What is metamerism ? How to solve metamerism?

What is Metamerism ? How to solve Metamerism?

lWhat is metamerism?

lCauses of Metamerism

lHow to measure metamerism

lHow to Reduce Metamerism

What is Metamerism?

Metamerism is a visual phenomenon where two objects with different spectra are perceived as the same color by the human eye under the same light source, but the colors of the two objects appear different under another light source. This phenomenon is very common in our lives, such as wearing a pair of black socks before going out in the morning, and finding that one of the socks is dark blue after going out; or matching clothes of the same color in a clothing store, and finding a big color difference after returning home.

The causes of Metamerism 

The creation of color requires three factors: object, light source, and observer.

Objects : The surface properties of an object determine how it reflects and absorbs different wavelengths of light, which is called spectral reflectance . Different objects reflect light in different ways, causing us to perceive different colors.

Light source : Light sources provide illumination, and different light sources have different spectral power distributions (the intensity of light emitted by the light source at different wavelengths). For example, natural light , fluorescent lights, LED lights, etc. all emit different light, which directly affects the wavelength of light reflected by objects, thereby affecting the perception of color.

Natural light spectrum and white light LED spectrum

Observer : The physiological structure of the human eye (mainly the three types of cones on the retina , which are most sensitive to red, green, and blue wavelengths respectively) and the brain's processing of these signals determine how we perceive color, because our visual system only presents an overall perception of color and cannot directly perceive the spectral details that constitute the color stimulus. Therefore, the observer's own visual characteristics will also affect the perception of color: two color stimuli may have completely different spectral distributions, but because they have the same stimulating effect on our cones, the final perceived color looks the same.

In summary , the color perception of an object is the result of the spectral characteristics of the object, the spectral distribution of the light source, and the visual system of the observer . Any change in any one factor will affect the final color perception, and the three must work together to accurately present and perceive the color. This can explain the phenomenon of metamerism: when the object itself and the observer remain unchanged, changes in the light source will lead to different color perception . Back to the example at the beginning of the article: the socks you wear and your perspective have not changed, but you walk from indoors to outdoors, the observation light source has changed, and the color of your socks has changed from black to dark blue in your perspective .

In short, because the spectral composition of the light source is different, the balance of wavelengths will change, and the light reflected by the object will also change, causing the color to change.

If the curves of two physical samples intersect each other at least three times, they are a metameric pair. When the objects are a metameric pair, the phenomenon of metamerism is obvious .

Metamerism is very common in our lives, especially in industrial production. Whether it is the textile industry, printing industry or coating industry, as long as it involves color matching and color control, this phenomenon will affect product consistency and quality control. Since different light sources will cause changes in color perception, it is crucial to ensure that the color remains consistent under different light in these industries. Therefore, in industrial production, professional instruments such as colorimeter and spectrophotometer are usually used to measure the spectral characteristics of color to minimize or avoid the impact of metamerism.

Especially before the product finally enters the market, it is necessary to conduct color matching tests under different light sources to ensure that the colors seen by consumers under various lighting conditions remain consistent. This can not only improve the quality of the product, but also reduce the risk of returns or complaints due to color inconsistency.

How to measure Metamerism

Metamerism Index ( MI ) is an indicator used to quantify the degree of metamerism. The higher the metamerism index, the greater the difference between the two colors under different light sources.

formula:

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explain:

ΔL*1 , Δa*1 , Δb*1 : the differences in L* (lightness), a* (hue red-green axis), and b* (hue yellow-blue axis) of the two samples under the first light source.

ΔL*2 , Δa*2 , Δb*2 : the differences in L*, a*, and b* of the two samples under the second light source.

Calculation steps:

Step 1 : Measure the L*, a*, and b* values of the color sample under two different light sources, and calculate the differences in ΔL*, Δa*, and Δb* (color difference) under each light source.

Step 2 : Substitute the color difference between the two light sources into the formula, calculate the difference in each dimension separately, and add up the squares of these differences.

Step 3 : Calculate the square root to get the final metamerism index MI.

The meaning of the formula:

If ΔL*1=ΔL*2 , Δa*1=Δa*2 , Δb*1=Δb*2 , it means that the two colors appear the same under different light sources, and the MI value is close to 0, indicating that there is no metamerism.

If the differences among |ΔL*2-ΔL*1|, |Δa*2-Δa*1|, and |Δb*2-Δb*1| are small, MI<ΔE* (color difference under the main light source), indicating that the metamerism index is relatively low; on the contrary, when the differences among ΔL*, Δa*, and Δb* are large, MI>ΔE*, indicating that the metamerism phenomenon is significant.

How to Reduce Metamerism

1. Choose the right light source

In production and quality control, choosing the right light source is the key to reducing metamerism. Here are some specific measures:

· Use standard light sources : Use standard light sources such as D65 (daylight standard) and A (warm light) for color matching and evaluation to ensure consistent color performance under common lighting conditions.

· Multi-light source testing : Test color samples under different light sources, including sunlight, fluorescent light, LED and other common lighting conditions to ensure color stability under different light sources.

2. Choose stable color matching materials

In the production process, choosing dyes, pigments or coloring materials that are insensitive to different light sources is an important means to avoid metamerism. For example:

· Use materials with similar spectral characteristics : Choose dyes or pigments with similar spectral reflectance curves. Even under different light sources, their performance will remain relatively consistent, reducing color differences caused by changes in light sources.

· Avoid using raw materials from different sources : Raw materials (such as dyes or pigments) in the same batch of products should be kept consistent as much as possible. Avoid mixing raw materials with different formulas or sources, which will increase the risk of metamerism.

3. Perform color management and colorimeter calibration

Metamerism can be effectively reduced by using accurate color measurement equipment and color management systems:

· Use a colorimeter or spectrophotometer : Accurately measure the spectral reflectance curve and metamerism index of color samples to ensure that color differences under different light sources are controllable.

· Color Management System (CMS) : Use a professional color management system to calibrate equipment and monitor, such as PeColor. The software has a built-in metamerism index function to ensure color consistency during the design, production and printing processes, and reduce color differences in different environments.

4. Use a light source with a high CRI

Light sources with a higher CRI (Color Rendering Index) can more accurately reproduce colors, thereby reducing metamerism. Using a light source with a CRI close to 100 can help reduce color variations under different light sources.

5. Optimize the color matching process

During the color matching process, several measures can be taken to reduce the risk of metamerism:

· Precisely adjust the formula : By repeatedly adjusting the formula, the color matching under different light sources is as close as possible. Some color matching software and tools can simulate the color performance under different light sources to help colorists optimize the formula.

· Add light source simulation equipment : Equip the laboratory or production site with a standard light source box to simulate different lighting conditions, detect metamerism in a timely manner and make adjustments.