A red umbrella, a yellow beach chair or a clear blue sky - our environment shines in colorful diversity. The white balance is used in digital cameras to capture subjects in the right colors. Different semi-automatics and manual controls are usually available in addition to the automatic color-adjustment.
What color does light actually have? In physics, light is described as a part of the electromagnetic radiation and has different wavelengths. These are associated with diverse color perceptions. So light appears bluish, if its wavelength is in the range of approximately 400 to 500 nm. A wavelength of about 600-700 nm, however, delivers a red impression. White light is not associated with any particular area because it is composed of light from all wavelengths. This becomes clear when we look at a rainbow.
Depending on the light source used, the color perception of an object may vary. But why? Each light source has its own color spectrum, which leads to a certain color impression. The Kelvin unit is used for measuring color temperature. This defines which color impression is associated with a light source. The heating of a black body is used as the basis for color temperature. The body emits different color spectrums with increasing temperature, which in turn brings a corresponding color impression with it. By increasing the temperature, we would perceive the black body turning from red to white to blue. Whereas a light bulb has a color temperature of about 2700 Kelvin (yellowish), daylight corresponds to about 5500 Kelvin (white).
Eye versus camera
For color vision, our eye is equipped with receptors (cones), which are sensitive to the range of wavelengths that we call visible spectrum (R/G/B). Together with our experience, it is possible for us to perceive color and to classify it properly. We know that a white wall is white. Regardless of the color temperature of the light source, our eye automatically makes a color matching (chromatic adaptation) with help from the brain, such that, independently of the light source, the wall is perceived as white.
Whereas we have the ability to perceive colors correctly, it is a different case with digital cameras. Filters and corresponding processes are used so that the subjects can be saved as a color photo. Most digital cameras have a Bayer-Pattern filter placed in front of the sensor, through which the light information is assigned to the color channels. Thus, the color itself is calculated during the image processing. Since sensors cannot benefit from experience or recognize the color temperature, they also cannot adjust the color automatically. Therefore, captured images would be often color distorted. Let us consider the white wall again. If we photograph the wall at afternoon and again at sunset, the wall would have different colorcasts without adapting for the color temperature at the time.
If the camera's white balance is set to “Daylight”, a yellowish impression would arise at afternoon and a reddish at sunset. So that this does not happen, a color adjustment must take place within the image processing, depending on the daytime or the color temperature of the light source. The white balance is responsible for the camera adjustment to the present light source.
Image taken in the afternoon
Image taken at sunset
White balance methods
Besides the automatic color matching, different choices (pre-sets) and manual white balance (selecting Kelvin values) are often available. In addition, most camera models offer the possibility to set the color temperature in accordance with the present light source (custom white balance with a gray card). With these features, the white balance can be used for correct color reproduction and creative purposes.
Automatic white balance (AWB)
Within the automatic white balance (AWB), the camera determines the adaptation to the given light source independently. It focuses on the brightest points in the scene and adopts them as white or neutral gray. Based on the adaptation, the color adjustment and the assignment of color temperature takes place. Fundamentally, the automatic white balance is a reliable tool and provides accurate results. Every now and then, the automatic white balance might not meet expectations. Situations where mixed light, artificial light or generally low light exists could complicate the white balance coordination. In addition, scenes in which one color dominates can be problematic (i.e. at sunset). However, in these cases it can be recommended to either use a pre-set, manual or custom white balance to achieve the desired result.
Semi-automatics and manual white balance
Let us first consider the options provided by the camera. Almost all models offer pre-sets for the most common light sources including daylight, shade, cloudy, tungsten and fluorescent light. Every pre-set is linked to a corresponding color temperature. Photographers can adjust the white balance to reflect a more accurate or individual taste for any given light situation.
The pictures below show three different white balance settings at mixed light, on the example of a Sony SLT camera. Usually, the basic white balance settings are given at most cameras of other manufacturers. However, individual pres-sets may vary.
In many cases, it is also possible to enter the corresponding Kelvin value for the color temperature manually. If the existing light conditions still influence the resulting color, customizing the white balance is another option.
Using the "Custom" function, the photographer offers the camera a white or neutral gray card within the recording scene as a reference to its own measurement of color temperature. Based on this measurement, the camera can determine the color temperature of the light condition and apply it to subsequent shots.
Color and color
Each person perceives color somewhat differently. For example, a green apple does not appear to be the exact same green for every viewer. Color should be considered relatively, but should reflect the basic color perception. Because deviations appear unnatural to us, we expect photos to correspond to the color sensitivity of human perception. It is possible to selectively manipulate this and it should be used. Purely from an artistic perspective, color is often used as a design tool to produce a cooler image impression, for example.
Therefore, we may sometimes turn a blind eye to the color reproduction.