White Balance Control Techniques in Digital Cameras

White balance control is a problem that must be noticed and most easily overlooked in digital camera shooting. With the increase in automation, technical indicators have become increasingly specialized. The concept of white balance is not available in ordinary cameras because the film has been fixed, and only the CCD is used as a photosensitive element. Usually it is automatically controlled, but as a semi-professional use, it is better to have white balance manual control function, in order to shoot a lot of unexpected results. Digital cameras detect and capture images by a series of photodetectors with color filters, but it is not as accurate as human eyes. In order to make the color reproduction of the image correct, we must correct the white balance to ensure the fidelity of its color reproduction.

When using a digital camera without white balance, people will find that the fluorescent light looks white, but the color of the image taken with a digital camera is a bit greenish. Similarly, if it is under incandescent light, the color of the image will be obviously reddish. The reason why human eyes regard them as white is because the human eye has made corrections. Therefore, the color of the image captured by the digital camera must be exactly the same as the color seen by the human eye, and hope cannot be pinned on the CCD body. This is because the CCD sensor itself does not have such a function, so it is necessary to perform the signal output from it. A certain correction, this correction is white balance control.

White balance control, also known as color balance, brightness correction, is the white formed by the multi-color mixture as a standard, the purpose of which is to restore white objects in the scene to white in the screen. It is also through such image adjustment that the colors of the photograph taken under various lighting conditions are in close agreement with the colors of the scene seen by the human eye.

To sum up, there are two reasons for digital camera white balance adjustment: First, because the sensor itself does not have the same sensitivity to the visible spectrum, it will inevitably produce a hue difference in the white background. Second, the human eye can adjust the hue difference according to the illuminance. Even if the white object can be restored to the same white color under different color lighting, it is necessary to make the digital camera have such an adjustment function, otherwise the image will appear to have A color projection.

The operation principle of the white balance control in the digital camera: By controlling the white balance, it is guaranteed that the white in the scene is restored to white in the image. If the color components are equal, the printer and monitor will output white. In other words, when the RGB signals of the computer monitor are equal, that is, R=G=B, the monitor displays white. So for a captured image, white balance is obtained when the color signal data is equal. The white balance is achieved on the white elements in the background (or in other related color spaces) R=G=B. If the color data in the white elements on the background are not equal, white balance correction is needed to make the signals. Equal, the image achieves white balance. The digital camera white balance control can be performed as follows: Rwb=A1×R0, Gwb=A2×G0, Bwb=A3×B0.

Here, Rwb, Gwb, and Bwb are white balance output signals, and R0, G0, and B0 are RGB output signals of the digital camera before performing white balance, and A1, A2, and A3 are coefficients that make the color signal components equal. It is based on whether the white balance is adjusted before or after the color insertion. There are two independent green signals, one for each green pixel in the Baye pattern and the other for each white balance coefficient.

The key technology of white balance control in digital cameras is how to select color channel coefficients or gains. In white balance correction, people often find a piece of white paper as a white balance correction jam. In fact, this is unscientific. Because some white paper is in the manufacturing process, manufacturers add some fluorescent materials artificially to show the whiteness of the paper. Since these fluorescent materials absorb light of certain wavelengths in white light, the light reflected by these white papers is no longer white light. After the digital camera calibrates the white balance using the white paper as a reference, the captured image will inevitably cause a color cast. The correct method is to capture a white image card or other white element, check the digital camera's output, and calculate the corresponding coefficients or channel gains that make the color signals equal.

White balance correction can be implemented as a post-processing step. In order to equalize the color signal to restore the white background, the white balance coefficient needs to be calculated. The color signal is multiplied by these coefficients to make the final image appear white. Each image needs to be processed as such. In the Motorola Image MOS array, the white balance is achieved by adjusting the gain of each color signal so that there is no need for post-processing of the image, and the white balance coefficient can be converted into a corresponding color channel gain before the exit of the sensor array sheet. , adjust the color signal.

What needs to be explained here is that the white balance needs to be adjusted under different illuminances. The digital camera is in a white balance state outdoors, and it is not white balance under indoor lighting, and vice versa. Most users do not often or even never perform white balance adjustments, so digital cameras need to be able to make white balance adjustments automatically. To achieve this function, the digital camera needs to be able to calculate the white balance coefficient, or can set the white balance under different lighting standards and program this information. The image MOS sensor can implement any of the above functions, can post-process the white balance, or program the color channel gain. The white balance coefficient or color channel gain is controlled by the white balance automatic algorithm and can be modified at any time.

In general, digital cameras usually preset automatic, daylight, cloudy, fluorescent, incandescent, and other adjustment methods. In actual use, setting them to be consistent with the light camera at that time can basically maintain the hue. In addition to these kinds of settings, the more advanced products have also added manual white balance methods to deal with the more complex situations of light, which can be of great help to those who are more professional.

The digital cameras produced by various manufacturers now have both automatic white balance correction and manual operation. However, even if it is done automatically, its correction ability is also different. Therefore, the author believes that if a digital camera has both manual and automatic modes and has multiple white balance control modes, that is the best choice.

Researchers at Motorola Labs have measured the white balance coefficients and color channel gains under typical illuminance through repeated experiments and used it to achieve basic white balance (these settings are used with RGB image MOS sensor arrays). The user can refer to these values ​​when making white balance adjustments. However, it should be noted that these values ​​are only approximations. They use special lights from Motorola Labs. If different IR filters or spectra are used, these values ​​will change.


Song Xingping

Source: Digital Printing January 2003 No. 1