shift captured by RGB sensors across different illuminants.

III. Image Reference White Point
When viewing images on a display, the human visual system cannot obtain the necessary environmental information because displays are self-luminous devices that are independent of surrounding illumination, causing the color constancy mechanism to fail.

Therefore, displays require a fixed reference white point. To render neutral colors such as white or gray accurately, the condition R=G=B must be satisfied.

When viewing images captured under different illuminants directly on a display, the white garment appears in three different colors due to variations in its RGB values. In contrast, when observing the same garment in the actual scene, it remains white regardless of the lighting conditions.

IV. Automatic White Balance — Enabling Color Constancy in Cameras
Given that image sensors lack the color constancy mechanism inherent in human vision, digital color cameras are typically equipped with an automatic white balance (AWB) function.

This function automatically adjusts the proportional relationships among the pixel values of the three RGB channels through algorithmic processing, ensuring that the RGB values corresponding to neutral colors in the scene become equal. Consequently, these neutral colors are accurately rendered as neutral tones on displays, thereby simulating the color constancy mechanism of human vision.