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Color Management
In 1666, Isaac Newton Discovered that when a beam of light of sunlight passes through a glass Prism, the emerging beam of light is split into a spectrum of colors ranging from violet at one end to red at the other.
Three basic quantities are used to describe the quality of chromatic light source.
1. Radiance: radiance is the total amount of energy that flows from light source measured in watts (W).
2. Luminance: Luminance gives a measure of the amount of energy an observer perceives from a light source – measured in lumens (LM).
3. Brightness: Brightness is a subjective descriptor that is practically unmeasurable.
Hue: Hue and saturation taken together are called chromaticity.
A color may be characterized by its brightness and chromaticity.
The amount of red, green, and blue needed to form any color are called the tristimulus values and are denoted, X, Y, and Z.
A color is then specified by its trichromatic coefficients which means
X + Y +Z = 1
The use of color is important in image processing because:
a. Color is a powerful descriptor that simplifies object identification and extraction.
b. Humans can discern thousands of color shades and intensities, compared to about only two dozen shades of gray.
Color image processing is divided into two major areas:
a. Full-color processing: images are acquired with a full-color sensor, such as a color TV camera or color scanner.
b. Pseudocolor processing: The problem is one of assigning a color to a particular monochrome intensity or range of intensities.
Figure 15.1 Primary and secondary colors of light
RGB colors are used for color TV, monitors, and video cameras.
However, the primary colors of pigments are cyan (C), magenta (M), and yellow (Y), and the secondary colors are red, green, and blue. A proper combination of the three pigment primaries, or a secondary with its opposite primary, produces black.
Figure 15.2 Primary and secondary colors of pigments
CMY colors are used for color printing.
Color characteristics
The characteristics used to distinguish one color from another are:
a. Brightness: means the amount of intensity (i.e. color level).
b. Hue: represents dominant color as perceived by an observer.
c. Saturation: refers to the amount of white light mixed with a hue.
The CIELAB color space is the most common used color space in the industry. The vertical L* axis reflects the lightness of a color. Here L*=0 represents absolute black and L*=100 represents perfect white. The positive a* axis represents the red parts of a color and the negative a* axis represents the green shares. The positive b* axis is for the color yellow and negative b* values mean blue. Thus, in this three-dimensional structure you can "address" all real existing colors at one light type, measurement geometry, and standard observer.
Another way to display the a* and b*-axes is the representation in polar coordinates. C* is named chroma and shows the difference from the neutral gray axis to the sample. H is called the Hue angle. It is always measured counterclockwise starting from the positive a* axis. This set of definitions is mainly used for saturated colors, since values in numbers are easier to understand. For instance, the color orange is more saturated than the sample, not more red and more yellow. The difference from a sample to the standard is stated in delta values (Δ or d):
- ΔL* = 0.5 sample is 0.5 units brighter - Δa* = -1.5 sample is -1.5 units greener - Δb* = -3.6 sample is -3.6 units bluer - ΔC* = -3.9 sample is -3.9 units less colored - ΔH = 0.7 sample is 0.7 depending on the location of the color shades
Optical brightening agents (OBAs) absorb invisible UV radiation and emit in the visible range. This can create reflectance values of more than 100%, i.e.: at specific wavelengths more light is reflected by the sample than had come in at these wavelengths. This effect is utilized in white papers or textiles, where blue light is excited. The CIE whiteness correlates better to the visual assessment than ISO whiteness, since the entire visual range is taken into consideration.
ORIGINALS
Any copy whether it is a mechanical, artwork or other material from which reproductions are to be made is called as a Original. Original is a term which can include camera ready artwork, drawings, paintings, photographs, transparencies, black-and white or colour prints and even three-dimensional objects. The term original commonly refers to photographs used for haftones or to original line art.
1. Reflection Originals
Any original copy which is to be reproduced that exists on an opaque substrate (such as photographic print) is called as Reflection Original. Reflection Originals must be scanned by reflecting light from their surface. Photographic color prints, paintings, wash drawings are termed reflection originals.
2. Transmission Originals
Any original copy which is to be reproduced that exists on a transparent substrate (such as photographic transparency) is called as Transmission Original. Transmission Originals must be scanned by transmitting light through their surface. Color transparencies are termed as transmission originals.
2. CLASSIFICATION OF ORIGINALS
The process of graphic reproduction, whether through traditional or digital means, starts with an assessment of originals. Originals may be monochrome, for single-colour reproduction, or colored for multi-coloured reproduction.
Originals can be further classified as follows:
1. Line originals
Line originals have no gradation of tone - that is, they possess no intermediate tones. The image is produced by clear distinct lines, or other shapes of uniformly solid areas. Text or artwork containing no tonal values or shades of gray and which can be imaged and printed without the need for halftone screens are called as line originals.
Examples of line originals include: paper paste-up from phototypesetters, typewritten or laser-printed line copy, dry transfer lettering; pen and ink effect drawings (in black ink on white paper or board): or their digital equivalent, produced electronically as described previously, using a word processing or similar based program, also draw or paint software program on a host computer system
Line Original
1. Monochrome line originals
Line originals prepared for single color are called as monochrome line originals. Typical examples of monochrome originals are technical drawings, figure drawings, architectural plans, etc.
When produced traditionally, it was generally accepted that flat artwork should be prepared in a size larger than that of the finished size, probably 1.5 to 2 times, as photographic reduction gave a sharper result and any minor irregularities tended to be lost. Excessive reduction if required should be avoided, since this causes loss - of details. For example, fine lines can fill in on thin type reversed out of a solid area.
At present line originals are reproduced using graphic software packages and/ or word processing software, which allow relatively simple and Straightforward graphic forms to be created: an alternative is the use of clipart or similar systems. These software packages allow the operator to view originals on screen, at for example 200% and 400%, and thus ensure at least relatively fine definition and correct butting up of line edges, etc. These can then be checked and adjusted, so that when reproduced at the correct size, the desired results are achieved without visible imperfections.
2. Colour line originals
Line illustrations may be produced for printing in two, three, four or more colours, with a separate colour split for each colour are called as color line originals.
Different techniques are used to isolate, or separate, the original into the required number of colours. Traditionally, using flat artwork, this would take the form of a coloured original, or a key drawing with the different colour areas indicated on an overlay or series of overlays. In computer-generated illustrations, the illustrator/operator will simply highlight or mask off the coloured original by use of the cursor, or pressure pen and pad system, tracing around the required areas, and instructing the software program in use to split for colour as requested.
2. Continuous Tone Originals
Continuous tone originals, consist of a variety of gradations between highlights (lightest areas), mid tones (neutral/mid-way areas) and solids (darkest areas).
Tone originals may be,
1. Monochrome Tone Originals (eg. Black and White Photographs)
2. Color Tone Originals (eg. Color Transparencies, Color Prints or Color artwork)
Continuous tone is a photographic image that is not composed of halftone dots, or in other words, an image that consists of tone values ranging from some minimum density (such as white area) to maximum density (such as dark area). An example of a continuous tone image is a photograph or a color transparency.
Other examples of continuous tone originals include: photographic prints and transparencies: plus wash drawings, pencil, charcoal and crayon sketches - all of which are increasingly prepared and reproduced by electronic means.
Continuous Tone Originals
Transparencies are still one of the most popular mediums for colour reproduction of continuous tone originals, although digital media such as Photo CD and digital picture libraries are increasing in popularity and use. Ideally, transparencies should be sharp and with a fine grain structure - that is, free from excessive grain - without colour bias or cast, and with good tonal and density range (from 1.8 to 2.8).
Photographic colour prints, paintings, wash drawings and the like, are termed reflection copy. If they are to be reproduced on a colour scanner, ideally they should be of an overall size small enough to fit comfortably on the desktop flatbed platen or drum scanner’s analyse unit; and flexible enough to bend, when a drum scanner is used. Cleanliness in handling continuous tone originals is even more important than in line originals because smudges and stains, like tones, will be reproduced.
Originals with uneven surfaces, such as drawings or paintings on heavy grained paper, board or canvas, require careful lighting and in such cases it is often worth getting a commercial photographer to produce a transparency or photograph, which will constitute a more suitable original for reproduction.
1. Color Originals
Pictures representing line and tone in color are called color originals. Eg: Color Transparencies, Color Prints, Color Paintings, Color Line Drawings. The type of original used for a given purpose depends upon the degree of realism or abstraction desired by the designer. Photographs are generally preferred when a high degree of realism is required. The more abstract design usually employs hand drawn artwork, although some photographs also can be used for this purpose. For the ultimate in realism, the actual object or piece of merchandise may be submitted for use as an original.
Figure: Photographic originals: (A) 4x5-inch color transparency; (B) 35-mm color negative;
(C) 35 mm color transparency.
1. Photographic Color Prints
The Photographic color print is commonly used for reflective color reproduction originals. One form of this material consists of a paper base that is coated with red, green, and blue-sensitive layers that form, during processing, cyan, magenta, and yellow dye layers. This is known as the dye coupling process. Depending on the film type and the processing technique, tripack materials can be used to make prints from color negatives, color transparencies, or directly from the original scene in the case of “instant” photography. The dye bleach process, on the other hand, consists of predyed emulsion layers that are selectively removed in processing. The prints can be made only from transparencies in this process.
One possible problem that can be encountered when reproducing color prints is due to the fluorescence of the substrate, a factor that can affect the reproduction of light tones. Substrate fluorescence can be countered by mounting UV absorbers over the illuminating light source.
It may be desirable to use prints instead of transparencies when the photographer has no control over the lighting of the original scene. It is possible to make adjustments when making the color print in order to make it conform more closely to the desired appearance. The print is, furthermore, a reflection original with a contrast range and gamut close to the photo mechanical printing processes. Such originals are, therefore, generally easy to match. Customer comparisons of original to reproduction are relatively uncomplicated when color prints have been supplied.
2. Photographic Color Transparencies
Color transparency films all have integral tripack types of emulsions coated onto film bases. Unlike photographic print materials, color transparency materials vary greatly in terms of resolution, sharpness, graininess, speed, and color rendition. In general, the lower the speed the higher the image quality.
The creative demands of the job may determine the color transparency format. Large-format 8Xl0-inch. (20X25-cm) cameras cannot be used satisfactorily for high speed action photography. The 35-mm camera with its light weight and motor drive is preferred in these situations. On the other hand, 35-mm cameras are not suitable for architectural photography. The swings and tilts of large-format cameras must be used to overcome the converging parallels common with 35-mm and other fixed plane cameras.
Films that are designed for viewing by projection in a darkened room, such as 35-mm transparencies, tend to have a higher-contrast range than sheet film transparencies. All transparency materials, however, have a range greater than 3.0 optical density. Each individual color film distorts the original colors in its own way. No one color film can be selected as the best for all photographic assignments, but whenever possible, the same fIlm should be used throughout a given job.
Transparencies have several advantages over prints. Higher resolution, higher sharpness, and the ability to wrap around a scanner drum (a factor if supplied prints are mounted on a rigid base) are the more important factors.
3. Artist’s Color Originals / Paintings
Artist’s originals exist either as fine art, which is created with no thought of reproduction, or as commercial art, which is created specifically to be reproduced. A wide varies of artist’s mediums is available as a carrier and binder for the pigments.
The medium chosen to produce a given piece of art depends upon the creative intent of the artist. Certain materials convey a particular mood, sensation, or color more successfully than other materials. Some problems may arise, however, when trying to reproduce artwork that has been prepared using a given technique.
The heavy intensities and saturations of oil paintings may be difficult to reproduce. Especially if there is a lot of dark shadow detail. The clear, light colors of pastels also may cause problems in reproduction, especially when coarser halftone screens are used. Extra colors may have to be used to achieve satisfactory reproductions.
Nongamut colors should not exist in commercial artwork. The graphic artist or designer should understand the gamut restrictions of average process inks on coated and uncoated papers. The colors that are selected for the artwork in question must fall within the range that is reproducible by the production conditions. If some colors outside the gamut are chosen, these colors will be reproduced at lower saturation. Those other colors within the gamut will be reproduced correctly; therefore, with some correct and some incorrect colors in the final reproduction, the designer’s original intent becomes distorted.
Figure: Fine art originals may contain colors that fall outside the color gamut of the reproduction system.
3. Halftone Originals
Originals in which detail and tone values are represented by a series of evenly spaced dots of varying size and shape, the dot areas varying in direct proportion to the intensity of the tones they represent are called as halftone originals.
1. Black-and-White Halftone Originals
A black-and-white halftone original consists of a pattern of black dots of various sizes that represent tones of gray. Examples of halftone originals are printed pictures in newspapers or magazines. Small dots with ample white space between them produce an illusion of a light tone or highlight. Large dots that are close together produce the illusion of dark tones or shadow areas. Because the dots are all the same tone (black), halftone originals can be copied as line originals. This type of original can also be copied as a continuous-tone original, depending on the use of the final product.
2. Color Halftone Originals
Color photographs printed in magazines, newspapers, or books consist of a series of dots in cyan, magenta, yellow, and black (CMYK) that fool the eye into seeing the millions of colors that make up the original image.
3. Digital Halftones
When using scanned images or images from a digital camera, you can produce digital halftones direct from the software to the printer. Digital halftoning depends on the lpi (lines per inch, or screen frequency) and the resolution of your output device (printer). The screen used may be specified in your printers PPD (PostScript Printer Driver) or set specifically in your software program.
4. Merchandise (Product) Samples
In those cases where a very accurate color match is required, an actual sample of the product is sometimes supplied for use as an original. Examples are paint chips, fabric swatches, linoleum squares, or upholstery samples.
HANDLING OF ORIGINALS
a. Any dust, finger marks, scratches, or other defects on an original have to be avoided during the reproduction, as they will be magnified during printing.
b. Care should be taken in handling all originals, preferably keeping them in mounts or sleeves except when they are being scanned.
c. A quick wipe over the surface of an original before scanning can save a great deal of time pixel cloning later in an image-editing program.
d. On some scanners, transparencies are mounted in oil to improve the contact with the scanner drum and reduce the likelihood of scratches and Newton’s rings appearing on the scanned image. If scratches or other marks appear on an image they can be removed in an image-editing application
Other requirements of good originals include good tonal gradation and good tonal separation between areas of detail (bearing in mind that this tonal separation will be compressed when the image is scanned).
3. LIGHT AND COLOR
To understand the process of color reproduction, it is first necessary to gain an appreciation of the phenomenon of color. To do this, we must examine the nature of light, without which color would not exist.
Light is radiant energy that is visible to the average human eye. For the purposes of this discussion it can be assumed that light travels in wave motion, with the color of light varying according to the length of the wave. The wavelengths can be measured and classified along with other forms of energy on the electromagnetic or energy spectrum. Light can either be a wave as was first proposed by Christian Huygens, or as a series of discrete particles as was first proposed by Sir Isaac Newton. Eventually it was decided that light could be both a wave and a series of particles.
The intensity (or luminosity) of a light source is measured in candles. The intensity of light reflected from a surface (or luminance) is measured in candles per square meter, or foot candles or foot lamberts.
Color is a complex visual sensation that is influenced by the physical properties of the illuminant and sample, but it is determined largerly by the physiological characteristics of the individual observer. Insights into the process of color perception may be gained through examinations of these distinct elements (illuminant, sample, human observer) and the manner in which they interact.
Color is an optical phenomenon, a sensory impression conveyed by the eye and the brain. Light reflected or transmitted by an object is received by our eyes and transformed into nervous impulses, which trigger the colour sensation in our brain. Color is not a physical variable, accordingly it has no physical unit. An object is not colored, but the sensation of color is produced as a result of irradiation by light, Sunlight, which appears to be white, radiates on to an object and is partially reflected. Consequently an object that reflects the red area of the spectrum appears colored. An object that reflects completely in the entire visible spectrum usually appears to be white and a completely absorbent body appears to be black.
When perceiving and describing colors, physical and physiological effects are always involved. The physical components are measurable, where as the physiological components are not measurable.
The mixing of certain basic colors produces all of the colors we can perceive. There are three categories of colors: primary colors, secondary colors, and tertiary colors. Primary colors are those that are not formed by mixing of any other colors and can be said to be “pure” colors. Secondary colors are those formed by the mixing of two or more primary colors. Tertiary colors are those produced by mixing of two or more secondary colors. What constitutes a primary color differs depending on whether one is talking about light or pigments.
Interestingly, according to Hope and Walch in The Color Compendium, polls have consistently found that in Western Europe and North America over half of the adults surveyed name “blue” as their favorite color, while children under eight consistently name “red” as their favorite. (In Japan, however, over half of the people surveyed named either white or black as their favorite color).
Color preferences tend to vary by culture, not unexpectedly. This may seem like a trivial matter, but it is an important consideration in planning multinational advertising campaigns, designing products such as clothing for other markets, and other such endeavors. It also manifests itself in appropriate dress when visiting other cultures; white is not universally accepted as the bride’s dress color at a wedding, for example, nor is black universally appropriate for funerals or other mourning rites. In other words, color is a cultural specific concept; various colors are symbolic of different things, and these symbols are not universally consistent.
Here, the biological vision of human beings is contrasted with the process of physical measurement, as performed by a measuring system. Light falls on a sample. The sample absorbs part of the light, while the rest is reflected or re-emitted as diffused radiation.
We perceive this re-emitted light with our, eyes. In the process of seeing, cones in the retinas of our eyes are stimulated. Different cones are sensitive to blue, green, and red. The stimuli are transformed into excited states, in turn, causing signals to be sent along the optic nerve to the brain, which interprets them as colour.
This same process can be emulated in a measuring instrument. One such measuring instrument is the spectrophotometer. Of course, a measuring instrument cannot actually perceive anything, but it is able to perform calculations on predefined and measured values.
Thus, during the measuring process light also falls on the printed sample. The reflected light, also known as spectral reflectance, passes through a series of lenses to strike a detector. This then relays the values it registers to the computer. There, digital filters that simulate the visual sensitivity of our eyes are used to calculate values, referred to as standard stimuli, or tristimulus values.
The standard stimuli are equivalent to the excitation of cones in our eyes. These tristimulus values are then converted and mapped onto a colorimetric system. With the aid of the figures thus determined, a colour can be precisely described and compared with other colors. This, in very simplistic terms, is the measurement principle underlying a colorimetric instrument.
Colour is a very complex issue and there are many factors which need to be considered in order to understand how we perceive and reproduce it.
We can see the visible wavelengths between 380 and 760nm (one nanometre equals one millionth of a millimeter). If one particular wavelength dominates or, more specifically, the spectral power distribution is unequal we see a particular colour - if there is a balanced distribution of all wavelengths we see white or gray - i.e. - neutral. Light with a wavelength of 380nm appears as violet, 760nm ‘as red and 570nm as green. Colour, as we know it, can be in the form of a ‘physical’ solid, such as printing ink or colored toner; or in the form of an energy light source, such as with a TV or computer colour monitor.
The sensation of colour is the effect of light upon the eye interpreted by the brain. White light is composed of a mixture of all colours of the rainbow or spectrum, and most objects are visible by the light reflected or transmitted from them, depending upon whether the object is opaque or transparent. The colors of the visible spectrum include (in order of increasing wavelength) violet, indigo, blue, green, yellow, orange and red.
White light appears to have no color because all the wavelengths are present in equal amounts, effectively “cancelling” each other out. Objects appear colored because they reflect or transmit some parts of the spectrum and absorb the others. For example, a red object appears red as it reflects the red light and absorbs most of the violet, blue, green and yellow lights. White objects reflect or transmit almost all parts of the spectrum, while tones of gray absorb equal proportions of all its constituents and black absorbs almost the whole of it.
The perception or sensation of color, despite attempts to objectively quantity it, is a highly subjective phenomenon. We speak of, for example, a “red apple,” but the redness of the apple is more dependent on our own peculiar visual systems than any inherent “redness” in the apple. (To organisms with different types of photoreceptors, it could appear to possess a much different color.) Even among different humans, the redness perceived is not absolute, varying according to minute physiological differences in visual acuity or according to the illumination used.
1. THE PROPERTIES OF COLOUR
The colorimetric properties of color are those that describe its three dimensions:
hue, saturation, and lightness.
a. Hue
Hue is the name given to a specific colour, to differentiate it from any other.The hues blue, green and red; yellow, magenta and cyan form the familiar colour wheel- see Figure Color Wheel.
The hue identifies whether a color is red, blue, green, yellow, or some combination term as greenish yellow or bluish red. Such other terms as magenta or crimson are often used as hue names. Hue may have an infinite number of steps, or variations, within a color circle. A circle displays all the hues that exist; indeed, it can be said that any reproduction process is capable of matching any given hue.
The circle is a modified version of a structure suggested by Frederick T. Simon.
Saturation, similar to chroma, indicates the purity of a colour. It refers to the strength of a colour, - i.e. - how far it is from neutral gray.
A gray-green, for example, has low saturation, whereas an emerald green has higher saturation. A color gets purer or more saturated as it gets less gray. In practice this means that there are fewer contaminants of the opposite hue present in a given color. To illustrate this concept, imagine mixing some magenta pigment with a green pigment (the opposite hue). The green will become less and less saturated until eventually a neutral gray will be produced. A gray scale has zero saturation. The figure below shows the magenta-green saturation continum. Magenta becomes desaturated by the addition of green in the same way green becomes desaturated by the addition of magenta.
As a color becomes less saturated, it is said to be dirtier or duller, and as it becomes more saturated, it is described as cleaner or brighter. There is a limit to how desaturated a color can be (it will always reach neutral gray) and there are practical limits in reproduction processes to how saturated a color may appear. These practical limitations in printing are due to the characteristics of the chosen ink-substrate combination.
In fact, the terms lightness and darkness are synonymous. Lightness or darkness of a solid color may be changed by mixing either white or black ink with the color. In process- color printing this is achieved by
printing a color at various halftone percentages from 0 to 100 (mixing with white), then overprinting the 100% solids with increasing percentages of black (mixing with black). The figure below shows the lightness aspect of color.
In practice both lightness and darkness have limits. In printing, the lightness of a color is limited by the properties of the substrate. It is generally possible, for example, to achieve lighter colors on a good coated paper than on newsprint or uncoated recycled paper. The darkness of a printed color is limited by the gloss of the substrate and the ink, and the amount of ink (and pigment) that can be physically
transferred to the substrate. Drying, trapping, dot spread, and economic factors restrict the thickness and number of ink films that can be sequentially printed.
Table: Variations of hue, saturation and lightness
A simplified illustration of how hue, saturation and lightness operates is shown opposite:
Reproduced below is a colour wheel, showing the additive primary colours of blue, green and red as well as the subtractive primary colours of yellow, magenta and cyan - note blue, green and red; yellow, magenta and cyan appear opposite to each other on the colour wheel.
4. THE ELECTROMAGNETIC SPECTRUM
Of the overall spectrum of electromagnetic waves, the human eye is only able to perceive a narrow band between 380 and 780 nanometers (nm). This visible spectrum is situated between, ultraviolet and infrared light. If the light of this visible range is passed through a prism, then the individual spectral colors can be seen.
However, light is not absolute. For example, if a printed image is compared with a proof under artificial light, the two may seem identical, but regarded in daylight, differences may suddenly appear.
Light is a small portion of the much larger electromagnetic spectrum, a broad range of different types of generated energy, ranging from radio waves and electrical oscillations, through microwaves, infrared, the visible spectrum, ultraviolet radiation, gamma rays, and high-energy cosmic rays. All of these sources of electromagnetic radiation exist as waves, and it is the variations in wavelength and frequency that determine the precise nature of the energy. These wavelengths range in size from many meters (such as radio waves) to many billionths of a meter (gamma and cosmic rays). Visible light is technically defined as electromagnetic radiation having a wavelength between approximately 400 and 780 nanometers (one nanometer is equal to one billionth of a meter).
The electromagnetic spectrum ranges from the extremely short waves of gamma rays emitted by certain radioactive materials to the radio waves, the longest of which can be miles in length. Light, the visible spectrum, ranges from about 400 to 700 nm (nanometers, or billionths of a meter) in length. Some sources suggest that the visible spectrum could range from about 380 to 770 nm, but the exact limits will depend on the visual system of a given observer. Below 400 nm are the ultraviolet rays, which are important when dealing with fluorescent materials. Above 700 nm are the infrared rays, which have significance in certain kinds of photography or image capture.
The visible spectrum occurs in nature as a rainbow. It can be duplicated in a laboratory by passing a narrow beam of white light through a glass prism. The spectrum appears to be divided into three broad bands of color-blue, green, and red-but in fact is made up of a large number of colors with infinitesimal variations between 400 and 700 nm. The colors in the spectrum are physically the purest colors possible. The splitting of white light into the visible spectrum, and the recombining of the spectrum to form white light, was first demonstrated and reported by the English scientist Sir Isaac Newton in 1704.
The reason that a spectrum can be formed by passing white light through a prism has to do with the refraction of light as it passes from one medium (air) to another(glass).The prism bends light of the shorter wavelengths more than light of the longer wavwlengths, thus speading the light out into the visible spectrum. In nature, drops of rain act in a manner similar to that of a prism: when a beam of sunlight breaks through the clouds it is refracted by by moisture in the air and a rainbow is formed.
Color reproduction is the process of making color images of an original scene or object. Generally speaking, it involves the use of an optical system, a light-sensitive material, an image processing method, and an electronic or colorant-based rendition system.
In the case of the printing industry, the process typically involves making reproductions from existing photographs or artists’ originals. Electronic camera images also are commonly used as the starting point for the printed color reproduction process Originals in full colour, such as transparencies and colour photographs, are mainly reproduced by four-colour process, using yellow, magenta, cyan and black printing inks. A separate screened negative/positive, printing ‘plate, cylinder or stencil is required for each colour, so that the printing combination of colours reproduce the full effect of the original. For the most faithful reproduction possible, special colours may be necessary, particularly in packaging and labels, where they may be used for overall solids or house colours. These are often specified as a PANTONE Matching System (PMS) reference.
There are two types of colour reproduction - 1. Additive Color Theory. 2. Subtractive Color Theory.
Photomechanical color reproduction is the traditional term that describes the printing industry’s color reproduction production process. This process may include the production of intermediate film, plate, or cylinder images prior to the stage when the colorants are physically transferred to a substrate. Some of the processes used by the industry form the image directly from digital data without the need for intermediate film or plates.
The yellow, magenta, and cyan subtractive primaries, plus black, that are used for making printed color reproductions are known as process colors. The term process color printing is often used to mean photomechanical color reproduction, but it also means the production of flat color tones by combined process colors.
The term color printing is a broad one that includes flat solid color (nonpictorial) package printing and fine art printmaking, as well as the photomechanical color reproduction process. Color printing may also be used to describe the production of photographic color prints or the generation of output from computer-driven desktop color imaging systems.
As previously mentioned it is possible to divide the spectrum of white light into three broad bands - blue/violet, green/yellow and orange/red - which appear essentially blue, green. and red to the eye: these are in effect the additive primary colours. If these colours, in the form of beams of coloured light, are in similar proportions upon a white screen then white light is created. With the overlapping primary colours of blue, green and red, the secondary colours of yellow, magenta and cyan are produced.
An additive mixture of colours is a superimposition of light composed of different colours. If all colours of the spectrum are added together, the colour white results. Red, green and blue are the additive primary colours. They are called one-third colors because each represents one third of the visible spectrum. The additive system starts with darkness (for example, a blank TV screen) and adds red, green and blue to achieve white.
The principle of additive color mixture is used in color TV and in the theaters to produce all the colors of the visible spectrum.
When wavelengths of light are combined or added in unequal proportions, we perceive new colors. This is the foundation of the additive color reproduction process. The primary colors of the process are red, green, and blue light.
Secondary additive colors are created by adding any two primaries:
a. red and green combine to produce yellow;
b. red and blue combine to produce magenta; and
c. blue and green combine to produce cyan.
d. The presence of all three colors will produce white, and
e. the absence of all three colors will result in black.
Varying the intensity of any or all of the three primaries will produce a continuous shading of color between the limits.
Two methods for adding colors may be used: (i) red, green, and blue-light image records either overlap each other, or (ii) are placed side by side within a mosaic structure. The overlapping-primaries method of additive color reproduction has certain practical limitations that restrict its use. The side-by-side red, green, blue image element approach to additive color reproduction has, however, proved to be quite successful for certain applications.
Color television works on this basis: a magnifying glass will reveal the red, green, and blue mosaic structure of the screen (figure below). Many early color photography processes were also based upon the mosaic-structure type of additive color reproduction.
Additive color photography processes, however, have certain disadvantages when compared to subtractive methods. The drawbacks of the additive color reproduction photographic process are due to the fact that the red, green, and blue-filter mosaic absorbstwo thirds of the light in the whitest areas. Additive-process transparency photographs appear to have low contrast and saturation unless they are viewed using a relatively intense light within a darkened room.
Satisfactory reflection color photographs and color printing cannot be produced by the additive process. Red, green, and blue rotating reflection disks are often used to demonstrate the principles of additive color reproduction, but it is necessary to illuminate the disk with an extremely intense light to achieve satisfactory results.
The additive color reproduction process works for television and computer monitor imaging processes because the intensity of the self-luminous display screen is sufficient to overcome the room lighting effects. For best results, however, television and monitor displays should be viewed under dim ambient lighting conditions, and the viewing distance must be sufficiently great so that the eye cannot resolve the mosaic structure of the screen.
Subtractive Color Theory
The limitations of the additive process for reflective light viewing can be overcome with the subtractive color reproduction process. The subtractive system starts with white (white paper illuminated by white light, for example) and subtracts red, green, and blue to achieve black.
The majority of commercial work is printed in four, rather than three colours, adding black to the process set. Black Color is included to compensate for deficiencies in the yellow, magenta and cyan pigments, and to allow type to print in only one dense, high contrast colour. Although the way in which the black separation is made can radically affect the final result, the theory of subtractive reproduction relates to the three primary colours of yellow, magenta and cyan. Subtractive color mixing operates by “subtracting” out one or more colors of light.
In ideal subtractive colour behavior, each of the primary colours would subtract one third of the spectrum. The yellow ink would absorb the blue portion and reflect a mixture of red and green light appearing yellow to the eye, which cannot analyse it into its component parts; the magenta ink would absorb the green portion and reflect blue and red; with the cyan ink absorbing the red portion and reflecting blue and green.
The subtraction of red, green, and blue is achieved by using colorants that are their opposites.
a. For red, this is a color made up of blue and green (i.e., minus red), called cyan.
b. For green, this is a color made up of red and blue (i.e., minus green), called magenta.
c. For blue, this is a color made up of green and red (i.e., minus blue),called yellow.
Colors are achieved by subtracting light away from the white paper (which reflects red, green, and blue). A combination of yellow (minus blue) and cyan (minus red) will, for example, result in green. Table below shows the possible combinations.
A continuous blend of colors between the gamut limits is obtained by varying the quantity of any or all of the primary colorants deposited within the image. In color photography, this is achieved in a purely subtractive manner, by varying the density of the cyan, magenta, and yellow dye layers. Most color printing, however, relies upon a combination of a fixed density (ink film thickness) and a variable area coverage to adjust the quantity of ink deposited. The “halftone” structure that results from the combination of inked dot areas printed upon a white paper base is optically fused by the eye to produce a continuous-tone appearance.
To produce a set of four colour separations the original is scanned/input on an, electronic colour scanner using RGB (red, green, blue) light sources and output for printing purposes as CMYK (cyan, magenta, yellow, black) separations.
The figure below, illustrates the use of BGR colour lights/ separation filters to produce YMC separations or printing plates; K (black) is reproduced from a yellow / orange combination-type filter.
Figure: Colour separation lights/filters and their respective printing plates
The principle of colour separation is probably best considered from the traditional method, where the blue filter is dense in the areas of the image representing the parts of the original reflecting or transmitting blue, less dense where there is less blue light and transparent where there is none; the printing plate therefore produced from the blue filter is the yellow plate. On the same basis the green filter produces the magenta plate and the red filter the cyan plate.
The key objective in the photomechanical color reproduction process is to produce cyan, magenta, and yellow images that are negative records of the amount of red, green, and blue in the original. This is achieved by initially photographing the original, in turn, through red, green, and blue filters. The subsequent image records or signals are adjusted as required prior to generating a halftone image that suits the chosen printing process. The images are then used to generate image carriers, which may be plates, cylinders, or stencils. Each plate is inked with its appropriate color which is sequentially transferred, in register, to a white substrate. The more direct electronic (“digital”) printing systems eliminate films, or even plates, from the production process.
There are practical considerations that limit the thicknesses of cyan, magenta, and yellow inks that may be printed by most processes; consequently, a black printer is normally employed to compensate for the resulting loss of image contrast. The black printer is made by photographing the original sequentially through red, green, and blue filters, and then following procedures similar to the other colors. Below figure shows the complete process in schematic form. The exact nature of the printed image will depend upon the process used to form and transfer the image.
Reproduction: Color reproduction is the science of creating colors for the human eye that faithfully represent the desired color. It focuses on how to construct a spectrum of wavelengths that will best evoke a certain color in an observer. Most colors are not spectral colors, meaning they are mixtures of various wavelengths of light. However, these non-spectral colors are often described by their dominant wavelength, which identifies the single wavelength of light that produces a sensation most similar to the non-spectral color. Dominant wavelength is roughly akin to hue.
There are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (mixtures of red and violet light, from opposite ends of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray, and white) and colors such as pink, tan, and magenta.
Two different light spectra that have the same effect on the three color receptors in the human eye will be perceived as the same color. They are metamers of that color. This is exemplified by the white light emitted by fluorescent lamps, which typically has a spectrum of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although the color rendering index of each light source may affect the color of objects illuminated by these metameric light sources.
Similarly, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photography, printing, television, and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colors. Each method has its advantages and disadvantages depending on the particular application.
No mixture of colors, however, can produce a response truly identical to that of a spectral color, although one can get close, especially for the longer wavelengths, where the CIE 1931 color space chromaticity diagram has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.
Because of this, and because the primaries in color printing systems generally are not pure themselves, the colors reproduced are never perfectly saturated spectral colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut.
Another problem with color reproduction systems is connected with the initial measurement of color, or colorimetry. The characteristics of the color sensors in measurement devices (e.g. cameras, scanners) are often very far from the characteristics of the receptors in the human eye.
A color reproduction system "tuned" to a human with normal color vision may give very inaccurate results for other observers, according to color vision deviations to the standard observer.
The different color response of different devices can be problematic if not properly managed. For color information stored and transferred in digital form, color management techniques, such as those based on ICC profiles, can help to avoid distortions of the reproduced colors. Color management does not circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colors into the gamut that can be reproduced.
Color Standard: Certified Color Standards are a combination of a visual reference and a master electronic standard in the form of spectrophotographic refletance data. The measurement conditions are determined by the brand or retail program account and are typically stored in a qtx-formatted file. Certified color standards are customized for the brand or retailer with their individual color names or IDs, their logo, and their own layout.
Digitizing color:
Colors can be represented by digital bits. One method is to first convert the color into a set of three numbers, and then convert the numbers into binary. The three numbers represent the amount of red, green, and blue in the color. This system is called an RGB representation (for Red-Green-Blue). We might decide to rate how much of each of the three colors is present by giving it a number from 0 (meaning none) to 255 (meaning a lot of that color). Here are some examples:
Red: 255 Green: 0 Blue: 0
Red: 0 Green: 0 Blue: 255
Binary: 111111110000000000000000
Binary: 000000000000000011111111
Red: 0 Green: 255 Blue: 0
Red: 128 Green: 0 Blue: 255
Binary: 000000001111111100000000
Binary: 100000000000000011111111
If you look closely, you'll note that the binary representation uses 24 bits, 8 for each color. The first 8 bits are for red, the second 8 bits are for green, and the final 8 bits for blue. For example, in the last color above (a bluish purple), the red value of 128 written in binary is 10000000, the green value of 0 is 00000000, and the blue value of 255 is 11111111. Because computer memory is usually arranged in groups of 8 bits (called a byte), most color schemes use a total number of bits that is a multiple of 8. Twenty-four bits are often considered sufficient to represent colors with enough shades to faithfully reproduce a photograph (called "photo-realistic"), although some people believe 32 or even 48 bits are necessary.