The environment we live in is vibrant and colorful, featuring a red umbrella, a yellow beach chair, or a clear blue sky. White balance is utilized in cameras to ensure accurate color reproduction of subjects during capture. In addition to automatic color adjustment, semi-automatic and manual adjustments are also available.

What color exactly is a beam of light? In physics, light is a portion of electromagnetic radiation and consists of different wavelengths. These are associated with different color perceptions. If the wavelength range is around 400-500 nm, the light appears bluish. If the wavelength range is 600-700 nm, it appears red. White light is not associated with any specific region; it is a combination of all wavelengths. This is clearly demonstrated when we see a rainbow.

1. Color Temperature

The color perception of an object may vary depending on the light source used. Why? Because each light source has its own spectrum, which dictates different color perceptions. The unit used to measure color temperature is Kelvin. It defines what color perception should be associated with each light source. The heating of a black body serves as the foundation for defining color temperature. As the temperature increases, the object emits different spectra, and the corresponding temperature at that time is the respective color temperature. As the temperature rises, we can perceive the color of the black body changing from red to white and then to blue. The color temperature of a light bulb is 2700K, while a sunny day corresponds to 5500K.

2. Human Eye and Camera

For color vision, the human eye possesses photoreceptors that are highly sensitive to light within a certain range of color wavelengths, namely visible light (R/G/B). Combined with our own experience, we can easily perceive and distinguish colors. We know that a white wall is white. Regardless of the color temperature of the light source, our eyes, with the help of the brain, can always automatically recognize colors.

Although we have the ability to perceive correct colors, cameras do not. Color filters and corresponding processing procedures are used to obtain a color image. Most cameras have a Bayer pattern filter in front of the sensor. This way, light information is distributed to the corresponding color channels. During image processing, colors are calculated. Since the sensor cannot recognize color temperature based on prior experience, it certainly cannot automatically adjust colors. Therefore, there are always color errors in captured images. Let's take the white wall as an example again. If we photograph the white wall at noon and at sunset, the white wall will exhibit different color casts, as it cannot automatically adjust to adapt to the prevailing colors.

If the camera's white balance is set to sunny, a pale yellow cast will appear in the afternoon, and a reddish cast at sunset. During image processing, color adjustments must be made depending on the time of day or the color temperature of the light source; doing so prevents the aforementioned situations. White balance is adjusted according to the current light source.

3. White Balance Modes

Camera White Balance Measurement In addition to automatic color matching, there are other options such as presets or manual white balance adjustment. Additionally, most cameras allow setting the color temperature based on the current light source and using a gray card for custom white balance. Through these methods, white balance can be used to correct colors.

Auto White Balance (AWB) With auto white balance, the camera adapts to the current light source. It focuses on the brightest area in the scene, assuming it to be a white or gray reference. Based on this adaptation, color adjustments and color temperature evaluations are performed. Fundamentally, auto white balance is a reliable method that provides accurate results. Occasionally, auto white balance fails to meet our expectations. Situations such as mixed light sources, artificial lighting, or generally low light conditions make white balance coordination more complex. Furthermore, problems can arise in scenes dominated by a single color. However, in these scenarios, we recommend using presets, manual, or custom white balance to achieve the desired results.

Semi-Automatic and Manual White Balance We must first consider the options provided by the camera. Almost all camera models provide presets for the most common light sources. Examples include sunny, cloudy, shade, halogen lamp, and fluorescent lamp. Each preset corresponds to a specific color temperature. Photographers can adjust the white balance in any given situation to reflect accurate or aesthetically pleasing results.

The following three images were shot under mixed lighting with different white balance settings, using a Sony DSLR camera. Generally, cameras from other manufacturers also have basic white balance settings. However, specific presets may vary.

In many cases, the corresponding color temperature value can also be entered manually. If the current lighting conditions still affect the resulting colors, another method can be chosen—custom white balance.

Using the custom function, the photographer uses a white or neutral gray card in a reference scene to measure the corresponding color temperature. Based on this measurement, the camera can determine the color temperature under the lighting conditions and apply it to subsequent shots.

4. White Balance Testing
To evaluate whether the white balance of an imaging system meets the standards, it is necessary to photograph professional test charts in a standard image quality laboratory and obtain data results through software analysis to check the white balance values. Therefore, Yanding recommends using the 24-color chart for testing and analysis. The color patches of the ColorChecker test chart include chromatic, primary, and grayscale patches, etc. Most of these patches represent natural objects, such as human skin, foliage, and blue sky. Since they represent the colors of their respective counterparts and reflect light in the visible spectrum in the same way, they can be used not only for color reproduction but also for cameras to create white balance, ensuring standard white is produced under any lighting conditions.

4.1 Equipment Selection:
The test equipment used for white balance testing includes the vertical LS-CCXL-2S06-IR Vertical Multi-Color Temperature Fill Light Source and the TC-24Color 24-Color Test Chart.

4.2 Test Procedures

1. Attach the color chart to the neutral gray backboard, centered left and right, and aligned horizontally;

2. Adjust the light source color temperature and brightness level to the target illuminance conditions (Illuminant A: 30 Lux, 800 Lux; TL84: 30 Lux, 800 Lux; D50/D65: 30 Lux, 800 Lux; if conditions permit, D75 can be verified: 30 Lux & 300 Lux). Normally, 6500K and 1000 Lux can be tested, ensuring that the surface uniformity of the test chart reaches over 90%;

3. Fix the device in the fixture. Check whether the device lens is dirty; if so, wipe it with a lens cloth. If the lens is damaged, the device must be replaced;

4. Level the tripod and pan-tilt head, and adjust the frame to ensure the image covers no less than 1/2 to 2/3 of the frame. The lens surface and the chart surface should be relatively parallel, and the frame should be symmetrical vertically and horizontally;

5. When shooting, wait until the frame is stable before capturing. It is recommended to capture 3 sample images.

4.3 Software Analysis
1. Open the RIQA image quality testing software, click on the color test item, and select the 24-color test according to the captured image.

2. After importing the image into the RIQA image quality testing software, select the corresponding chart type and color reference value file, then click “Start”. The software will automatically frame the analysis area. The framed area should not be less than 38*38 pixels, or the analysis area can be selected manually;

3. Click “Analyze”, and the results are shown in the figure below.

The test results include multiple metrics, such as image saturation, color difference, signal-to-noise ratio, color coordinate analysis charts, etc. PDF, TXT, and Excel reports can be saved to view the test results and diagrams.

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