Industrial Coloration of Plastics

2	Color and Color Measurement

Countless color variations surround us in everyday life. The term “color” is so evident that it is easy to overlook the role that color plays in everyday life. Although color has a fundamental impact on our lives, our knowledge of “color” and how to use it in a controlled manner is often not sufficient – which can lead to problems when defining or communicating product colors. The assessment often takes place under the influence of personal feelings and experience, making a uniform assessment impossible.

The human eye perceives light in the visible range. However, “light” is not the same as “color”, but, first of all, electromagnetic radiation, which triggers nerve impulses on the retina and which makes “seeing” possible at all. The spectrally sensitive sensory cells of the retina transmit their signals to the brain, where this information creates the color impression. So, color is a sensory impression in the human brain. Visible light is a small part of existing electromagnetic waves. Different wavelengths are perceived as different colors. Color is, therefore, a perceptual phenomenon that depends on the observer and the conditions under which the color is observed. However, there is no physical quantity for the determination of colors like for length or weight.

Three things are required to perceive colors [1]:

       an object,

       a light source (illuminant), and

       an observer.

The human brain successfully uses the color impression for object tracking and recognition. However, the color of an object changes when the color of the illuminant changes or when the object is against a different colored background.

The energy in the form of light is required to see colors. To better understand the “color” phenomenon, we need to recognize the origin of light. Light comes from a variety of sources and consists of electromagnetic radiation, a form of energy that propagates in a wave motion. All visible light consists of a mixture of colors that are put together in different proportions to form every distinctive visible light. The way we measure light is spectral energy distribution, in Figure 2.1 the visual color spectrum begins at 400 nm and ends at 700 nm wavelength. Light with wavelength below 400 nm is called ultraviolet (UV), and that above 700 nm is called infrared (IR). The human eye cannot perceive ultraviolet or infrared light.

White light consists of a mixture of colors, which are characterized by a specific wavelength range. These are the colors of the spectrum – red, orange, yellow, green, blue, and violet.

Incandescent lamps and luminescence are two ways of generating light. The incandescent lamp generates light from thermal energy. Heating the filament of an incandescent lamp to a sufficiently high temperature will cause it to glow. Luminescence, also known as cold light, is light from other energy sources. It can be generated at room temperature or even lower temperatures. Quantum physics explains luminescence as the movement of electrons from their primary state (lowest energy level) to a state of high energy. When they return to their primary state, the electron releases the energy in the form of a light photon. If the time interval between the two steps is short (a few microseconds), the process is called fluorescence. If the interval is long (a few hours), the process is called phosphorescence.

The combination of these wavelengths in the light can change according to the light source. For this reason, colors can look different under the influence of daylight, fluorescent light, or sodium vapor lamps. Natural sunlight varies widely. It can be very blue, especially at noon in northern latitudes. Direct sunlight is usually seen as golden, but at sunset, it can be bright red. Artificial light from sodium vapor lamps is yellow, from mercury vapor lamps blue-green, or from an incandescent lamp more or less yellow.

Several phenomena can occur when the light hits an object. With transparent objects, the electromagnetic waves pass through the object. With colorless objects, the light passes through the objects unchanged. With colored objects, part of the light waves is absorbed. The unabsorbed light waves are perceived as color by the human eye.

With more or less opaque objects, part of the light waves is absorbed, the remaining part is reflected. With a blue object, for example, the red part of the color spectrum is absorbed. The human eye then perceives the reflected blue part as a blue color. The reflection curve of white shows approximately the same intensities near 100% reflection in all wavelengths of the spectrum.

Refraction or scattering means that the light changes direction when it passes from one medium to another, such as from the polymer to a pigment or filler particle in a plastic part. Scattering is influenced by the difference in refractive index between a particle and its surroundings, the particle size, and the wavelength of the light. An opaque color ensures high scattering. A translucent color shows a combination of reflection and scattering. Absorption occurs when most of the wavelengths in the visible spectrum are absorbed. Black objects absorb almost all light.

An object appears in a particular color because the light reflected from its surface is made up of precisely the wavelengths combined to produce the observed color. The object absorbs all other wavelengths. For example, a blue object reflects the blue light spectrum but absorbs red, orange, yellow, green, and violet, which are most other wavelengths. A red object reflects the red part of the spectrum, but absorbs most of the orange, yellow, green, blue, and purple.

Black and white colors are different from other colors in terms of how they reflect and absorb light. A white object reflects almost all colors, while a black object completely absorbs most colors.

Other significant influences on the color of an object are shape and surface effects. For example, an object can be spherical or square, dull or glossy, transparent, opaque, or translucent. It can also appear metallic, pearlescent, fluorescent, or phosphorescent. Viewing angles also affect our color perception.

The human eye is the defining observer of color. An observer almost always bases the acceptance of a color on visual judgment. For this reason, color matching can become very subjective, since the color view varies greatly from person to person. Characteristics such as age, gender, inherited characteristics, and even moods can influence the color vision [2].

2.1	Basics of Color Measurement

People who believe that the eye is the most important observer of color argue that it is possible to judge color purely by referring to color cards through visual adjustment. Since each person perceives the color differently, this method is very subjective and is not suitable for an objective assessment.

In addition to genetic abnormalities, color vision changes with age due to the build-up of yellow macular pigmentation in the eye. For this reason, it is argued that all original colors must be based on physical measurements. However, these measurements and their interpretation must be closely related to the reactions of the visual observers.

The color control is, therefore, divided into two segments: visual and instrumental.

The color measurement method using the CIELAB method, including measurement with a spectrophotometer, is described below [3].

2.1.1

Spectrophotometer

One method for color measurement is the spectrophotometric method. Spectrophotometers measure the spectral characteristics of light and use them to calculate the standard color values based on the CIE normal observer functions. In addition to the numerical results in different color systems, spectrophotometers allow the graphical representation of the spectral properties of the object color. Colors are created by mixing the different wavelengths in certain proportions. A spectrophotometer evaluates the light reflected by the object for different wavelengths or wavelength ranges and can display the results as a graphic.

For the measurement, the visible spectrum is broken down into small, narrowly defined wavelength ranges, each of which is evaluated by another segment of the sensor (e.g., 39 segments). With this method, the smallest color differences can be determined, which remain hidden from the human eye.

2.1.2

CIELAB Method

The Commission Internationale de l’Eclairage (CIE) has set certain standard values that are used worldwide for color measurement.

The CIELAB system, which is widely used today, consists of the Cartesian coordinates a*, b* (hue), and L* (lightness). Each color can be represented by the L*, a*, and b* values. In certain cases, the polar coordinates L*, C* (chroma, chromaticity), and h° (hue angle) are also used.

The Following Points Must Be Taken into Account when Measuring Color:

       Is the...