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颜色与光技术篇






时间:2009-01-07 12:44来源: 作者: 点击:

核心提示: The Nature of Light "Light" is a special, narrow range of electromagnetic energy. It is a special range because it alone can stimulate the two types or receptors within the eye which permit vision. Therefore, we call light "visible energy,


The Nature of Light

"Light" is a special, narrow range of electromagnetic energy. It is a special range because it alone can stimulate the two types or receptors within the eye which permit vision. Therefore, we call light "visible energy," even though we cannot see the energy itself.

Light and Energy

Electromagnetic energy is only one form of energy known today. Other forms are thermal, chemical, kinetic, atomic, electrical, etc. Electromagnetic energy is also referred to as radiant energy because it exists only in the form of repeating wave patterns traveling in straight paths, as rays, in all directions from its source. So light, being a special form of radiant energy, also is called "visible radiant energy."

Considering that energy cannot be destroyed - only changed from one form to another, and considering the physiological composition and functions of the eye, we can now understand that light is transformed from electrical energy to radiant electromagnetic energy within a light source, travels in a high-speed, high-frequency wave form, and becomes useful to man when a sufficient amount of it is transformed into chemical energy within the receptors of the eye.

There is a very broad spectrum of radiant, electromagnetic energy, of which light is but one narrow band. All radiant energy travels at the speed of 3x10**8 meters per second (186,000) miles per second) in air or in a vacuum. At one end of the spectrum are cosmic rays, and at the opposite end are electrical power waves. The individual types of radiant energy are identified by their particular ranges of frequencies, or number of wave cycles per second. The average wavelength of the shortest cycles of radiant energy known (cosmic rays) is 0.00001 nanometers. (One inch contains about 25.4 million nanometers. A nanometer is one thousand-millionth of a meter.) At the other end of the known spectrum of electromagnetic radiation are electric power waves - with an average wavelength of almost 5 million meters (3100 miles).

The spectrum of radiant energy waves we call light is very narrow, ranging from approximately 380 nanometers to 760 nanometers (or from 15 to 30 millionths of an inch). Wavelengths shorter or longer than these do not stimulate the receptors in the eye. Beyond this range is darkness, for, while the eye may be exposed to many other wavelengths of radiant energy, they are not capable of initiating responses in the eye.

Light Sources

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The sun - and electric lamps, are considered light sources because they transform energy from another form in to the radiant energy wavelengths which we call light. But these light sources also emit useful energy at wavelengths both shorter and longer than light waves. Ultraviolet energy - valuable for its germ-killing, suntanning, and photochemical properties, has wavelengths shorter than light waves; and infrared energy waves (often referred to as heat rays) are longer than light waves. All radiant energy, when absorbed, can be transformed into heat.

The Color Spectrum

A light source emitting radiant energy relatively balanced in all visible wavelengths will appear white to the eye. However, passing a narrow beam of white light through a prism of transparent material will spread and separate the individual wavelengths of visible energy so that the eye can distinguish between them - the resulting visual phenomenon is called a color spectrum. The normal eye will see three wide bands of blended color - violet, green, and red, with several narrower bands (blue, yellow, and orange) blended between the wider bands. The "colorblind" eye will see only graduations of gray, or perhaps some of the colors and some gray - depending upon the extent of physiological impairment of the eye.

A lamp designer is more concerned with the cause of these wavelengths than he is with the names we have given them. However, it is vitally important that he know that wavelengths longer than 610 nanometers produce that effect we call "red" - and those between 440 and 500 nanometers are called "blue," and so on, in order to control both the appearance of light sources themselves and the effects light sources have on the appearance of colors in objects around us.

THE NATURE OF COLOR

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"Color" is a term which describes an imbalance of visible radiant energy reaching the eye from light sources and objects. . . imbalance being defined as any deviation from the averaged amount of energy at all wavelengths. Such deviations, or imbalance combinations, are almost incalculable in number - which accounts for there being so many "colors," or names, to describe the various "mixes" or combinations of visible energy.

WHAT COLOR IS

A colored light source radiates much more energy at some wavelengths than at others, and a colored object reflects or transmits some wavelengths more readily than others. In either case, there is an energy imbalance, occasionally to the point where some wavelengths actually are missing, in the mix that reaches the eye. Thus we see that color has both qualitative and quantitative characteristics. The qualitative characteristics refer to information on which wavelengths are present; while the quantitative characteristics refer to how much energy is present at each wavelength. The qualitative characteristics are a specification of chromaticity, and are termed dominant wavelength and purity. The quantitative characteristic is a specification of luminance - formerly called photometric brightness.

Dominant wavelength is the wavelength, or color,, which appears to be most abundant. However, it need not be the wavelength which is actually the strongest in intensity, although both wavelengths are usually very near to each other.

Purity may be described as the percentage of color vs the percentage of white in any color.

To demonstrate that color is the result of an Imbalance of visible radiant energy, consider two theoretical objects which both reflect half of the light from a perfectly balanced white light source. One reflects half of the energy at all wavelengths of the visible spectrum - it appears gray and produces no color sensation because all wavelengths are still present, though only half as intense. The other object reflects all the energy in half of the spectrum - say, the shorter wavelengths, from 380 manometers to 570 manometers - but no energy in the other half of the spectrum. This object will produce a strong color impression - blue, but only a secondary sensation that only half of the light source energy is being reflected. So it is apparent that color is not a result of any changes in volume of total radiant energy, but is a result of energy deficiencies at individual wavelengths.

An extreme case of imbalance of light waves would be monochromatic situation wherein only one color, or wavelength of visible energy, is present. The color perceived would be of the purest quality since no other wavelengths of energy (colors) are present to dilute its purity.

SEEING COLOR IN OBJECTS

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It is now apparent that without light there can be no colors, for colors are simply other names to describe the various mixtures of electromagnetic energy - which exists only in the transient state of radiation. And since colors are descriptions of dynamic, or moving, phenomena, they cannot also be physical properties of stationary objects. What, then, are colors in objects?

Colors we see in objects are still the result of waves of radiant energy that reach the eye - but after they have been modified in many ways by each object.

All physical objects have a modifying effect on light waves - reducing both the amount of energy and the types of light waves which reach the eye from the light source. Even the particles in the earth's atmosphere filter the sun's radiation before it reaches our eyes - which is a partial explanation for changes in the colors of the sky and clouds at sunset.

We call radishes "red," lemons "yellow," and pine trees "green" - in fact we have assigned color names to almost everything we come in contact with in our daily lives. That these objects appear to be the same colors under all lighting conditions is called "color constancy" - which simply means that those objects consistently reflect or transmit light waves only in a particular, narrow color range while absorbing all others. Water has no color constancy because it will reflect and transmit all light waves, hence, it appears to be whatever color is dominant in its surroundings.

The two fundamental ways in which objects and mediums modify the colors of light have already been mentioned-transmission and reflection, but objects and mediums usually are selective in how much energy, and at which wavelengths they will transmit or reflect light.

Just how much the color and intensity of the transmitted light is modified depends on the molecular composition of the materials through which the light passes. For example, in some colored lamps, coatings of colored pigments and dyes are used to selectively absorb unwanted wavelengths or colors and transmit the desired wavelengths. In other cases, the glass or medium itself is colored to achieve the same effect.

When light is evenly played on a diffuse (unpolished) surface, the effect is that light waves are reflected in all directions, but only after they have been modified by the absorption qualities of the surface. The result is that the surface then appears to have a color all its own . . . different from the color of the light source. But that is only because the surface has absorbed various amounts of various wavelengths of spectral energy. Sandstone is a highly diffuse material with relatively even spectral absorption qualities, hence, it appears tan in color throughout. A coat of paint on an object also has an evenly distributed quality of absorbing, and thus will evenly reflect whatever colors, or wavelengths of energy, that are not absorbed by the paint.

It is important to reiterate that since all light waves are modified in some way by all physical objects - that the color appearance of an object is determined by the mix and energy of light waves which remain intact to reach our eyes. Objects have a characteristic color only because of the way they selectively reflect or transmit or otherwise modify various wavelengths of light.

Take for example butter, which appears "yellow" because it absorbs blue light and reflects a high percentage of all other colors. The resultant combination, or dominant wavelength, is yellow. Similarly, lettuce reflects light with wavelengths primarily in the 500 to 600 manometer (green) range and absorbs most of the energy at other wavelengths. A tomato, then, is red only because it reflects radiant energy at 610 to 780 manometers while absorbing most of the energy at other wavelengths.

But just as important to the apparent color of objects is the character of light waves being radiated onto the objects by the light source.

COLOR IN LIGHT SOURCES

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Colored light sources emit energy in selective wavelength bands, and "white" light sources generally emit energy at all visible wavelengths. But some light sources actually are deficient in energy at various wavelengths and still emit what is considered "white" light. This deficiency affects the perception of object colors (color rendition) and color differences, graying some colors while increasing the relative vividness of others.

There are "warm" sources such as all incandescent and some fluorescent lamps. These produce white light that tends to be strong in the red, orange, or yellow wavelengths. Conversely, there are "cool" sources such as clear mercury lamps and other fluorescent sources which produce white light that is strong in blue and green. Lighting a surface alternately with warm and cool lamps, then, will produce an apparent change in the perceived color of that surface, despite the fact that in both cases, so-called white sources are used. This effect is most pronounced if the changes are rapid, and the observer does not have time to adapt to the difference in whiteness.

Some light sources, of course, are deliberately made with only one color predominant, to achieve a specifically desired effect. For example, if a wall that appears white under a white light is lighted with a light source that is predominantly red, the wall will appear to be red because only red wavelengths of visible energy are present to be reflected from the wall toward the observer's eye. If the same white wall is lighted with green light, the wall will appear green.

As we have seen with the tomato example, lighting a red surface with a "white" light source will make the surface appear red - because only red wavelengths of light are reflected toward the observer's eye . . . all other wavelengths are absorbed. However, if the same tomato is lighted with a green light, it will appear much darker and essentially colorless (brownish gray) because there is little red energy in the green light to be reflected. The important point is that regardless of the characteristics of a surface finish, the eye cannot see colors from it that are not contained in the source of illumination.

THE PSYCHOLOGY OF COLOR

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Up to this point we have treated color in the vernacular of the illuminating engineer or physicist. The definition given was: "Color" is a term which describes an imbalance of visible radiant energy reaching the eye from light sources and objects. And this definition is true - as far as it goes. But consider this definition - "Color" is a concept or human interpretation of the neural impulses transmitted to the brain from the normal eye when it is stimulated by various imbalances of visible radiant energy. This latter definition is not more true - it is just more complete - because it encompasses all three sciences involved: physics, physiology, and psychology. Paraphrased, color is a concept resulting from the interaction of light source, object, eye, and brain.

COLOR PERCEPTION

The process and mechanisms by which the brain "perceives" color, or any other concept, is not yet fully defined, but science and technology are being continually advanced toward that end. Perhaps someday we will understand how we think "color" - but for the moment we are concerned with what is already established knowledge in the field of light and color.

The more complete definition of color as a concept is accurate when we consider that color does not exist independent of normal color vision. As previously mentioned, the totally color-blind person cannot distinguish between various wavelengths of light - he can only distinguish between various amounts of light. To him there is no "color" everything is either black or white or shades of grays in between. Most important is the fact that the physical light waves received by both a colorblind person - and by a person with normal color vision, are not changed by the condition of the receptors in the eyes of either person. Only the concept (perception or interpretation) of what is seen by each person is changed. Therefore, red, or any other color, is exclusively the mental concept resulting from the brain's interpretation of special visual stimuli.

COLOR ASSOCIATIONS

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Now that we have brought color into the realm of the abstract concept, it immediately becomes a part of what we think about everything else - objects, situations, attitudes, moods, environmental conditions, etc. Color is unconsciously assimilated into all our impressions or concepts because our eyes are constantly supplying our brain with color information which is automatically associated with all other information on any given subject or situation.

Perhaps more than any other single element in design, people's reactions to color affect their individual preferences. But, some color reactions are universal. Certain colors, for example, are associated with certain moods. Reds, oranges, and yellows, are generally accepted as stimulating while blue, blue-violet, and violet are considered the least exciting.

People's reactions to the colors associated with materials do not always correspond to their reactions to the same colors associated with light. As a demonstration, the green in foliage is generally accepted as refreshing, cool, and undisturbing, so people have come to think of trees and shrubs as somewhat neutral or quiescent in effect; hence, green background materials in man-made objects are psychologically restful. But green in a light source is unnatural; and used alone, tends to produce a macabre or sinister effect.

Where the "proper" appearance of people is important, there is a strong unconscious preference for white light sources which are rich in red light; these lamps help impart a healthy, ruddy or tanned impression of the skin and flatter the complexion. There also are indications that people prefer warm light in areas where lower levels of illumination are involved, while cool light seems to be more acceptable for higher levels.

Where stronger, more saturated colors are involved, people generally agree that warm colors appear to advance, while cool colors recede and help support a feeling of spaciousness. In a more subtle sense, changes in the color of light appear to alter the moods of a space - the impressions associated with warm sunlight and cool shadows; the pinks and purples of a sunset; the similar restful qualities of the interior of a cathedral where light is tinted by stained glass windows; the gaudy, overstimulating effects of rapidly moving, colored carnival lights. People invariably feel the psychological impact of light and color without ever realizing that they do - or even analyzing the reasons for associating colors with moods.

Color Vision

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As previously learned, the process of seeing light and color is a complex process involving physics, psychology, engineering, photometry, etc. - and now we come to the eye itself - physiology.

Eye Functions

There are many theories to explain the phenomenon of color vision. The most easily understood is Young's three-component theory which assumes three kinds of light sensitive elements (cones) - each receptive to one of the primary colors of light - an extreme spectrum red, an extreme spectrum violet and an imaginary green.

The cones in each eye number about seven million. They are located primarily in the central portion of the retina called the fovea, and are highly sensitive to color. People can resolve fine details with these cones largely because each one is connected to its own nerve end. Muscles controlling the eye always rotate the eyeball until the image of the object of our interest falls on the fovea. Cone vision is known as photopic or daytime vision.

Other light receptors, called rods, are also present in the eye but they are not involved in color vision. Rods serve to give a general, overall picture of the field of view, and are receptive only to the quantity of light waves entering the eye. Several rods are connected to a single nerve end; thus they cannot resolve fine detail. Rods are sensitive to low levels of illumination and enable the eye to see at night or under extremely low lighting conditions. Therefore, objects which appear brightly colored in daylight when seen by the color-sensitive cones appear only as colorless forms by moonlight because only the rods are stimulated. This is known as scotopic or night vision.

DAY-NIGHT VISION

As the adjacent spectral sensitivity curves show, the eye is not equally sensitive to all wavelength. In dim light particularly, there is a definite shift in the apparent brightness of different colors. This was discovered by Johannes von Purkinje. While walking in the fields at dawn one day, von Purkinje observed that blue flowers appeared brighter than red, while in full daylight the red flowers were brighter than the blue. This is now called the Purkinje effect and is particularly important in photometry the measurement of light.

COLOR DEFICIENCIES

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The totally color-blind person cannot distinguish between color and quantity because the cones are either partially or totally impaired -only the rods are functioning. This person's eyes are only sensitive to luminance, or quantity of light. As a result, light sources appear as "brighter or dimmer," and objects appear as "lighter or darker."

A totally color-blind person has full appreciation of his surroundings - but in values of grays, much as a person with normal color vision has full appreciation of a color TV program which is viewed on a black and white TV set.

The most prominent type of color deficiency is known as Deuteranomaly - or red-green blindness, wherein the person sees yellows and blues normally, but has trouble differentiating reds and greens. Only about five percent of the male population has this deficiency, and only 0.38 percent of the females. An even smaller number of people - 0.003 percent males and 0.002 percent females, are totally color blind.

COLORIMETRY

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Colorimetry is the science of measuring and systematically designating colors. It is an important science, since precise systems of color measurement are required to identify, duplicate, and standardize the thousands of colors in use today - and many useful systems have been set up to organize and specify these colors. But before any of the systems can be described, it is essential to understand the relationship which exists between the primary colors of light and the primary colors of pigments.

PRIMARY AND SECONDARY COLORS

The primary colors of light (red, green and blue) can be added to produce the secondary colors of light - magenta (red plus blue), cyan (green plus blue), and yellow (red plus green). Thus, the colors of light are called "additive." A secondary color of light mixed in the right proportions with its opposite primary will produce white light. For example, a mixture of yellow and blue light will result in white light. Thus, yellow and blue are complementary colors of light - as are cyan and red, and magenta and green.

In pigments or colorants, however, a primary color is defined as one that subtracts or absorbs a primary color of light and reflects or transmits the other two. So the primary colors in pigments (sometimes called subtractive primaries) are magenta, cyan, and yellow - the secondary colors of light.

This subtractive nature of pigments is easily demonstrated by placing magenta, cyan, and yellow pigment filters over a source of white light (see adjacent illustration). Each of the pigment filters absorbs or subtracts one of the primary colors from the light. Where two filters overlap, one of the primaries of light is transmitted. For example, the yellow filter absorbs blue (transmitting red and green) and the magenta filter absorbs green (transmitting red and blue). Together, the filters transmit only red - having, in effect, subtracted the other two primary colors from the white light. Where the three pigment filters are superimposed at the center, all light is absorbed. Complementary pigment colors are the same as those in light - yellow and blue, cyan and red, magenta and green.

Color television reception is an example of the additive" nature of light colors. On the interior face of the picture tube is applied approximately 100,000 triangular dot patterns of electron-sensitive phosphors - each triad consisting of one phosphor dot each that will radiate red light, blue light, and green light, respectively. In operation, all red emitting phosphor dots in all triads are stimulated by electron pulses from an electron gun inside the picture tube which generates "red" pulses corresponding to "red" energy seen by the TV camera. Blue and green electron guns inside the picture tube stimulate the blue and green triad phosphors in the same manner.

The effect, viewed on the home color TV receiver, is that the three primary colors from the phosphors are "added" together and received by the color sensitive cones in the eye, and a full color image is perceived. Thirty successive image changes per second - in all three colors, complete the motion illusion in color television.

THE COLOR TRIANGLE

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Although black and white pigments are not considered true colors, their addition to colored pigments produce tints, shades, and tones. There is a triangular relationship involved here - that adding black to a pigment color produces a shade, whereas adding white produces a tint. When gray (a mixture of black and white pigments) is added to a color pigment, a tone is produced.

OSTWALD COLOR SYSTEM

Similar in arrangement to the Color Triangle is the Ostwald Color System which arranges lettered color chips in triangles and describes them in terms of purity, whiteness, and blackness. The purest colors (or hues) contain no white or black pigments. The system as originally published divided the spectrum of colors into 24 basic numbered hues with 28 variations of each in lightness or darkness (tints, shades, and tones).

MUNSELL COLOR SYSTEM

The Munsell system of color notation is based on a theoretical solid form - much like an irregular globe. The vertical axis is graduated into nine shades of gray - with black at the bottom as zero, and white at the top as ten. The colors of the spectrum are divided into 20 basic hues which are represented as vertical pie sections of the solid with their purest colors located around the perimeter, or equator. The Munsell system also uses a set of coded, standardized color chips for each color. Variables in the Munsell system are hue, value, and chroma.

Hue is the classification of a color by which the eye sees it as red, blue, green, yellow, etc. A Munsell hue is designated by a single letter - R for red, G for Green, or pairs of letters, such as YG, for yellow-green.

Value (similar to the gray scale in the Ostwald System) indicates lightness or darkness of a color on a scale ranging from 0 for black to 10 for white. Thus, a color can be dark or light red, indicating a position in a light-to-dark scale, (Munsell value is approximately equal to the square root of the percent reflectance of the color.)

Chroma indicates the purity or saturation of the color, or conversely, its freedom from dilution. Chroma is indicated by a number preceded by a slant line, following the value notation.

ISCC-NBS Color System

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The Inter-Society Color Council - National Bureau of Standards (ISCC-NIBS) has standardized 267 names for describing the colors of paint. Each name is matched to a color chip, with the boundaries of each name fixed with limits defined in the Munsell color notation system.

The following 28 basic hue names are used:

Additional adverbs and adjectives are used in combination with the above hue names to completely identify the 267 basic color chips in the system. The additional modifiers are:

C.I.E. Color System

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The C.I.E. color system was devised and adopted the C.I.E. (Commission International de l'Eclairage - the International Commission on Illumination) in 1931 and has since become an international standard for measuring, designating, and matching colors.

In the C.I.E. system, the relative percentages of each of the theoretical primary colors (red, green, blue) of a color to be identified are mathematically derived, then plotted on a Chromaticity Diagram as one chromaticity point, the dominant wavelength and purity can be determined. All possible colors may be designated on the Chromaticity Diagram, whether they are emitted, transmitted, or reflected. Thus, the C.I.E. system may be coordinated with all other color designation systems.

To specify the chromaticity of a color by the C.I.E. system, it is first necessary to measure the color's spectrophotometric values (reflectance, emittance, or transmittance) at each wavelength. These values then must be weighted by the values of the three theoretical primaries and the resultant computation will represent the amount of the theoretical primaries (red, green, and blue) needed to produce for the standard observer the color of the spectrum at that wavelength.

The sums of each of these calculated red, green and blue calculations are called the tristimulus values for that color. Tristimulus values are denoted by capital letters, X (for red), Y (for green), and Z (for blue). The Y (green) value also is that color's luminosity. The tristimulus values are then used to calculate the color's chromaticity coordinates.

A color's chromaticity coordinates represent the relative percentages of each of the primary colors present in a given color. Lower case letters are used to designate the coordinate values: x = red, y = green, and z = blue. The fractional values are easily computed from the tristimulus values, X, Y, Z, by the following equation:

X/(X+Y+Z)=x

By substituting Y and Z, respectively, in the numerator of similar equations, the chromaticity coordinates for green (y) and blue (z) may also be calculated. Since the coordinates represent fractional values, then the sum of x, y, and z will always equal unity, 1.0.

The present C.I.E system is more accurate than both the Ostwald and Munsell systems because it specifies color on a physical basis - eliminating the need for human or subjective comparisons or judgments. The most complete measurement of color, then, would necessarily have to include the C.I.E. chromaticity, dominant hue, and purity data together with the spectral energy distribution data in order to pinpoint all key variables,

COLOR TEMPERATURE

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All objects will emit light if they are heated to a sufficiently high temperature. Also, as an object is raised in temperature, the color of the light emitted from it will change. An iron bar, for example, appears dull red when first heated, then red-orange, then white, and finally blue-white as it is heated hotter and hotter. In the same way, a tungsten filament in an incandescent lamp changes color when different voltages are applied. This phenomenon was studied by Max Planck in 1900 and is the basis for his law of blackbody radiators. This law, in essence, predicts the distribution of thermal radiation as a function of temperature, and defines the upper limit of thermal radiation. A blackbody is defined as one which will absorb all radiation falling on it.

This law can be used to designate the relative color temperature of any heated object. A color temperature designation, applied to a light source, refers to the absolute temperature in degrees Kelvin of a theoretical blackbody or full radiator whose color appearance matches that of the source in question. Such a body is black at room temperature, red at 800K, and brilliant blue at 60,000K. Tungsten filament lamps used for general lighting have color temperatures in the 2600K to 3000K range. Low wattage lamps used where luminance is not too important operate at about 2000K. Such lamps as TV and studio floods operate in the 3100 - 3400K range, just short of the tungsten melting point of 3500K. In most cases, actual filament temperature is slightly lower that the apparent color temperature.

Technically, a "color temperature" designation can only apply to incandescent sources, and, as such, it is a specification of both the degree of whiteness and the spectral energy composition of the source. However, the term "apparent color temperature" is often used to specify the degree of whiteness of fluorescent lamps as well as sky light, mercury vapor lamps, etc.

Typical examples of "apparent color temperature" values are as follows:

  • Warm White (WW) and Deluxe Warm White (WWX) Fluorescent Lamps - 3000K
  • Cool White (CW) and Deluxe Cool White (CWX) Fluorescent Lamps - 4000K
  • White (W) Fluorescent Lamp - 3500K
  • Daylight (D) Fluorescent Lamp - 7000K
  • Sunlight at sunrise -1800K
  • Sunlight at noon - 5000K
  • Sky - overcast - 6500K
  • Sky - extremely blue (clear northwest) - 25000K

Unfortunately, the apparent color temperature designation for any light source does not give information on its specific spectral energy distribution. For example, Cool White and Deluxe Cool White lamps appear to be the same color but their spectral distribution curves are quite different and their effects on colored objects and materials are definitely different. The same limitation applies in using color temperature notations to specify sky light, mercury vapor lamps, etc.

Standard Light Sources

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Three sources have been selected by the C.I.E. as standards for use in colormetry. These sources are designated source A, B, and C. Calibrated lamps may be obtained from the National Bureau of Standards.

Source A is a tungsten filament lamp operating at 2854K. Source B utilizes a lamp having the spectral quality of source A in combination with a specified filter. The apparent color temperature of source B approximates that of noon sunlight (4870K). Source C is source A, the tungsten-filament source, but with a different filter. The filter and lamp combination produces a color quality which approximates daylight at 6770K.

COLOR MEASURING INSTRUMENTS

The two most common color measuring instruments in use today are the spectrophotometer and the colorimeter. Their operating principles are quite different.

The spectrophotometer gives the radiant energy at each unit wavelength across the entire visible spectrum, while the calorimeter gives only the sum of radiant energy for each primary. Spectral energy data from either may be used to calculate the C.I.E. chromaticity of a color, but it is obvious that the spectrophotometer calculations will be more accurate since the input data are more complete.

Spectral energy distribution curves from spectrophotometers are often used for color matching where high accuracy is essential. Conversely, colorimeters are not as accurate as spectrophotometers over broad ranges, but they are effective for general color matching where minor color variations are immaterial. Colorimeters are quite accurate within narrow color ranges, though, and are often built into production color control operations.

Both devices will perform in either additive or subtractive modes, and thus will provide color measurements from similar light sources or object colors.

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