Color Theory

A thorough review of color theory is beyond the scope of this article; however, several references have been published (1-5). The portion of the electromagnetic spectrum that comprises visible light falls between 380 and 750 nm, bordered by ultraviolet light on the low end and infrared light on the high end. The visible spectrum is represented by the colors seen in the rainbow with blue existing at wavelengths less than 480 nm; green roughly 480-560 nm; yellow, 560-590; orange 590-630; and red, at wavelengths longer than 630 nm (1). Purple is achieved by mixing blue and red and is considered to be a nonspectral color. If white light is dispersed by a prism a spectrum is obtained representing all the visible colors at appropriate wavelengths. The relative power or energy (power multiplied by time) emitted at these wavelengths can be plotted to produce the spectral power distribution curve of the light source. A group of light sources, black bodies, change from black through red to white when heated, the color produced being dependent on the temperature reached. This is referred to as color temperature. Tungsten filaments are close approximations of black bodies, although their color temperature is not exactly equal to their actual temperatures. Real daylight and fluorescent lights do not approximate black bodies.

When light falls on an object it may be reflected, absorbed, or transmitted or a combination of these may occur. When light passes through a material essentially unchanged it is transmitted. Wherever light is slowed down, as occurs at the boundary of two materials, the light changes speed and the direction of the light beam changes slightly (refractive index). The change in direction is dependant on the wavelength and accounts for the dispersion of light into a spectrum by a prism. The change in refractive index results in some light being reflected from the object. Light that is absorbed by the material is lost as visible light. If light is absorbed completely the object appears black and opaque. If some of the light is not absorbed but transmitted the object appears colored and transparent. Lambert's law, which states that the fraction of light

Technologist

Process Consistency Stability Oxidation that is absorbed by a substance is independent of the intensity of the incident light, is always true in the absence of light scattering. Beer's law states that the light absorption is directly proportional to the number of molecules through which the light passes. In practice, Beer's law is often not exactly observed, possibly due to chemical changes in some of the absorbing molecules.

When light interacts with matter it may travel in many directions. The sky is blue because of light scattered by molecules of air. Scattering caused by larger particles produces the white of clouds, smoke, etc. Scattering results in light being diffusely reflected from an object. Translucent materials transmit part of the light and scatter part of the light, if no transmittance occurs, the object is said to be opaque. The color of the object is dependent on the amount and kind of scattering and the absorption occurring. The amount of light that is scattered is dependent on the difference between the refractive indexes of the two materials. The boundary between two materials having the same refractive index cannot be seen because no light scattering occurs. Large particles scatter more light than small particles until the particle size approaches the size of the wavelength, at which point scattering decreases. Many foods are not completely reflecting or transmitting materials, which introduces an element of empiricism into an attempt to measure the color (6). With these foods absorption and scattering are factors affecting visual judgments. The absorption coefficient K and the scatter coefficient S in the Kubelka-Munk equations have been used in attempts to deal with this (7,8).

The chemist tends to think of color rather simplistically as being determined by the amount of light absorbed at a specified wavelength. But that is not how color is seen, for it is the light that is reflected from an object that determines color. The appearance of an object may vary over the entire object because of the angle of viewing and the light falling on the object. Appearance attributes have been divided into two categories: color, and geometric or spatial attributes (3). Color refers to the light reflected from the object to provide a portion of the spectrum as well as white, gray, black, or any intermediate. Geometric attributes result from the spatial distribution of the light from the object and are responsible for the variation in perceived light over a surface of uniform color, such as gloss or texture. The mode in which the eye is operating affects the visual evaluation. There are three modes: the illuminant mode, in which the stimulus is seen as a source of light; the object mode, in which the stimulus is an illuminated object; and the aperture mode, in which the stimulus is seen as light. The aperture mode may be thought of as a lighted window where first the light is seen, but at a closer range the object mode takes over and the contents of the room can be seen. The aperture mode views an object through a smallish aperture removing the effects of spatial distribution of light. This method is usually used for visual color-matching experiments. However, it is the object mode that is of practical importance in assessing the appearance of a food. The everyday evaluations of color, haze, clarity, gloss, and opacity are made by observing what the object does to the light falling on it.

The human eye is an incredibly sensitive and discriminating sensor; it can detect up to 10,000,000 different colors (6). The basic units of sight are the eye, the nervous system, and the brain. The cone light receptors located in the fovea of the retina of the eye are responsible for the ability to see color. It is generally agreed that there are three types of cone receptor responding to red, green, or blue and that these responses are converted in nerve-signal-switching areas within the eye and the optic nerve to opponent-color signals as proposed by Muller. Three opponent-color systems, black-white, red-green, and yellow-blue, were first proposed by Hering (3). The other type of light receptor, the rods, are also located in the retina; they increase in density as the distance from the fovea increases. The rods are responsible for black-and-white vision, the ability to see in dim light. Rods do not contribute to color vision. Approximately 8% of the population, predominately men, have abnormal color vision. For all humans the mode of presentation including viewing angle, source of illumination, and background, affect the color perceived by the viewer.

COLOR MEASUREMENT Tristimulus Colorimetry

It is possible to match colors with a simple laboratory setup. Three projectors with a red, green, and blue filter, respectively, can be focused with the image superimposed on a screen. A color to be matched is projected on the screen by a fourth projector. An operator can match the desired color by determining the amounts of red, green, and blue required for a match. Unfortunately, not all colors can be matched by the red, green, and blue primary colors, so the researchers were given free rein to choose other primaries. They chose X (roughly corresponding to red), Y (roughly corresponding to green), and Z (roughly corresponding to blue). X Y and Z are unreal in that they cannot be produced directly in the laboratory, but they are very useful mathematically. This concept was accepted by the Commission Internationale de Eclairage (CIE) and became known as the CIE system. These tristimulus (three-stimuli) values for the equal-energy spectrum were used to define the 2° 1931 standard observer and were designated as x, y, and z, which are special cases of X, Y, and Z; the 2° refers to the angle of observation. Later systems of measurement were developed with a 10° angle of viewing and are thus called 10° 1964 standard observers. The definition of the XYZ system offered a number of advantages. First, all colors lie within the primary coordinates. Second, the Y value contained all the lightness, and third, many of the coordinates for orange and red colors lie along a straight line, which is a great advantage in color formulation.

The XYZ values, sometimes presented as Y,x,y or xyz as described below, provided a measure of the color of a sample when viewed under a standard source of illumination by a standard observer. The CIE organization defined a number of standard sources of illumination. CIE Source A represented incandescent light and a color temperature of 2,854 K; CIE Source B simulated noon sunlight; and CIE Source C simulated overcast-sky daylight. These light sources were supplemented by the CIE in 1965, and illuminant D66 with a color temperature of 6,500 K is now widely used. In 1931 the CIE adopted the standard observer as having color vision representative of the average of that of the population having normal color vision. These data were used to obtain CIE standard-observer curves for the visible spectrum for the tristimulus values r, g, and b, for a set of red, green, and blue primaries. The red, green, and blue data were transformed into XYZ values and provided the basis for the standard observer curves shown in Figure 2 (1). The definition of the standard observer curves made it possible to design a simple colorimeter. Figure 3 shows white light falling on a sample to be measured. The reflected light is passed through a glass filter to determine the X component, the Y component, and the Z component, respectively. The signal from each filter is measured electronically and constitutes a tristimulus reading for the sample. The accuracy of the reading depends on the ability of the manufacturer to produce a filter/photocell combination that duplicates the response of the human eye, that is, the standard observer curves. They have been very successful in this aspect, and several commercial colorimeters available today are based on this principle.

Spectrophotometry

The CIE values of a color can be calculated by multiplying the energy spectrum of the illuminant by the reflectance, or transmission, spectrum of the sample and the standard observer curves. They can be mathematically represented by the following integral equations:

Figure 2. Curves for 2° standard observer expressed as tristimulus values X, Y, and Z.

Wavelength (nm)

Figure 2. Curves for 2° standard observer expressed as tristimulus values X, Y, and Z.

Figure 3. A simple tristimulus colorimeter. The light reflected from the sample passes through the three tristimulus filters, X, Y, and Z, and the signals are recorded by the photocell. The combined outputs from the filter/photocell combination duplicate the standard observer curves xy, and z and represent the color of the sample.

Tristimulus filters

Figure 3. A simple tristimulus colorimeter. The light reflected from the sample passes through the three tristimulus filters, X, Y, and Z, and the signals are recorded by the photocell. The combined outputs from the filter/photocell combination duplicate the standard observer curves xy, and z and represent the color of the sample.

J380 r750

J380 /-750

Z = REzcfe

Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

Thousands Have Used Chemicals To Improve Their Medical Condition. This Book Is one Of The Most Valuable Resources In The World When It Comes To Chemicals. Not All Chemicals Are Harmful For Your Body – Find Out Those That Helps To Maintain Your Health.

Get My Free Ebook


Post a comment