Color and Appearance

Color and Appearance

Color and Appearance can be quantified with the right instrument. The applications are wide ranging from consumer products to food, fashion, paint, and more. TEquipment offers a variety of instruments to meet these needs. First let’s understand more about each instrument and its applications. At the end there will be a brief tutorial on color. You can learn more about color from Precise Color Communication by Konica Minolta. It is the reference used in this article.

What is a Colorimeter?
A Colorimeter quantifies color. Colorimeters have sensitivities corresponding to those of the human eye, but because they always take measurements using the same light source and illumination method, the measurement conditions will be the same, regardless of whether it’s day or night, indoors or outdoors. This makes accurate measurements simple. Minute color differences are the biggest headache anywhere that color is used. But with a colorimeter, even minute color differences can be expressed numerically and easily understood.

Key Features of a Colorimeter

• Built-in Light Source. Ensures uniform and consistent illumination
• Constant Illumination/Viewing Angles
• Constant “Observer”. The “observer” of the colorimeter is a set of three photocells filtered to closely match internationally adopted color standard (Standard Observer).
• Elimination of Area Effect Ad Contrast Effect. Since the colorimeter measures only the specimen (provided the specimen is at least the specified minimum size), the effects of different specimen sizes or backgrounds are eliminated.
• Color-Difference Measurement. Color difference from a target color can be measured and instantly displayed in numerical form.
• Data Memory and PC Communications

What is a Spectrophotometer and How does it differ from a Colorimeter?
1. Spectral Reflectance. Subjects measured with a tristimulus colorimeter can only obtain numerical color data. If a Spectrophotometer is used, not only can the same types of numerical data be obtained, but also the spectral reflectance graph for that color. Further, with its high-precision sensor and the inclusion of data for a variety of illuminant conditions, the spectrophotometer can provide higher accuracy than that obtainable with a tristimulus colorimeter.
A tristimulus colorimeter measures the light reflected from an object using three sensors filtered to have the same sensitivity as the human eye. On the other hand, the spectrophotometric method utilizes multiple sensors to measure the spectral reflectance of the object at each wavelength or in each narrow wavelength range. The instrument then calculates the tristimulus values.

2. Spectrophotometers provide high accuracy and the ability to measure absolute colors. They are typically larger, more expensive, and less portable than colorimeters, so better suited in research and complex color analysis. Colorimeters are better suited to production and inspection applications for the color difference measurements and color chart comparisons.

3. Spectrophotometers can spot Apparent Color change. Color changes depending on the source light. A relatable experience is looking at clothing in the department store lighting versus outdoors. In order to quantify color, the industry has defined Standard Illuminants for daylight, fluorescents and more. Colorimeters have a built in light source to standardize the reported measurements. A spectrophotometer actually measures the spectral reflectance of the object. The instrument can then calculate numerical color values in various color spaces using the spectral power distribution data for the selected illuminant and data for the color-matching functions of the Standard Observer.

4. Spectrophotometers can handle Metamerism. Color depends on the light source. A related problem is the color difference that can occur between two seemingly identical objects viewed together in different light sources. For example, two hand bags viewed outside look identical but viewed under indoor lighting look different. This problem is often due to use of different pigments or materials. Looking at the spectral reflectance curves from a spectrophotometer will spot the difference immediately, while a colorimeter would report the same color for both objects.

Key Features of a Spectrophotometer

• Illuminant Conditions. data for a wide variety of illuminants (light sources) are stored in memory to allow meassurement results to be calculated under various illuminant conditions
• Fixed Illuminant/Viewing Angles. The illumination/viewing geometry is fixed to ensure unform condisions for measurements
• Spectral Sensor. Spectral sensor consisting of various segments to measure light at each wavelength interval for high accuracy
• Color Spaces. Measurement data can be displayed numerically in a wide variety of color spaces, including Yxy, L8a*b, Hunter Lab, and other formats
• Color-difference Measurement and Graphing. Color difference from a target color can be measured and instantly displayed in numerical form or on a spectral reflectance graph
• Data Memory and PC Communications

Color and Gloss (SCE and SCI Methods)
Even for objects composed of teh same materials, variances may be seen in the colors due to differences in the gloss of the surfaces. For example, why is a duller blue color seen when sandpaper is applied to a shiny or high gloss blue sample?

Light which reflects directly back (but opposite angle) from the light source, as if by a mirror, is called specularly reflected light. The rest of the light that is scattered in many directions is called diffuse reflectance. The sum of the two is called total reflectance. Shiny objects have higher specular reflectance/lower diffuse reflectance and the opposite is true for dull objects. The total amount of reflected light is always the same if the materials and color are the same. Therefore, if a glossy blue plastic part is sanded, the specular reflectance is reduced and the diffuse reflectance increases.

Two key terms when talking about gloss are:

• SCE, Specular Component Excluded
• SCI, Specular Component Included.

The names are counter intuitive. SCE values include the gloss and texture while SCI exclude them. SCI values represent the true color that lies beneath.

Why a Gloss Meter instead of a Spectrophotometer?
A Gloss Meter, or Glossmeter, will measure the specular reflection of the object at multiple angles. Results will correlate to known standards unlike a spectrophotometer which can only give relative values between samples. The intensity is dependent on the material and the angle of illumination. In case of nonmetals (coatings, plastics) the amount of reflected light increases with the increase of the illumination angle. The remaining illuminated light penetrates the material and is absorbed or diffusely scattered dependent on the color. Metals have a much higher reflection and are less angle dependent than non-metals.

Spectrophotometers measure SCI and SCE but only at one angle. The results they present are only relative between samples. This may be fine for a lab comparing their own samples. But as soon as comparison to another lab is needed, then a dedicated Gloss Meter is needed.

Information of Gloss

• Gloss Measurement. Gloss is a visual impression that is caused when a surface is evaluated. The more direct light is reflected, the more obvious will be the impression of gloss.
• High Gloss. Smooth and highly polished surfaces reflect images distinctly. The incident light is directly reflected on the surface, i.e. only in the main direction of reflection. The angle of incidence is equal to the angle of reflection
• Matt to Semi-Gloss. On rough surfaces the light is diffusely scattered in all directions. The image forming qualities are diminished: A reflected object does no longer appear brilliant, but blurred. The more uniform the light is scattered, the less intense is the reflection in the main direction and the duller the surface will appear.

Basics about Color Attributes
The world of color is a mixture of three attributes: Hue, Lightness, and Saturation.  Let's discuss each.

• Hue forms the color wheel. Apples are red, lemons are yellow, teh sky is blue. That's how we all think of color in everyday language. Hue is the term used in the world of color for teh classifications of red, yellow, blue, etc.
• When the Lightness (how bright) of a color is compared, it can be separated into bright and dark colors. This lightness can be measured indepentandly of hue.
• Saturation compares vivid colors and dull colors. How do you compare the yellos of a lemon and a pear? you might say the yellow of the lemon is brighter, but more to the point in this case, it is vivid, while the yellow of the pear is dull. There is another big difference, but this time one of color saturation or vividness. This attribute is completely separate rfom those of both hue and lightness.

There are various methods, called Color Spaces, to express colors numerically. Here are a few:

• Munsell renotation System/Munsell Color Charts. It is a letter/number combination (H# V#/C#), where H is Hue, V is Value (lightness), and C is Chroma (saturation).
• Yxy Color Space developed by Commission Internationale de l'Eclairage (CIE) in 1931.
• L*a*b* Color Space, (or CIELAB) also developed by Commission Internationale de l'Eclairage (CIE) in 1976 is widely adopted.
• Hunter Lab Color Space. Developed as a more visually uniform color space than the 1931 Yxy and similar to the L*a*b* color space, it remains in use in various fields, including the paint industry of the U.S.