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Файл:Color Theory. Tutorial
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2. COLOR: PHYSICAL AND CHEMICAL PHENOMENON
Object colors. Spectral composition
The color of an object (a body) is a certain subjective characteristic
of the radiant flux passing from the object to the human eye. It is
characterized by the spectral composition of this flux.
Acoustics is based on the similar relationships. The main physical
characteristics of sound are its frequency and intensity. They correspond to
certain physiological characteristics of their perception (pitch, sound
volume). However, there is no such strong relationship between the spectral
composition of the incident radiation and the sensation of color.
There are self-luminous and nonself-luminous objects.
The Sun, an electric incandescent lamp, special luminous
compositions for covering watch dials and other devices are the examples of
self-luminous objects in which thermal, chemical and electrical energy or
energy of radioactive decay of products is continuously converted into the
energy of emitted light waves. The spectral composition of the radiation of
self-luminous bodies is quite definite and depends on their state at a given
moment. For example, it depends on the temperature of an incandescent
object. The constancy of the spectral composition of the radiation
determines their certain color under unchanged emission conditions.
Nonself-luminous objects practically do not emit visible light. In the
absence of an external source, they look to us like black objects. These
objects can direct a radiant stream into the human eye. This stream comes
from a luminous source and reflects from the surface of a nonself-luminous
object. The stream can be scattered by the surface or passed through its
thickness. The spectral composition of this flux will depend both on the
spectral composition of the external radiation incident on the body and on
the ability of the body to reflect and absorb different parts of this spectrum.
Therefore, the color of a non-luminous body depends not only on its optical
properties, but also on what light it is illuminated with. For example, if an
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object is capable of reflecting only red rays and it is illuminated with green
rays, it will not reflect anything and will appear black to us. If this object is
illuminated with white light containing red rays in its composition, it will
reflect only the latter, and the body will appear red to us.
Fig. 1. Spectral distribution: a is spectrum of sodium vapors; b is band spectra;
c is continuous spectrum.
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The shape of the spectral distribution curve f (λ) depends on the
nature of the radiating object and its state.
Excited atoms of monatomic gases (He, Ne) or vapors of some
metals (Na, Hg) emit line spectra. Figure 1a shows the spectral composition
of light emitted by sodium vapor excited in an electric arc.
This spectrum consists of two very narrow intense lines close to each
other located in the yellow part of the spectrum.
Excited molecules emit band spectra. Series of very close lines are
grouped in separate sections of the spectrum and fill entire bands, as shown
in figure 1b.
Incandescent solid bodies emit a continuous spectrum (figure 1c). As
the temperature rises, the radiation intensity of different regions increases
and the curve shape changes.
Thus, the spectral composition of visible radiation is characterized
not by a certain value, but by the entire function f(λ), i.e. countless number
of its values for all visible wavelengths:
λ
< λ < λ
violet
. (4)
red
The function f(λ) can be estimated quantitatively by passing
radiation through a prism or diffraction grating and spatially separating rays
of different wavelengths. However, all these rays come to the human eye
together causing the sensation of a particular color.
The Sun has the brightest and richest spectrum. This spectrum
allows people to perceive the world in all the diversity and charm of its
colors.
The spectral composition of solar radiation reaching the Earth
through the atmosphere has a character close to that shown in figure 1c. It
varies depending on the day time and atmospheric conditions. It does not
represent a combination of a small number of individual monochromatic
waves. Due to these conditions, the organs of hearing and the main
photosensitive elements of the eye are not analyzers tuned to resonance with
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certain frequencies of radiation. The radiant flux of a given spectral
composition, which is determined by the type of function f(λ), coming from
a certain part of the object to the human eye, causes certain chemical
transformations of visual purple and creates a single impression of the color
of this area in the brain.
At the same time, due to certain physiological properties of the eye,
the relationship between the spectral composition of radiation and
the sensation of a certain color is ambiguous.
Primary and secondary colors
All colors can be divided into two large groups. There are
achromatic and chromatic colors. The group of achromatic colors includes
white, black, and colors resulting from their mixing (a great variety of gray
colors). All other colors are chromatic ones.
Chromatic colors and their shades are created due to their mixing
with each other.
Primary colors are three colors: red, blue, yellow. They are used to
create other colors. The process of creating different colors is called color
synthesis. It is based on the mixing of primary colors (additive synthesis) or
on the extraction of primary colors from white (subtractive synthesis).
Secondary colors have the radiation of white color after they are
mixed. They are created by additive and subtractive synthesis.
Primary colors of subtractive synthesis are colors created by
extracting three primary colors from white color.
Color characteristics
If you know the color characteristics you can distinguish colors and
combine them into a harmonious unity, creating a certain artistic image.
The main characteristics of colors are hue, brightness, saturation.
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Color hue (shade) is the quality of a color that allows you to give it
a name and is denoted by such terms as “yellow”, “green”, “blue”, etc. It is
used to characterize only chromatic colors. The spectrum of sunlight is
the natural scale of color hues.
Brightness (lightness) is the degree of color difference from black or
white. The lightest one is white, the darkest one is black. In the gamut of
spectral colors, yellow is perceived as the lightest color, and purple is
perceived as the darkest color. Lightness ratios are very important in
composition, as the same color hues evoke different sensations depending on
their degree of lightness. Achromatic colors differ only in lightness.
The lightness of colors is associated in our minds with the amount of black
and white paint in their mixture. Lightness is used to characterize the
illumination of various details. The subjective assessment of the lightness of
differently colored parts is determined by comparing them with achromatic
colors of different lightness.
Saturation is the expression degree of a color hue. It is the degree of
difference between a chromatic color and an achromatic one, which are
equal to their lightness. The most saturated colors are the spectral ones,
which do not have achromatic impurities and the color hue is perceived
sharply. The closer the color is to gray, the less saturated it is. Saturation
determines the intensity of a color. Spectral colors are more intense than
low-saturated ones.
This color characteristic indicates the amount of dye or the dye
concentration. Saturation is a subjective characteristic of a color. We usually
say “strong color” or “faded color”.
There is a dual interpretation of the concept of color. It may be
explained by coloring and differences in lighting. Comparing differently
colored objects, people describe their color hue. In this case, the color hue
is described by light-and-shadow ratios.
In addition to color hue, saturation and brightness, other subjective
characteristics are also used. For example, yellow and red colors are called
warm, blue and green are called cold.
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Coloration
Coloration is a system of color correlation that forms a certain unity
and is an aesthetic manifestation of the colorful diversity of reality.
Coloration is one of the most important means of emotional expression, an
essential component of the artistic image of a work.
The nature of the coloration is associated with the content and
general design of the work of art, with the era, style, and individuality of the
person.
The system of coloration combines colors according to the principle
of their subordination. All colors are in close interaction and cooperation.
There are the main and secondary colors. The main colors are distinguished
quantitatively and qualitatively. These colors determine the coloristic mood
of the whole composition. The secondary colors are used to achieve color
richness and make the main colors more active.
By the nature of the prevailing color combinations, the coloration
can be calm and intense; cold and warm; light and dark; joyful and gloomy,
etc. According to the degree of saturation and color intensity, coloration can
be bright or faded, active or restrained.
In any case, the colors that make up the coloration must be
consistent with each other according to the laws of harmony.
Simple and complex light emissions
Light radiation that affects the eye and causes sensations of color is
divided into simple and complex.
Simple (monochromatic) emission cannot be decomposed into any
other colors.
If the spectral composition of two colors is the same, the colors are
called isomeric. If the radiations of the same color have a different spectral
composition, such colors are called metameric. This phenomenon is the basis
for all systems of color synthesis.
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Light emitted by ordinary sources, as well as light reflected from
non-luminous objects, has a complex spectral composition, i.e. it consists of
the sum of various monochromatic radiations.
Color temperature
Color temperature is the temperature at which a black body emits
light of the same spectral composition as the studied light. It indicates
the spectral distribution of the radiation energy. It does not indicate the
temperature of the source. Thus, the light of the blue sky corresponds to
a color temperature of about 12,500–25,000 K, i.e. much warmer than
the Sun. Color temperature is expressed in Kelvin (K).
The concept of color temperature is applicable only to hot light
sources. The light of an electric discharge in gases and metal vapors (sodium,
mercury, neon lamps) cannot be characterized by the value of the color
temperature. The natural radiations of the sky are not fully temperaturebased (i.e. coming from incandescent objects). Nevertheless, they are
characterized by color temperature quite accurately.
LECTURE QUESTIONS
1. What is the color of an object?
2. What colors are called achromatic?
3. What colors are called chromatic?
4. What are primary colors?
5. What are secondary colors?
6. What color radiation is produced when secondary colors are mixed?
7. What is color hue?
8. What is brightness (lightness) of a color?
9. What is color saturation?
10. What is coloration?
11. What light emissions are called complex?
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12. What light emissions are called simple?
achromatic color
ахроматический цвет
chromatic color
хроматический цвет
color combination
сочетание цветов
color richness
насыщенность цвета
color temperature
цветовая температура
external source
внешний источник
frequency and intensity
частота и интенсивность
incandescent object
раскаленный предмет
incident radiation
падающее излучение
luminous source
источник света
metameric
метамерный, сегментный
monochromatic radiation
монохроматическое излучение
optical properties
оптические свойства
primary color
основной цвет
radiant flux
лучистый поток
radioactive decay
радиоактивный распад
secondary color
вторичный цвет
self-luminous and nonself-
luminous objects
самосветящиеся и
несамосветящиеся объекты
solar radiation
солнечное излучение
spectral band
спектральные полосы
spectral distribution curve
кривая спектрального
распределения
13. What is color temperature?
LECTURE VOCABULARY LIST
3. LIGHT PROPAGATION
In a homogeneous medium, light propagates in a straight line. This
property of light is the basis of geometric or ray optics, which studies
the laws of light propagation by the methods of geometry.
These methods are not in conflict with reality. They significantly
simplify all calculations related to the construction and use of optical
instruments. They are used to explain the formation of images, calculate
aberrations of optical systems, etc. Phenomena associated with the wave
nature of light are not considered in geometric optics.
Ray (beam) and luminous point are accepted as basic concepts in
geometric optics. A beam is a line that determines the direction of
propagation of light waves and light energy. A luminous point is an infinitely
small light source.
In those cases where the light source or the illuminated surface has
certain dimensions, they are considered as a collection or a place of luminous
points with their properties.
Light reflection
It is known that everything that surrounds us either emits light or
reflects it (or transmits light, in the case of transparent objects). If the
spectrum of energy emitted by the body coincides (or overlaps) with
the spectrum of visible radiation, a person perceives it as a luminous object.
The color of this body depends on the spectral composition of the radiation.
As noted, all visible bodies in nature can be divided into selfluminous (light sources) and nonself-luminous (reflecting and transmitting
light).
The brightness of a reflective surface depends on its illumination and
on its reflective properties. The color of non-luminous objects is determined
by their optical properties: spectral reflection, spectral transmission and
light scattering.
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Selective spectral reflection is explained by the fact that coloring
light is reflected from the surface of a painted object to a lesser extent than
from an absolutely white surface, which completely reflects all the light
falling on it (figure 2a). The gray surface absorbs light waves of different
wavelengths evenly. The light reflected from it does not change its spectral
composition, only the radiation intensity changes (figure 2b). Black surfaces
in nature almost completely absorb the light incident on them (figure 2c).
A perfectly black surface does not reflect light at all. Similar surfaces that
reflect and absorb various color rays are called achromatic (colorless). All
other surfaces reflect light differently at different wavelengths.
Spectral reflection, as well as spectral transmission, is explained by
the fact that coloring matter contained in the object absorbs monochromatic
radiation in different ways, i.e. it has different spectral absorption.
The intensity of the reflected light depends both on the nature of the
reflecting surface and on the angle of incidence of the light beam. White
paper, for example, reflects more than 80 % of the light falling on it. While
black ink used to print text reflects only 2–3 %.
Shiny and matte surfaces also reflect light differently. For example,
shiny polished surfaces reflect light like a mirror, i.e. the angle of incidence
of a light beam is equal to the angle of its reflection (the law of light
reflection). While a matte surface reflects light evenly in all directions.
In this case, we observe light scattering.
Selective scattering of light (dispersion) is explained by the fact that
different monochromatic radiations are scattered in different ways. Selective
scattering depends on the size of the smallest particles of the surface
reflecting light. Certain monochromatic radiation is reflected from a particle
only when its wavelength is less than the diameter of the particle. If the
smallest particles are small enough to scatter longwave radiation but large
enough to reflect shortwave radiation, the scattering will be selective. Red
and orange spectral radiation will pass through the surface, while blue and
violet ones will scatter.
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