Color Tolerance Explained: From MacAdam Ellipses to ANSI C78.377 and (u',v') Circular Color Tolerance Standards
Color tolerance is an important parameter in LED lighting product testing, which characterizes the degree of deviation between the light source color and the standard color. This article will explain in detail the concept of color tolerance, the relationship between correlated color temperature and color coordinates, MacAdam ellipses, color tolerance standards (including ANSI C78.377 standard, (u',v') circular color tolerance, etc.), and industry requirements to help everyone better understand and apply this important indicator. Yiming Technology, as a professional LED lighting testing equipment manufacturer, has rich experience and technical accumulation in the field of color tolerance testing.
I. What is Color Tolerance?
Standard color matching deviation, also known as color tolerance, characterizes the difference between the measured x, y values and the standard coordinates, generally represented by ellipses. The smaller the SDCM value, the closer the product's color coordinates are to the standard value, the smaller the difference between the spectrum emitted by the light source and the standard spectrum, the higher the accuracy, and the purer the light color.
Figure 1: Color Tolerance Diagram
II. Relationship Between Correlated Color Temperature and Color Coordinates
To answer the customer's question: Why do luminaires have similar color temperatures but very different color tolerances? The reason can be seen from the conversion formula between correlated color temperature and color coordinates:
- Color coordinates and color temperature have a one-to-many relationship, the same color temperature has different xy values.
- The same color temperature can produce different color sensations.
The conversion formula between color temperature and color coordinates is:
T = -437n³ + 3601n² - 6861n + 5514.31
Where: n = (x - 0.3320) / (y - 0.1858)
T is color temperature, n is coefficient, x, y are color coordinates
From this formula, it can be seen that color coordinates and color temperature have a one-to-many relationship, and the same color temperature can correspond to different color coordinate values. This explains why luminaires have similar color temperatures but very different color tolerances. Color tolerance reflects the deviation between color coordinates and standard coordinates, not just the deviation of color temperature.
III. MacAdam Ellipses and the Meaning of "Step"
To answer the customer's question: How did the ellipses in color tolerance standards come about? What about color consistency?
When a supplier asks whether you need LED light sources within a 4-step MacAdam ellipse or a 5-step MacAdam ellipse, how should you choose?
1. MacAdam Ellipses
Scientists MacAdam selected 25 representative color points on the x-y chromaticity diagram and studied the maximum range where the human eye cannot detect color changes. The results were ellipses of varying sizes and aspect ratios, where points on the ellipse are all 1 SDCM away from the center point, called MacAdam ellipses. Although these ellipses differ in size and orientation, they can all be represented by the equation:
g₁₁dx² + 2g₁₂dxdy + g₂₂dy² = SDCM²
The color coefficients in the formula can all be found in the ANSI C78.376 standard.
Figure 2: MacAdam Ellipses Diagram
2. Meaning of MacAdam Ellipse "Step"
MacAdam ellipses are usually described by "steps", where "step" refers to standard deviation. A 1-step MacAdam ellipse refers to the standard deviation of color matching results at 1 times the distance from the target color. Similarly, the meanings of 3-step, 4-step, etc. can be understood.
When a supplier indicates that they can provide LED light sources within a 4-step MacAdam ellipse, we should know that it is indeed better than LEDs within a 5-step MacAdam ellipse. Color consistency: 4-step > 5-step;
The smaller the step number, the stricter the color tolerance requirements, and the better the color consistency of the light source. Therefore, when selecting LED light sources, the appropriate step requirements should be chosen according to actual application needs.
IV. Main Categories of Color Tolerance Standards
To answer: What are the coordinate points for European and American color tolerance standards? Are the center coordinate points selected correctly when using the ANSI C78.377-2017 standard?
1. Color Tolerance Standard Comparison
Currently, the main color tolerance standards are ANSI C78.376-2014, IEC60081-2002, and ANSI C78.377-2017. The corresponding color tolerance coordinate points are summarized as follows:
Figure 3: ANSI C78.376-2014 Color Tolerance Coordinate Points
Figure 4: IEC60081-2002 Color Tolerance Coordinate Points
As can be seen from the figure: IEC and ANSI standards have different coordinate points, with 2700K showing obvious differences. Therefore, the standard to be used must be confirmed before ordering.
2. ANSI C78.377 Standard and (u',v') Circular Color Tolerance
Many customers now use the coordinate points in ANSI C78.377 as new standard points. Here, we should choose circular standard coordinate points, don't make mistakes.
ANSI C78.377-2017 is a color tolerance standard developed for SSL light sources such as LED, OLED, and LD. ANSI C78.377-2017 requires meeting a 7-step quadrilateral standard, while also proposing 4-step quadrilateral and 4-step (u',v') circular color tolerance consideration standards. Among them, (u',v') circular color tolerance, as an important content in the ANSI C78.377 standard, provides a more precise and unified evaluation method for color tolerance testing of LED lighting products.
Yiming Technology, as a professional LED testing equipment manufacturer, deeply understands the importance of the ANSI C78.377 standard, especially the application value of (u',v') circular color tolerance in the industry. Yiming Technology's integrating sphere testing system software has added support for (u',v') circular color tolerance standards, meeting the requirements of some customers who want to stay ahead. Yiming Technology's testing systems fully comply with ANSI C78.377 standard requirements and can accurately test (u',v') circular color tolerance, helping customers ensure their products meet the latest color tolerance standards.
Figure 5: ANSI C78.377-2017 Standard Color Tolerance Coordinate Points
Figure 6: Yiming Technology Software Support for (u',v') Circular Color Tolerance Standard
V. SDCM Requirements in the Luminaire Lighting Industry
- Energy Star ANSI C78.376, color tolerance ≤7 SDCM, divided by LED characteristics.
- EU Standard ERP, color tolerance ≤6 SDCM, standardized LED division according to lighting technical requirements.
- National Standard GB10682-2002, double-ended fluorescent lamp performance requires color tolerance ≤5 SDCM, which can be used as a reference for LED lamp color tolerance requirements.
Different standards have different requirements for color tolerance. In practical applications, appropriate standards should be selected according to target markets and application scenarios. Generally speaking, the smaller the SDCM value, the stricter the color tolerance requirements, and the better the color consistency of the product.
VI. Summary
Color tolerance is an important parameter in LED lighting product testing, which characterizes the degree of deviation between light source color and standard color through SDCM values. Understanding the concept of color tolerance, the relationship between correlated color temperature and color coordinates, MacAdam ellipses, and various color tolerance standards (including ANSI C78.377 standard, (u',v') circular color tolerance, etc.) is of great significance for the design, production, and testing of LED lighting products. In practical applications, appropriate color tolerance standards and requirements should be selected according to target markets and application needs to ensure that products meet relevant specifications and have good color consistency.
This article on color tolerance hopes to help solve some questions about color tolerance. As a professional LED lighting testing equipment manufacturer, Yiming Technology has deep technical accumulation in the field of color tolerance testing. Yiming Technology's integrating sphere testing systems fully support testing requirements for various color tolerance standards, including ANSI C78.377 standard, (u',v') circular color tolerance, and other latest standards. Yiming Technology is committed to providing customers with accurate and reliable color tolerance testing solutions to help customers ensure their products meet industry standards and regulatory requirements.
