“Develop success from failures. Discouragement and failure are two of the surest stepping stones to success.”
Digital Printing
ON DEMAND PRINTING
Digital printing is generally defined as any type of print reproduction method that utilizes electronic files to produce a printed pieces from spots and dots of ink, toner or dye. Applying it to the print workflow has eliminated most of the manual steps involved with conventional print processes.
Digital printing technology can be divided into two main classification: variable imaging and direct imaging.
1. Variable imaging: variable imaging also known as computer-to-print, is a totally digital workflow including the design process, prepress function, and print output. Computer-to-print system, such as digital presses, utilize a digital print engine that allows the image carrier to be, reimaged for each printed impression. Common-to-print system is the ability to produce printing application in which every page that is printed is different. This is known as variable data publishing. It is plate-less technology are electrophotography, ink jet, ion or electron deposition, magneto-graphy, thermal transfer, and thermal dye sublimation.
2. Direct imaging: direct imaging uses electronic files to create films or plates for printing. Direct imaging technology includes the following.
a. Computer-to-images-setter: computer-to-image-setter involves the electronic preparation of a print application including the design process and then outputting it to film from an image-setter device. The film is then used to create an image on printing plates which are mounted on a conventional press.
b. Computer-to-plate (CTP): digital platemaking can divided two group (1) computer to plate (2) computer to plate on press.
1. Computer to plate: CTP workflow includes electronic design and direct imaging of printing plate from the computer files.
2. Computer-to-plate-on-press: the image is transferred directly from the digital files to the image carrier already mounted on a press. An offset press with the capability is known as direct imaging press. A direct imaging press is much like a conventional offset press expect that the image carrier (plate) can be reimaged for every press run.
Digital printing workflow:
There are three main processes to occur to print to the customer.
Pre-press- form design to produce of image carrier.
Press: the process for transferring image to substrate, electronic or conventional.
Post-press: final manufacturing processes: folding, conversion, binding, die-cutting etc.
Computer to press (direct imaging and computer to print): there two different computer to press technologies with computer to press-direct imaging technology, a printing plate as is required for example, In conventional offset printing is produced directly in the printing press. For instance, four printing plates for multicolor printing can be imaged inside the printing press. The register alignment that can be achieved in this manner depends only on the quality of the press and of the imaging process and is no longer subject to operator-related influencer another computer to press technology is computer to print (also referred to as computer to paper).
This technology does not require production of printing plate for each job. This technology used (non-impact printing) makes it possible to introduce job data directly into the replication process without a printing plate. Ex- electrophotography. In this non-impact process, a laser creates a charged image corresponding to the printing image on a unit attracts an appropriate medium, for ex-powder toner, and is subsequently transferred to paper.
Includes another computer to print process in which imaging takes place without any. Intermediate image carrier, as it the case in inkjet system. In these processes, the ink is applied directly to the printing sheet via electronically controlled nozzle systems.
Computer to press/direct imaging printing systems: it is the technological comparison of these computers to press-direct imaging technologies with conventional plate making outside the press as well as with computer to press/ computer to print technologies where no plate is required.
It is evide not from the illustration that with the increasing degree of automation some special additional equipment is necessary inside the printing unit. In the case of direct imaging with the once usable plate, the imaging unit must be installed inside the press. If a technology comes into play where a plate can be used several times, then additional erasing and preparation system are needed in addition to the imaging unit.
So far direct imaging technology has been used in offset printing, however, there are also attempts to employ direct imaging in gravure printing. For screen printing simple single-color printing system using direct imaging technology exist.
Configuration of direct imaging machine 74 Karot:
Shows the operating console near the delivery the direct imaging press 74 karat. Each plate cylinder of the press can take up two plates, and inking is performed by a keyless inking unit in waterless offset.
That only two plate cylinders are used for four color printing. Two plates are mounted on each of the double-sized plate cylinders. The two plate are imaged with the desired image using only one laser system in a laser ablation process. The inking keyless units consists of an anilox roller and chambered doctor blade system, adjustment on the ink zones in not possible, which placed greater demands on the quality of the original and color matching.
Through adequate temperature control of the ink form rollers, it is possible to influencer the entire ink film, thickness slightly. The color separations for yellow, cyan, magenta, black are transferred to the two double diameter blanket cylinders. The plates on each plate cylinder are inked by alternation engaging the two ink form rollers, for ex-the cyan form roller is engaged for the first plate when the second plate comes In, the cyan inking unit is disengaged and the ink form rollers for yellow is engaged.
The short inking unit has the advantage (has only two rollers) that it can react quickly, which means in principles that there is a better control of start-up (make-ready) waste. The ink supply takes place via ink cartridges similar to the ones used in conventional sheet offset presses.
The imaging system of the direct imaging press 74 karat presented in 1997 is a laser system moving axially and having thirty-two beams imaging can range from 1524 to 3556 dpi (600 dpcm and 1400 dpcm; dot per centimeter). These press is designed for A2+ (four page), format. The sheet pass through the press in ‘landscape’.
The direct imaging press for A3+ format, the quick-master DI, print format A3+ with the sheet passing through in ‘portrait’.
The direct imaging press 74 karat has an output of 10000 sheet (maximum A2+) per hour.
Concept for re-imageable masters with material application/ablation): offset plate making by means of thermal transfer.
Dico web litho: a plate suitable for offset printing is fused on metallic surface of a plate cylinder by thermally transferring a suitable polymer. The polymer material, stored on a roll is fused on the metallic cylinder surface by thermal laser to create the image via laser ablation.
Depending on the lasser imaging head, image can be burned In e.g. by 64 beams simultaneously at a resolution of up to 2400 dpi, while the cylinder rotates, the plate is imaged across the entire press width through axial shifting of the imaging system and the storage unit for the polymer material (width of the ribbon material about 15mm).
After the imaging process the material is fixed on the cylinder by supplied heat (e.g. hot air 150C).
Web offset printing unit with components for imaging and erasing the plate: after the print job is completed the Ink-accepting polymer material is removed from the cylinder surface in a chemical and mechanical cleaning process. After this cleaning process the water-accepting/hydrophilic base cylinder is neutralized and ready for imaging the next job.
The difficulties here are the basic material properties of the base cylinder required for offset printing, the stability and quality of the ink-accepting surface areas that are thermally transferred to the base cylinder and the additional equipment needed with in the offset printing unit.
Computer to print (job based digital printing): this technology does not require production of printing plates for each print job. This technology of the printing method used (non-impact printing) makes it possible to introduce job data directly into the replication process without a printing plate, as for ex-in electrophotography.
In this non-impact printing process, a laser creates a charged imaged corresponding to the printing image on a unit comparable to a plate cylinder.
The charged image attracts an appropriate medium, for ex-powder toner, and is subsequently transferred to the paper.
Another computer to print process in which image takes place without any, intermediate image carries as is the case in ink jet system. The ink is applied directly to the printing sheet via electronically controlled nozzle systems.
Computer to print: Docu colour 40, Nexpress 2100
Printing system for electro-photographic multicolor printing using liquid toners. This main feature of this system is that a single printing unit is sufficient to produce a four-color printing.
One sheet for multicolor produce held on the impression cylinder during four rotations.
This intermediate cylinder receives the individual color separation from a photoconductor drum that has been imaged by means of a laser to carry the particular color separation. The liquid toner is applied to photo-Condutor drum by means of a special types of inking unit. The intermediate cylinder the individual color separation are transferred directly to the paper sheet. The different process of liquid toner are transferred to the sheet.
Computer to print machine: E- print 1000
This computer to print system forms the basis of a series of models for 4-color printing on sheet material (E-print pro) and six-color printing including a powderful front-end system (turbo stream). A model with increased productivity (ultra-stream) has been presented in the year 2000 with a modified printing unit design as well as for printing on web material (publisher).
The turbo stream model is illustrated in, together with an outline view of the E-print 1000 and a more detailed explanation of the components.
The system works with liquid toner. It is (Multi-pass system) and an intermediate cylinder that is fitted with a blanket is used for the ink transfer onto the substrate (sheet material).
Imaging takes place via a scanning, Multi-beam laser system. The printing system can also be used for duplex printing. A finished unit can be connected.
Four color printing the productivity is 33 A4 pages per minutes (equivalent to approximately 0.15m/s the speed for a color separation is approximately 0.6m/s).
Computer to print for multicolor printing in one printing contact (one shot technology).
The onmius model for printing on web material.
All color separations are collected on an intermediate carrier, a cylinder covered with a flexible blanket, before the entire multicolor print-image is transferred to the web material in a single printing contact.
In this system the imaging also takes place by electrophotography and laser systems with a resolution of 800 dpi.
Advantage of E-print:
1. Economic.
2. No. films or plates.
3. No Proofing.
4. No. press make-ready.
5. No. expert operators required.
6. Electronic collation.
7. Automatic duplex printing.
SCANNING AND GRAPHIC INPUT
Digital camera: Two types of digital camera:
1. Digital camera: an electronic photosensitive sensor capture light that enters the digital camera, the saves the image date in memory card. The image store into memory as a bitmap image. The CCD is used to record image information.
The CCD has no. of elements photodiode which determine the resolution of a camera.
The scan the object by the scanning array CCD which record the image information as picture elements (pixel).
The no. of picture according available memory in the camera.
Array arranged in a rectangular box called pixel and linear array, an image is captured one row of pixels at a time whereas an area array captures an entire and any required three separate exposure to captured all color information.
With tri-linear array, which contain three linear arrays mounted side by side each array coated with a colored dye to act as a color filter (red, blue, green).
2. Scanning back on existing analog camera: a device that can capture photographic image and store them in digital form integrated circuit card. The scanning back is connected to standard film camera and replaces the film expose area. The camera takes the photo and scanning back convertor the image into electronic format.
Most digital photography is based upon charge couple device technology. A CCD is a solid-state device that consists of light-sensitive element in linear or area-array from row & column. Light fall on the element is convert into electronic signal and in turn, converted form and stored on a Ram chip, card or disk.
Scanner: Two types of scanner:
1. Flatbed: it is a device which used to scan the picture or photograph and upload in to the computer for editing.
Working of flatbed: the image is placed on the bed of gloss face down and is illuminated by florescent light from bottom. The incident light on heating the copy reflect or transmits and falls on to a light sensitive diode that form photo array called CCD. The CCD capture the image information and convert the light energy to electrical energy. The monochrome have single linear array and color have tri-linear array for RGB. The light moves across the original the scanner align of image date. The more pixel per inch (PPI). It has ability captured more resolution and more details image. A 8 bit scanner can detect 256 shades for black and white image only. The second part of scanning of a desktop scanner is the image data by the CCD from an RGB mode to CMYK mode.
In case of enlargement and reduction the number of the pixel will be generate mathematically through scanning software so, it not advised to make higher enlargement of picture to ensure quality of picture.
For text scanner utilizing (OCR) software can scan text and convert to ASCIL text.
Both scanner can scan (text and illustration) simultaneously.
2. Drum scanner: drum scan image data with photomultiplier tubes (PMT) placed (CCD). The original mounted on a cryclic cylinder, the scanner drum rotates high speed and optics light fall on the original and signal give (PMT). PMT split into separate R, G, B beam.
Scanning mode: line art, color, grey scale.
Scanning spot: where the scanning beam focusing.
Scanning velocity: the speed which a laser beam read the track on an optical disk.
Color separation: The most common way of printing color artwork is by producing a positive or negative image of the artwork on a paper or film. Color separation is required for preparing artwork into process color C, M, Y, K and spot color. To produce high-quality separation of printing, including line screen, resolution, process color and spot color.
To reproduce color and continuous tone image, usually separate artwork into four plates. When ink with the appropriate color and printed in register with one another, these color combine to reproduce the original artwork.
1. Spot color: spot color printing is the printing of line image in color. It used when the mixed four color and not give correct. The spot color is reproduced using a single plate. Spot color can also be used to add solid and screened blocks of color to an image. Screened is add some dot at time for duotone.
2. Process color: it is continuous tone color image in Cyan, Magenta, Yellow, Black is called process color. It create the illusion of full color with a precise halftone pattern. The any image produce by combining and blending one or more of these four color in different proportion and in different screen angles.
Fake duotone or single color printing on colored paper creates a two color effect without any extra cost. Two or three solid color with a variation of tints and overlapping give a reasonably good effect.
To get a two color effect of a photograph, it should be printed in black on a light color or through the duotone process, where two screened plate are prepared. One is printed in black and the other is printed in color but both screen angle are different.
On computer, color are separated by the command.
IMAGE - CMYK mode - channels
UNIT-2
FILE FORMAT:
1. GIF: (graphic interchange format) it format on the web and storing picture like-live drawing and simple cartoon.
2. JPEG: (joint photographer’s expert group) it stores full color information 24 bit/ pixel and much small than GIF’s.
3. TIFF: bitmap files (tagged image file format) it storing bitmap and does not oriented image or text.
It has different level based on no. of color or grey.
It transport for raster image and desktop publishing format.
4. PICT: (macintosh format) it based on quick draw, the mac’s native graphic language object and bitmap can be write white, black, cyan, magenta, yellow, red, green, and blue can hold resolution greater ’72 dpi’.
It is readily imported but poorly supported by publishing application.
5. DCS: (desktop color separation) DCS is a file format that creates four color separation by saving image as a set of ‘EPS’ file. It is used to exchange color data between retouching, separation and page layout.
The complete preview image is store in one file. The individual ‘CMYK’ color plate are stored in separate file. It advantage editing program easily support.
6. PDF: (portable document file) it used to represent a document of the application software, hardware. The PDF writer available both in MAC and WINDOW. It useful for interactive viewing.
7. Post script: a method to describe typographic as vector or outlines at same time introduce PS language. It used for raster based printer and in file include all type and graphic information.
8. EPS: (encapsulated post script) in the ‘EPS’ file special function like transfer curve, screen rulling and separation information embedded in the file that cannot be changed once it leave the creator of the file.
EPS used for storing object oriented and bitmapped software and it has two sub-type-ASCII (text) and binary (hexadecimal).
ASCII format contain two version one is high resolution ‘PS’ description for printing on ‘PS’ device and second is low bitmapped PICT preview and it can be display on the monitor without PS
POST SCRIPT LANGUAGE
RIP (raster image processor): A RIP operate by transforming the font-end instruction form the HOST DTP/EPS system, which are stored in post script language.
The post script language consist of 300 command that instruct the programme to move to certain point, draw lines, fill box. Select type. In an application programme files is create and stored in internal format when these file is converted in to post script and it is converted PS code.
The save file is send to output device, where the interpreter execute the code and translate the page into raster image in the coordinate system of the output device.
Create a list of all the object on a page, known as a ‘display list’.
The raster image sent to the marking engine which produces it into film or paper.
Two types of RIP:
1. Hardware and software (RIP)
a. Software: the process of converting the received image data into bitmap of device pixel values is known as RIP.
Each RIP has limit on the overall size of each single graphic. It can handle at any-one time with just below A3, for example being the maximum some RIP’s can handle. Adobe, the originate and developer of post script has issued licenses for RIP design which continue to improve to take account of development such as postscript levels 2 & 3.
Most forms of powerful output device are controlled by a RIP from, for example, color copiers transformed into color printers to image setters and CTP systems. RIP are designed to run apple mac, PC and UNIX platforms. General purpose RIP’s driving a wide range of output devices.
The MGI ‘JETSTREAM’ RIP has facility to support color copiers, electrostatic and inkjet printers, as well as image setters on the apple mac, PC or UNIX platform.
Major process companies such as linotype-hell (now Heidelberg prepress) and Agfa has developed RIP to drive their specific range of image setter.
a. Linotype-hell developed the ‘DELTARIP’ to drive some of image setter. It consist of three parts, delta software, delta workstation and delta tower.
Delta software is drives the A3+ Quas or B2 Herkules and dry-setter image setter.
Delta workstation is based on a PC running Windows NT.
The delta tower looks after the screening requirement.
b. Afga has developed the ‘COBRA’ software RIP run on a UNIX platform SPARC workstation which is capable of driving image setters and other input and output devices simultaneously.
Electronics for imaging (EPI) are well known in the industry for their fiery RIP’s converting color copier into color printers, the range has to cover large format digital printing.
Proprietary/Delicated RIP’s developed by harlequin company and produced its own postscript interpreters.
Rich RIP’s which are very popular in high-end workflows driving image setters, plate setter, digital proofing system and digital presses.
Function of RIP:
1. Trapping
2. Imposition
3. Color conversion to CMYK
4. Half-toning and dot gain compensation
5. OPI image storage.
FILE OUTPUT
For the output, the final files of all artwork, scanned, and page layout along with the relevant type used, bitmap image, and postscript font are stored and forwarded to the output station in a printing file format such as EPS. These files are opened, trapped and imposed if required and then made ready and passed on to the (RIP). The (RIP) converts the postscript date into rasterized date so that it is in a format that is readable for the image-setter and can be outputted on to a film.
Post script language: vector drawing in PS in based on three fundamental procedure on graphical primitives, straight line, arc and curves. These are the basic building block of every types of graphic object.
PS essential a graphic programming language used to create precise description of graphical objects for accurate transfer to an output device.
User space and device space
PS level 2: it provides method of handling all aspects of color specification and output.
It uses device specific parameter to control the way the color is rendered to output devices such as color printer, film recorder, Monitor.
1. Better memory management.
2. Better color handling.
3. Increased use of dictionary.
4. Ability to handle file decompression during output.
A dictionary is a look up table that holds a set of parameters used in defining a graphical object.
Item to check for output:
1. All the links in place.
2. Fonts and other resources available to output device.
3. Correct page size, screen Rulling.
4. Specified color correctly.
5. Items correctly sealed and cropped.
Avoiding output problem:
1. Reviewing links before output.
2. Renaming files.
3. Avoiding extended links.
4. Ensuring that all linked files are included.
5. Listing all the files used in the publication.
6. Copying the page layout file to the same folder.
Color Management: Color is an optical phenomenon, a sensory impression conveyed by the eye and the brain. Color is not a physical variable; accordingly it has no physical unit .An object is not colored per se, but the sensation of color is produced as a result of irradiation by light. Sunlight, which appears to be white, radiates onto an object and is partially reflected. Consequently an object that reflects the red area of the visible 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.
A color measuring instrument (colorimeter, spectrophotometer) primarily measures only the chromatic stimulus, from which the color stimulus specification and possibly also the color perception can then be deduced numerically by means of suitable interpretation models. These may, for example, be the standard color spaces defined by the CIE: CIELAB, and CIELUV.
What is a color management system? and its needs
A system that transforms data encoded for one device (such as scanner RGB) into that for another device (such as printer CMYK) in such a way that it reproduces on print the same colours as those scanned. Where exact colour matching is not possible the result should be a pleasing approximation to the original colours. In general the term colour management system is usually reserved for those systems that use the internationally accepted CIE system of colour measurement as a reference.
Color terminology
What is the definition of a color?
Colour is the sensation produced in response to selective absorption of wavelengths from visible light. It possesses the attributes of Brightness, Colorfulness and Hue. An international standard developed by CIE can be used for measurement of these attributes for any colour.
CIE Chromaticity Diagram
The CIE color diagram contains all colors. The colors described in the CIE system is plotted on a chromaticity diagram. The diagram is the horse shoe shaped “spectrum locus” (the line connecting the points representing the chromaticities of the spectrum colors). The two dimensional map of color obtained is known as Chromaticity diagram. The wavelengths of the visible spectrum are plotted on the outside. The x and y axes designate two of the three standard color value ratios in relation to the eye of a standard observer. Only a certain number of the colors that occur in nature can be reproduced in printing using the four process colors of CMYK. The colors produced in four color offset are within the polygon. Compare also the color spectrum of a positive slide and the spectrum used in newspaper printing. The vertical from the achromatic center of the color triangle represents the luminance axis ‘capital Y. if the luminance axis is also plotted, we talk about the CIE color space.
Spectral Reflectance curves
Densitometer
UCR an GCR
4.2 COLOR MEASURING INSTRUMENTS
What is spectrophotometry?
Spectrophotometry is the measurement of the reflectance or transmittance of a sample at discrete wavelengths. Spectrophotometers usually provide illumination of the sample by white light and then contain a diffraction grating to refract the reflected light and enable measurement of the amount of light reflected at discrete wavelengths
What is a colorimeter?
The word colorimeter is normally used for a device which uses three or more filters to produce a response similar to that of the eye, as opposed to a spectrophotometer which measures the amount of light reflected or transmitted at each wavelength. Both colorimeters and spectrophotometers can give the same tristimulus values though the spectral method is usually more accurate.
What is the CIE system of colorimetry?
A Colour is the sensation achieved when light falls on the retina of the eye. In the retina colour sensitive receptors are ‘triggered’ to produce electro-chemical signals, which are sent to the brain to produce the sensation of colour. The light reaching the eye is the product of the light reflected at each wavelength by the sample and that of the illumination source shining on it.
The three types of receptor each peak in sensitivity at different wavelengths - one at short wavelengths, one medium wavelengths and one at slightly longer wavelengths. This means that any colour can be reproduced by just 3 coloured dyes, pigments or coloured luminous stimuli - so long as their peak absorption or emission wavelengths are also separated. It also means that colours can be seen to match despite having different spectral composition - a phenomenon known as metamerism. Such a match will generally fail when the light source shining on the sample is changed.
Colour (whether coloured light or print) is traditionally measured by specifying the amounts of Red, Green and Blue lights which would be needed to match it. Based on experiments in which observers were asked to match various colours by mixing three coloured lights, the international colour standards body International Commission on Illumination (CIE) defined a ?standard observer? as the average of these observers for a specific set of ?lights?. They then defined a system of measurement units and measurement procedures which enable any colour to be specified in terms of the amount of the three standard lights that would be needed to match it. These are the CIE XYZ values, and other quantities such as CIELAB are calculated from them.
Scanners
Manufacturers of high-end and mid-price scanners offer the possibility to scan in an IT8 reflective test chart for the calibration, and the scanner profile is calculated by comparing the ascertained colorimetric actual values with the theoretical nominal data of the test chart. If the scanner is also suitable for transparent copy, the same procedure can be followed with an IT8 transparent test chart. External scanning programs or plug-ins that permit calibration via IT8 are also available for many scanners. Prior to generating the scan profile, the operating sequence and the scan parameters must be determined (e.g. linear scanning, highlight and shadow settings). The purpose of a profiled scan is always to reproduce the scanned copy as identically as possible.
Proof printers
In principle, device profiles of proof printers are generated in exactly the same manner as output profiles for print standards. However, fingerprinting proofers is very complicated since digital proof printers seldom work in a linear fashion. It helps when the proof software for the basic calibration permits linearization of the proofer via gradation curves and also allows maximum inking to be produce better profiling results. If there are any last-minute changes to the whiteness of the substrate to be printed, these changes to the existing ICC profiles can be optimized to a certain extent by profile editors. This does not require a test chart to be printed or measured.
IMAGE REPRODUCTION PROCESS
How to implement ICC colour management?
To apply colour management, you need a profile for each of your scanner and/or digital camera and another for your monitor and/or printing device. Each of these relates the device colour data to the standard colour space which allows them to be combined to produce an overall transformation.
To combine profiles you need a Colour Management Module (CMM). At its most basic this is nothing more than an interpolation engine for combining LUTs. ICC do not specifically recommend a single CMM as some CMMs attempt to ‘add value’ for specific applications by picking up private tag information in the profile.
Many colour management-aware applications such as high-end RIPs and Adobe Photoshop contain an internal CMM. CMMs are also built in to the OS on the Mac (ColorSync) and Windows (ICM and WCS).
THE “THREE Cs” OF COLOR MANAGEMENT
Many people use the term “calibration” to mean all steps necessary to achieve accurate color during the production process, perhaps implying that reproduced colors are “calibrated” to match the original. “Color management” is a more meaningful term for matching color on different input and output devices, since the calibration of each device is only the first of three steps necessary to achieve accurate and consistent color throughout the reproduction process.
Calibration ensures that all devices (scanner, monitor, and printer) perform to a known specification, be it RGB illuminance, CMYK density, or CMYK dot area.
Characterization is a way of measuring and quantifying the color space, color gamut, or color behavior of a particular device under known conditions. It is a way of determining how an input device captures color or an output device records color when it is calibrated.
Conversion (also known as color transformation or color correction) refers to translating a color image from the color space of one device to that of another under known conditions. Color conversion can be done by manually correcting the image or automatically by using color management software.
To achieve the goals of color management, calibration, characterization, and conversion must be done in this sequence. Calibrating a device to specification serves as a foundation for characterization and conversion, and a device must be characterized before color data can be converted for accurate rendering.
CALIBRATION
Color management is based on the assurance that all devices in a color reproduction system are performing to specification. Calibration alone does not guarantee color matching; it simply ensures that the scanner, monitor, and printer are performing to their respective specifications, and provides a way of ensuring they will be consistent over time.
Scanner calibration means that when a specific light level is measured from a film or paper target, the scanner consistently records a corresponding digital value in the image file for that spot on the original. Monitor calibration means that the display card consistently displays a pixel corresponding to the specific digital value received from the file. Other items that require calibration include the color printer/proofer and the platesetter.
CHARACTERIZATION
After devices are calibrated, they must be characterized. Characterization defines the color gamut, or set of reproducible colors, that an input device can capture or an output device can record. Device characterizations are stored as profiles, digital files of data describing the color gamut of a device. In page-layout software, color management systems keep track of the input, display, and output devices the user has specified using tags or data appended to color files.
A variety of models can be used to characterize input and output devices, including RGB color space, CMYK color space, and CIE color space, which includes two models based on the dimensions of hue, chroma, and value. These are the CIExyY and CIELAB color spaces. In both models, hues are arranged around the perimeter of the color space, saturation increases from center to edge, and value varies along the third color space axis.
Scanners are characterized by software that measures the values in a scanned ITS.7 target and compares them to corresponding values in a reference file. The ITS.7 target is the internationally standard input target developed by the ITS subcommittee of the Committee for Graphic Arts Technologies Standards. The basis of the ITS.7 target is the Q60, a series of photographic film and paper test images for characterizing the gamut of input devices developed by the Eastman Kodak Co.
The printer must also be characterized. Output targets are measured with a spectrophotometer in CIExyY and/ or CIELAB color space to characterize the color gamut of an output device. As with scanners, the characterizations are stored as device profiles.
Profiles for commonly available monitors are offered by the developers of color management software, although they are valid only when the monitor is performing to manufacturer’s specifications. Some software allows users to characterize their own monitors; other systems have built-in calibration.
CONVERSION
Conversion refers to translating color-image data from the color space of one device to that of another under known conditions. Color conversion is necessary so that a scanned image reproduces as a believable representation of the original on both the screen and the printer. Since output devices typically have smaller color gamuts than originals, scanners, and monitors, colors in the original must be fit into the gamut of the device, a process known as gamut compression.
Color management software converts or translates color from one space to another: from scanner to monitor, from monitor to printer, and from scanner to printer. Once color management profiling software has been used to characterize the scanner, monitor, and printer, it is necessary to apply the profiles to the image according to the desired “matching” objectives.
Three methods of color conversion are used-one for photographs, one for spot colors, and another for business graphics. Perceptual rendering, used for continuous-tone photographs, maintains the relative range of colors in a photograph. It causes the white portions of an image to have no ink on the paper, and the black portions to have the darkest color that the device can print.
Colorimetric rendering, most effective for spot colors, maintains an absolute color match. It renders colors that are within the device’s gamut identically, and brings colors outside the gamut to the closest color the device can print.
Saturation rendering is appropriate for bright saturated illustrations and graphs like those used in business presentations. This rendering style produces pure, saturated colors in print according to the printing device’s limitations. It does not try to precisely match printed colors to those on the monitor.