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Color Evaluation in the Coil Coating Industry

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Quantifying the color of an object, and then measuring color difference between the object’s color and a color standard, is an important factor in assuring color consistency and acceptance. Assembling parts from various lots and production runs requires that parts have minimal—or no—color difference, so a typical observer sees nothing objectionable.

Typically, a color instrument will deliver color values represented by three color coordinates:

  • “L” value, which describes the lightness or darkness of a sample
  • “a” value, which describes the redness or greenness of a sample
  • “b” value, which describes the yellowness or blueness of a sample

Color Difference

In the coil coating industry, the control of color on a coil line is always expressed in terms of delta L (ΔL), delta a (Δa), and delta b (Δb). Constituent color coordinates represent color difference as:                                              

ΔL = positive (+) values are lighter than the standard; negative (-) values are darker

Δa = positive (+) values are redder than the standard; negative (-) values are greener

Δb = positive (+) values are yellower than the standard; negative (-) values are bluer

The total color difference between a sample and a standard is always expressed in terms of delta E (ΔE), and there are many methods of measuring and quantifying color difference; however, all methods base the calculations on the difference between two samples in the ΔL, Δa, and Δb color coordinates.

The ΔE color difference between two samples is commonly calculated as follows:

ΔE = [(ΔL)2 + (Δa)2 + (Δb)2]1/2

This use of constituent color coordinates making up ΔE provides the coil coater with additional capability in producing and maintaining an acceptable color result.

Color vs. Appearance

It is important to understand the fundamental difference between color and appearance. Color is concerned with the physics of light interacting with pigments, which plays a critical role when color-matching software is used. Appearance, on the other hand, is how an individual (a human, not a machine) responds to an object’s color. For example, a black tire that has gone through a car wash looks considerably blacker than it did before being cleaned. The pigmentation that produces the color of the tire has not changed, but its appearance certainly has been affected by the removal of dirt and grime. In other words, we may declare the tire’s color to be blacker, but it is actually the appearance that has changed.

An excellent example of the difference between color and appearance is demonstrated in the figure below, where a glossy dark gray piece of plastic has had the upper half textured (labeled “structured”). The color (the physics of light striking pigments) is the same on both samples, but the appearance reveals a notable difference.

Source: BYK Instruments

When light strikes an object, some of that light bounces off the surface at the same angle as the incident light beam. We measure gloss in this fashion. The light strikes the surface at a 60° incoming angle, and some of that light bounces off at a 60° outgoing angle. This particular reflection angle is defined as specular (mirror-like) reflectance. The percent difference between the amount of light striking the object and the amount of light bouncing off the surface is described as the gloss of object.

In addition to specular reflectance, there is also diffuse reflectance. For a 30-gloss coating, for example, 30% of the incoming light reflects along the specular reflectance angle; the other 70%, however, is reflected into the diffuse region.

When measuring gloss, we do not consider diffuse reflectance, only specular reflectance. While we do not consider diffuse reflectance for gloss, it is critical to understand how the appearance of an object is affected by diffuse reflectance. This understanding is important when taking a color reading, because—depending on the color instrument being used—gloss can have an effect on appearance, but it may not affect color readings. The instrument type (discussed below)—and the setup of the instrument—can have a significant influence on the color readings that are generated.

When measuring colors, there are two types of instruments. One type utilizes a sphere to collect all the reflected light—both specular and diffuse reflectance—from a sample.

The other type of color instrument is defined as a 45/0 device. In the picture below, the light source illuminates the object as a 45° angle, but only light reflected perpendicular to the surface (the 0° angle) is collected.

When using an instrument that utilizes a sphere, you will have the option to capture all reflections—both the specular and diffuse reflectances. You also have an option to exclude the specular reflectance. When choosing this latter option, the spherical instrument will generate values very similar to a 45/0 instrument.

The best color instrument setup to use to match what an observer sees is either a 45/0 instrument or an instrument equipped with a sphere, but using a specular component excluded setup. You can see in the figure below that a 45/0 instrument reads the “color” difference (actually, the appearance difference) between the smooth and textured piece of plastic at 4.8 ΔE, but using an instrument with a sphere and including the specular component (designated as “SPIN” in the figure) yields a ΔE of 0.1. Clearly, the glossy and textured (structured) sides of the panel look significantly different.

Source: BYK Instruments

Additional information on the topic of color and gloss can be found in a variety of ASTM methods:

  • D523 Standard Test Method for Specular Gloss
  • D2244 Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates
  • D3134 Standard Practice for Establishing Color and Gloss Tolerances
  • D3794 Standard Guide for Testing Coil Coatings
  • E284 Standard Terminology of Appearance
  • E308 Standard Practice for Computing the Colors of Objects by Using the CIE System

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