In industrial production and quality control, color accuracy and consistency are paramount. The spectrophotometer calculates the internationally accepted Lab color space value by measuring the spectral data of the sample, so as to achieve objective and quantitative evaluation of color difference. Mastering how to interpret Lab values and their criteria is key to effective color management.
Lab color space vs. chromatic aberration formula
Lab color space is a uniform color space based on the visual perception of the human eye, which is composed of three coordinate axes: the L* axis represents the brightness, the a* axis represents the red and green direction, and the b* axis represents the yellow and blue direction. Chromatic aberration (ΔE) is calculated by calculating the geometric distance between the standard sample and the test sample in the lab space.
The most commonly used chromatic aberration formula is the CIELAB chromatic aberration formula, and its basic calculation method is as follows:
ΔEab = √[(ΔL*)2 + (Δa*)2 + (Δb*)2]
where ΔL* = L*Samples - L*Standard， Δa* = a*Samples - a*Standard， Δb* = b*Samples - b*Standard。 This formula calculates the total chromatic aberration. For more precise assessment of color shifts in different directions, more complex formulas such as CMC (l:c) or CIEDE2000 (ΔE) are often used in modern industrial standards00)。
Lab component values and chromatic aberration directions
When analyzing color difference, we cannot only look at the total color difference ΔE, but must judge the specific color bias based on the change direction of each component. The following table lists the color bias represented by changes in the values of each component:
| ΔL* is positive | Samples are lighter than standard (whiter) |
| ΔL* is a negative value | Sample is darker (darker) than standard |
| Δa* is positive | Samples are reddish (or less green) than standard |
| Δa* is a negative value | Samples are greener (or less red) than standard |
| Δb* is positive | The sample is more yellowish (or less blue) than the standard |
| Δb* is a negative value | Samples are more bluish (or less yellow) than standard |
This table allows you to quickly locate the specific direction of color deviation and provide clear guidance for process adjustments.
Reference for common industry color difference judgment standards
The requirements for color tolerance vary significantly between different industries and products. The criteria are usually based on the total chromatic aberration ΔE, and may set separate limits on the component differences of L*, a*, and b*. The following are common reference scopes for some industries, which should be subject to the technical agreements agreed by both supply and demand parties or relevant national and international standards.
| Textiles, printing and dyeing | ΔEab < 1.0 (demanding) |
| Coatings, plastics | ΔEab 0.5 - 2.0 (on request) |
| Printing, packaging | ΔEab < 3.0 (generally acceptable) |
| Exterior parts of the car | ΔE00 < 0.8 (extremely demanding) |
It is important to emphasize that the human eye has different sensitivity to different color regions, so a single ΔE numerical threshold is not absolute. For example, in neutral gray areas where the human eye is sensitive, even if the ΔE is small, it may be detected; In high-saturation regions, larger ΔEs are sometimes acceptable. Therefore, combining visual evaluation with instrumental data is a reliable practice.
For effective color control, it is recommended to establish the following process: First, determine the standard sample and measurement conditions (e.g., light source, observer angle, measurement aperture) that are mutually recognized with the customer or design department. Secondly, according to product characteristics and customer requirements, formulate clear internal control tolerance standards including total color difference and component difference. Third, calibrated spectrophotometers are used to regularly measure and record data on incoming materials, production processes, and finished products. Finally, when the color difference exceeds the tolerance, the reasons are analyzed according to the positive and negative directions of ΔL*, Δa*, and Δb*, and the adjustment of color matching or process parameters is guided.
The core of interpreting the Lab value and chromatic difference of a spectrophotometer is to understand the composition of the Lab color space, the meaning of the chromatic aberration formula, and the color bias indicated by the change of each component. An effective chromatic aberration standard must be developed in conjunction with specific industry practices, product requirements, and visual characteristics of the human eye. By systematically measuring, recording, and analyzing Lab data, enterprises can achieve digital and precise management of colors, thereby improving product quality and market competitiveness.
References
International Illumination Commission. CIE 015:2018 Colorimetry.
National Standardization Administration of China. GB/T 3979-2008 Measurement method for object color.
American Society for Testing and Materials. ASTM D2244-21 Standard Practice for Calculating Color Aberration Using Instrumental Measurements of Color Coordinates.
Bill Meyer, Saltzman. Principles of color technology.