What Is Colour Tolerance and Why Is It Important?

Colour tolerances set acceptable limits for the appearance of a product and are an essential part of quality assurance and quality control processes for colour. You can efficiently determine whether a product falls within the limits by establishing tolerances. If it does, the product passes. Failed goods fall outside the tolerances, meaning process adjustments may be necessary to create the desired appearance. 

Many industries, such as food, plastics, and chemicals, rely on accurate appearance. Tolerances enable you to establish a standard and decide how much variation to allow, giving you consistent colour outcomes. 

What Is Colour Tolerance?

Colour tolerances are the parameters within which a product's colour can vary yet remain acceptable. Typically, visual assessment is the initial step to define the acceptable colour range. Spectrophotometry then objectively and scientifically quantifies and manages those tolerances.

Before establishing tolerance values, you must decide on the observer, choose a colour scale, such as Hunter L,a,b, and then select illuminants for comparison, like D 65 and Illuminant A. Depending on what you're measuring, the scale you choose might come from the International Commission on Illumination (CIE), which sets global measurement standards, or from another entity. 

Once you have a comparison standard, you can gather products and measure their colour to determine acceptable variations called tolerance values. 

In this process, you start by selecting a relevant standard for your product, then determine how much this standard can vary before it creates visible differences in appearance. This test helps you set boundaries for future production to objectively decide whether products fall close enough to the original standard to conform to quality expectations.

Detecting Colour Differences

You can establish tolerances based on two levels of difference:

• Minimum perceptible difference: A visual comparison shows a just-noticeable difference between the standard and the sample. 
• Maximum acceptable difference: The level represents the largest acceptable difference between a standard and a sample. 

Focusing on the maximum acceptable difference means a broader range of acceptable values, while relying solely on minimum perceptible difference can lead to high rejection and material waste. As a result, choosing the right approach is typically industry-driven. For example, the food industry frequently uses the maximum acceptable difference, but pharmaceutical manufacturers usually opt for the minimum perceptible difference.

Range of Tolerance Definition

The range of tolerance is the acceptable level of variation related to an established colour standard. You will establish this standard using a measurement model like CIE L*a*b*, which results in three values measuring:

• L*: Lightness, from dark at zero to light at 100.
• a*: Position on the red–green axis, with the positive number indicating red and the negative indicating green.
• b*: Position on the yellow–blue axis, with yellow represented as the positive and blue as the negative.

Once you establish a standard, you can decide on each dimension's tolerance. Here are the formulas for determining the difference (∆) between a sample and a reference:

∆L* = L*sample - L*reference

∆a* = a*sample - a*reference

∆b* = b*sample - b*reference

You can also calculate the total colour difference across the three values in a single formula, with the results represented by delta E* (∆E*). Here's the formula to determine ∆E*:

∆E* = √((∆L*)2+(∆a*)2+(∆b*)2)

While ∆E* values can be helpful, use this method carefully. When the difference is spread over three dimensions, you may get a value that looks acceptable but has significant variation in one measurement, such as red vs. green. 

This variation will create a noticeable visual difference but may not impact the delta E* value if the other dimensions are close to the standard. If you calculate the difference values separately, you can more closely monitor each one to verify that none stray too far from the standard.

Types of Colour Tolerances

When establishing tolerances, it's helpful to remember how humans perceive colour differences. We will likely notice hue differences, while chroma might have less effect. Lightness differences are even less likely to affect our perception than chroma or hue. This knowledge helps you set up a standard. There are three main tolerance systems you might use:

• Box: This system sets a range of acceptable values using singular data (line), dual data (box), or triple data (cube). A value is acceptable if it falls on the line or inside the box or cube. Otherwise, you will reject the sampled material.
• Circular: This approach uses all three colour values to get a single value that determines the difference. In the L*C*h colour space, it will look like a sphere. 
• Elliptical: This technique accounts for variations in how the human eye sees colour differences. The limit values create an ellipsoid around the standard. The system compensates for hue, neutral colours, lightness, and chroma. 

Advanced spectrophotometers and analysis software are the most effective methods for setting and enforcing colour quality tolerances. They enable you to measure spectral data and compare values without relying on time-consuming manual calculations. 

The Benefits of Establishing Colour Tolerances

Defining tolerances brings many advantages to your production processes, including:

• Consistency: You will have clear boundaries that allow for objective evaluations so every product falls within a set range. 
• Consumer trust: Setting boundaries ensures product standardization, which makes customers more likely to buy a product and purchase from you repeatedly. 
• Efficient production: Your standard allows for quick and indisputable decision-making, allowing you to maintain quality while keeping production moving. 
• Cost savings: Catch and fix issues earlier in the production process, eliminating the costly process of correcting errors later.

How to Establish Colour Tolerances

Here's the process to establish tolerances. 

1.Assess Colours Visually
Gather samples with a similar appearance to a standard. Assess them and determine which are different but may still be an acceptable match. Use multiple people to confirm this assessment to account for differences in perception.

2.Measure the Samples With a Spectrophotometer
Measure each sample you have deemed acceptable using a spectrophotometer and record the difference as ∆E*, ∆L*, ∆a*, ∆b*, ∆C* (chroma), and ∆H* (hue), depending on the colour space you use. The samples with the most extensive variation will set the maximum tolerance limit.

3.Establish a Tolerance System
Choose a box, circle, or elliptical system to map out acceptable values visually. An elliptical system will provide the most accurate range of tolerances based on how the human eye perceives colour. Two prominent examples of such elliptical systems are the CMC (ΔECMC) and CIEDE2000 (ΔE00) colour difference formulas. Use the maximum limit tolerance values to create an ellipsoid around the standard. Any measurement that falls within the ellipsoid represents an acceptable tolerance. Anything outside is unacceptable. These samples get rejected. 

Make Colour Tolerance Easy With HunterLab Solutions

HunterLab is a leader in colour measurement solutions with over 70 years in the industry. We provide a range of high-quality spectrophotometers with advanced colour-matching software to help you collect and analyze data and establish adequate tolerances. Get in touch with us online to learn more about colour tolerance or the solutions we offer.