Visual Color Assessment - Standard Operating Procedures

The analysis of visual color and color variation is an important exercise for numerous markets and industries. Here, we describe in detail protocols and optimal practices for dependable and sturdy visual analysis of color. Specialist suppliers, trademark owners, and retailers rely on rigorous color analysis to select appropriate suppliers and partners, while suppliers can utilize their color analysis productivity to advance their high-level standards to potential customers.

Accurate information about color and color variation is particularly important with global supply chains. Companies must rely on partners in all parts of the world to control color quality by collecting detailed information with precision instruments and standardized techniques. The goal of “no perceptible color difference” is too expensive for most industries. It is therefore vital to agree and practice visual boundaries for acceptable color variations.

Overview of Lighting and Color

Individual perception of color occurs when light is reflected off an object and into the eye, where it is sensed and transmitted to the visual cortex. As different people experience color differently, one must account for all other variables at various sites to achieve consistent analysis.This requires a standard procedure and an initial step in controlling visual analysis is a standardized viewing area, like X-Rite’s SpectraLight QC.

The SpectraLight QC offers a repeatable visual workspace for color analysis with consistent, highly accurate light sources which simulate the internationally-recognized standard  lighting conditions.

Lighting Choices

When viewing samples in different lighting environments, the apparent color match may also change between objects. This is particularly important when your products will be seen in different environments, such as outdoors, in shopping malls, or at home. The SpectraLight QC offers six white light sources for flexibility  analyzing objects as they appear at the point of sale and in customers’ surroundings. The SpectraLight QC offers two widely-adopted artificial daylight light sources as well as incandescent sources, fluorescent sources, and, as soon as LED typical spectra are standardized and available, LED light sources.

Changing a light source changes the amount of energy at different wavelengths of the visible spectrum. Each light source has its own Spectral Power Distribution (SPD) which contains the proportions of visible light energies present, and above are graphs that show SPD curves for daylight, Cool White Fluorescent, and incandescent Illuminant ‘A’ light sources. By looking at the graphs, it is clear that Daylight has the most even energy across the entire spectrum, particularly in the blue wavelengths; whereas, incandescent ‘A’ is skewed toward the red. Sharp peaks in the SPD are characteristic of fluorescent light sources, and are created by the fluorescence of rare-earth phosphor compounds in the bulbs.

Sources and Illuminants

We observe objects and analyze color using actual lamps or “sources”. In contrast, “illuminants” are theoretical depiction of idealized sources, as internationally recognized by the CIE (Commission Internationale de l’Eclairag or International Commission on Illumination). For instance, Illuminant F2 is the theoretically ideal depiction of light from a cool white fluorescent tube, while the bulb used to simulate it is only considered the light source.

There exists a group of daylight illuminants of unique color temperatures, for example, D50 is a daylight illuminant that correlated to the color temperature of 5000 K and is popular in the graphic design industry; D65 illuminant represents the color temperature 6500 K, and is used by the textile and automotive industries and many others. The SpectraLight QC has light sources that best simulate illuminants like the D50 and D65.

What is the need to mimic daylight? Why is natural outdoor daylight not good enough for evaluating color and color variations?

Sunlight varies depending on the time of day, latitude, season, weather, and even pollution. As a result, a standard representation of daylight is what we may refer to as D65. D65 is fundamental to color calculations based on daylight and is globally accepted to control for so many variances between daylight in different locations.

D65-daylight varies between different light booths. The quality of D65 is graded using an international scale created by the CIE and known as Publication S 012/E (ISO 23603) “Standard Method of Assessing the Spectral Quality of Daylight Simulators for Visual Appraisal and Measurement of Colour”.

A CIE rating of A for visible light is achieved by the SpectraLight QC by using a filtered tungsten daylight system. This is the result of a tungsten lamp spectrum passing through a selective glass filter whichyields daylight.

Common light sources with related CIE illuminants are listed below.

Light Source Standard CIE Illuminant Simulated Notes
Daylight (filtered tungsten) D65 Most accurate daylight
Daylight (fluorescent) D65
U35 fluorescent Not a CIE standard Energy efficient fluorescent
U30 fluorescent Not a CIE standard Energy efficient fluorescent, limited availability
TL84 fluorescent Not a CIE standard Energy efficient fluorescent
Cool white fluorescent FL2 Fluorescent, broader-band than energy efficient lamps
Incandescent A Incandescent, limited availability in some markets

Visual Evaluation of Color and Color Difference

Reliable protocols are required for color analysis to be conveyed between consumers and suppliers. SOPs ensure that observations between samples are standardized between many different locations and users.

Sample Presentation

Many industries have similar methods for analyzing color variations between test objects and regulations. Typically, test objects will be held in place at the same angle as an SOP dictates.

Objects with a soft, directional surface like pile fabrics must be combed or smoothed so that the pile is facing the same direction, and would be required for similar materials.

No other objects should be present in a SpectraLight QC cabinet prior to analysis. It is also important for the inside of the light booth to be clean and free from dirt or debris. The cabinet is painted to match ‘Munsell N7,’ which is a neutral gray color, and it is important that it is not altered so that viewing conditions are consistent among all units worldwide.

It is important to guard against changes in samples caused by temperature or humidity as well. Steps to control the samples may include conditioning, carried out by a lab or an accelerated,  environmental-conditioning chamber treatment

Viewing Geometry

When arranging your samples for observation during color analysis, there are two different methods. Both of these positions produce equivalent results; however, there are practical reasons to favor one or the other. All supply-chain partners must adhere to the same viewing geometry whenever specified.

On the left, the illustration shows the sample is illuminated at a 0° angle, and the user observes the sample at a 45° angle. This is known as “0/45 viewing geometry”.

On the right, the illustration shows the sample is illuminated at a 45° angle, and the user observes the sample at a 0° angle. This is known as “45/0 viewing geometry”.

To create these types of geometry, angle tables are available to place inside the SpectraLight QC.

Something to consider when using 45/0 viewing geometry, is that the variation in user height could make it difficult to guarantee that each person is observing the sample at 0°. This can be partially overcome by making viewers sit whilst analyzing objects.

Alternative viewing arrangements exist when working in some applications, it is important to specific to all supply-chain participants the required method.

Observer

In order to implement a successful program, the training, skill and innate color vision of the user analyzing the sample is vital. The user should take an ability test to check for any difficulty noticing minute color variations.

This information has been sourced, reviewed and adapted from materials provided by X-Rite.

For more information on this source, please visit x-rite.com

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