Spectrophotometry’s applications are seemingly boundless. Color matching measurements are made every day by those comparing a reproduced object to a reference point. Spectrophotometry-assisted color measurement can be useful in areas such as:
Color testing of inks
Color control of paints
Control of printed colors on packaging material and labels
Color control of plastics and textiles throughout the development and manufacturing process
Finished products like printed cans, clothing, shoes, automobile components, plastic components of all types
Corporate logo standardization
Color and appearance instruments are used to measure the properties of paints and coatings including color, gloss, haze and transparency. Appearance is the manifestation of the nature of objects and materials through visual attributes such as size, shape, chroma, color, texture, glossiness, haze, transparency, opacity, hue, luster, orange peel, translucency, etc.
Color and appearance instruments generally fall into one of four categories, colorimeter, densitometers, spectral cameras, and spectrophotometer. Colorimeters measure color using three or four filters that match human color receptors. Colorimeters can show L, a, b or L*, a*, b* numbers but can only measure in one light source. Densitometers measure the density of ink films using one or more filters. Densitometers do not give complete color information, but are useful for specification and control of printed colors. Spectral cameras provide measurements with full spectral and spatial information. Spectrophotometers operate on the principle of reflected light. Spectrophotometer measures individual wavelengths and then calculate L, a, b or L*, a*, b* values from this information. These color and appearance instruments can measure in all standard illuminants.
To accomplish their readings, color and appearance instruments many use any of a number of measurement scales. These include:
Hunter L, a, b: a color standard that was finalized in 1958. ΔL=lightness, Δa=green and red and Δb=blue and yellow.
CIELAB: an international color standard adopted in 1976. CIE is a tricolor system that is based on the fact that any color can be matched by a suitable mix of the 3 primary colors. When a color is expressed in CIELAB, ΔL* defines lightness, Δa* denotes the red/green value and
Δb* the yellow/blue value.
CIELCH: a color standard developed from CIELAB. While CIELAB uses Cartesian coordinates to calculate a color in a color space, CIELCH uses polar coordinates. This color expression can be derived from CIELAB. The ΔL* defines lightness, ΔC* specifies chroma and h° denotes hue angle, an angular measurement. The L*C*h° expression offers an advantage over CIELAB in that it’s very easy to relate to the earlier systems based on physical samples, like the Mun sell Color Scale.
Delta CIELAB and CIELCH: Assessment of color is more than a numeric expression. Usually it’s an assessment of the color difference (delta) from a known standard. CIELAB and CIELCH are used to compare the colors of two objects.
XYZ: the XYZ space allows colors to be expressed as a mixture of the three tristimulus values X, Y, and Z. The term tristimulus comes from the fact that color perception results from the retina of the eye responding to three types of stimuli. After experimentation, the CIE set up a hypothetical set of primaries, XYZ, that correspond to the way the eye’s retina behaves.
Yxy: Yxy space expresses the XYZ values in terms of x and y chromaticity coordinates, somewhat analogous to the hue and saturation coordinates of HSV space.
When tolerancing with CIELAB, you must choose a difference limit for ΔL* (lightness), Δa* (red/green), and Δb* (yellow/blue). These limits create a rectangular tolerance box around the standard.
When comparing this tolerance box with the visually accepted ellipsoid, some problems emerge. A box-shaped tolerance around the ellipsoid can give good numbers for unacceptable color. If the tolerance box is made small enough to fit within the ellipsoid, it is possible to get bad numbers for visually acceptable color.
CIELCH users must choose a difference limit for ΔL* (lightness), ΔC* (chroma) and ΔH* (hue). This creates a wedge-shaped box around the standard. Since CIELCH is a polar-coordinate system, the tolerance box can be rotated in orientation to the hue angle. When this tolerance is compared with the ellipsoid, we can see that it more closely matches human perception. This reduces the amount of
disagreement between the observer and the instrumental values
CMC is not a color space but rather a tolerancing system. CMC tolerancing is based on CIELCH and provides better agreement between visual assessment and measured color difference. CMC tolerancing was developed by the Colour Measurement Committee of the Society of Dyers and Colourists in Great Britain and became public domain in 1988. The CMC calculation mathematically defines an ellipsoid around the standard color with semi-axis corresponding to hue, chroma and lightness. The ellipsoid represents the volume of acceptable color and automatically varies in size and shape depending on the position of the color in color space.
Visual Color and Tolerancing
Poor color memory, eye fatigue, color blindness and viewing conditions can all affect the human eye’s ability to distinguish color differences. In addition to those limitations, the eye does not detect differences in hue (red, yellow, green, blue, etc.), chroma (saturation) or lightness equally. In fact, the average observer will see hue differences first, chroma differences second and lightness differences last. Visual acceptability is best represented by an ellipsoid. As a result, our tolerance for an acceptable color match consists of a three-dimensional boundary with varying limits for lightness, hue and chroma, and must agree with visual assessment. CIELAB and CIELCH can be used to create those boundaries. Additional tolerancing formulas, known as CMC and CIE94, produce ellipsoidal tolerances.
In 1994 the CIE released a new tolerance method called CIE94. Like CMC, the CIE94 tolerancing method also produces an ellipsoid. The user has control of the lightness (kL) to chroma (Kc) ratio, as well as the commercial factor (cf). These settings affect the size and shape of the ellipsoid in a manner similar to how the l:c and cf settings affect CMC. However, while CMC is targeted for use in the textile industry, CIE94 is targeted for use in the paint and coatings industry.You should consider the type of surface being measured when choosing between these two tolerances. If the surface is textured or irregular, CMC may be the best fit. If the surface is smooth and regular, CIE94 may be the best choice.
Visual Assessment vs. Instrumental
Though no color tolerancing system is perfect, the CMC and CIE94 equations best represent color differences as our eyes see them.
Choosing the Right Tolerance
When deciding which color difference calculation to use, consider the following five rules (Billmeyer 1970 and 1979):
1. Select a single method of calculation and use it consistently.
2. Always specify exactly how the calculations are made.
3. Never attempt to convert between color differences calculated by different equations through the use of average factors.
4. Use calculated color differences only as a first approximation in settingtolerances, until they can be confirmed by visual judgments.
5. Always remember that nobody accepts or rejects color because of numbers – it is the way it looks that counts.
White and Yellow Indices
Certain industries, such as paint, textiles and paper manufacturing, evaluate their materials and products based on standards of whiteness. Typically, this whiteness index is a preference rating for how white a material should appear, be it photographic and printing paper or plastics.
In some instances, a manufacturer may want to judge the yellowness or tint of a material. This is done to determine how much that object’s color departs from a preferred white toward a bluish tint.
The effect of whiteness or yellowness can be significant, for example, when printing inks or dyes on paper. A blue ink printed on a highly-rated white stock will look different than the same ink printed on newsprint or another low-rated stock. The American Standards Test Methods (ASTM) has defined whiteness and yellowness indices. The E313 whiteness index is usedfor measuring near-white, opaque materials such as paper, paint and plastic. In fact, this index can be used for any material whose color appears white. The ASTM’s E313 yellowness index is used to determine the
degree to which a sample’s color shifts away from an ideal white. The D1925 yellowness index is
used for measuring plastics.
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