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Colour Analysis & Reproduction Technology
UNIT-1
LINE THEORY
Line photography: after inspection the originals to ensure that they are suitable for reproduction. The next stage in conventional graphic reproduction is to produce negative or positive film, which are the intermediate step to produce the printing plate or other means of print surface preparation.
Line reproduction is the most simple of all reproductions. Line reproduction is used for black and white copy that does not enquire tonal reproduction or the use of a halftone screen. This copy may be single colour or multicolour, it may be of a job that is completely done in line, or it may be part of a line-and-halftone combination job.
1. Line original: it have no gradation of tone and no intermediate tones. The image is produced by clear distinct lines, or other shapes of uniformly solid areas. Ex-pen and ink, paste up paper form phototypesetter, type written.
Two type of line original:-
a. Monochrome line: black and white.
b. Color line: Multi-color
1. Basic Principles of Reproduction Photography
Reproduction photography is used to capture, reproduce, and prepare images or artwork for printing. It ensures that the original image quality is maintained when transferred to film or plates.
Key aspects include line photography, contact photography, and handling special line originals.
2. Line Photography
3. Difficult Line Originals
Some originals require special attention:
4. Contact Photography
5. Line Separation from Black and White Artwork
Summary Table:
Aspect
Key Points / Purpose
Line Photography
Capture high-contrast line art; maintain sharpness
Density Range
0.8–2.0 for optimal reproduction
Exposure
Controlled via computerized camera & densitometer
Difficult Line Originals
Colored lines, fine screen, fluorescence effects
Contact Photography
Spreads & chokes to adjust for printing tolerances
Line Separation
Prepares black-and-white artwork for multi-color printing
UNIT-2
HALFTONE THEORY
Halftone photography: Development of halftone process: two problem dominate the history of picture reproduction.
1. Tonal value (solved by glass cross-line screen).
2. Combination of reading matter.
Halftone photography: any image such as a photograph-that exists as a series of small dots of varying size and colour density, which serve of simulate the appearance of continuous gradations of tone is known halftone.
Lightness and darkness of portion of an image are effective by varying the size and density of the dots; small dots spaced far apart produce (light area), while large dots clustered more closely together produce (dark area).
Halftone are produce either as film positive and negative by photographing a continuous tone original through a halftone screen or fine grid.
150 line screen = 22, 500 dots for per square (150 rows and 150 column).
Why halftone screen is necessary: halftone screen is the basic tool of halftone photography. Now support for example, we want to print a black and white photograph. The original is composed of a wide range of shades of grey, the various shades of tones are “continuous” that is they blend smoothly one to the other.
It is not possible for a printing press to apply different shades or tones of an ink to paper.
The method by which continuous tone copy is transformed through a halftone screen. The screen breaks up the continuous tone into tiny dots. These dots are equally spaced. The size or diameter of the dots will vary according to the original. When this halftone image is put on a press plate and printed. The ink printed by each dot, of course has the same density.
Area of continuous tone prints: three areas
1. Highlight area
2. Middle-tone area
3. Dark tone area
These three area of a print with the density of the steps on any graphic arts grey scale.
Halftone screen: all continuous tone image whether they are colour or black and white need to be converted to halftone before they can be reproduced because few printing process can lay down varying density of ink, and distribution in a process called halftone screening. When the film is exposed, the image will consist of thousands of tiny dots, dark, middle tone, Highlight. Each colour separation negative is processed as a halftone however, the when successively coloured dots are overprint, if the angle of the liner of dots is the same for all four colour, the lines will interfere with each other and produce an moire pattern. Consequently, each screen needs to be placed at a different angle.
On digital system, halftone is performed electronically. Computer output device need to create image as a series of tiny dots called spots.
Types of screen: two types of used.
1. Glass cross line screen (kept at a definitely).
2. The contact screen (direct contact to emulsion predetermined distance).
What the halftone screen does: on the process camera, light is either transmitted through a transparency or reflected from an opaque original copy. It then process through the lens of camera. It strikes the halftone screen before it falls on the light sensitive emulsion that will become the halftone positive or negative image. When the light coming through the lens strikes the halftone screen, the light either passes through the clear portion of the screen, or is absorbed by the opaque portion. The glass halftone screen act as a grating-allowing light to pass through the opening or blocking. It allows the light to pass through the different areas in varying amounts.
The various tone of the continuous tone copy determine the amount of light that passes through the halftone screen.
Contact screens: contact screens, used to create the illusion of continuous tone in film-based work with graphics art cameras, by other dots of lesser density.
It traditional film-based reproduction the contact screen is held in close emulsion-side-to-emulsion-side contact with the light sensitive material being exposed to create the halftone pattern required. Manufacturer produce a variety of contact screen suitable for different purpose-coarse, medium or fine screen, special effect, grey or magenta etc.
Halftone dot shapes: many different dot pattern can be produced by halftone screens. They are round dot, square dot and elliptical are in numerous dot shapes. The round dot is best for high-speed presses used for web-offset printing. The square dot result in sharper in sheet-fed offset litho printing. The elliptical dot, since it allows more gradual transition and better detail on the mid tones and very fine suitable colour blends or changes. For monochrome work, screen positions are such that the line of halftone dot falls at an angle 45 degree across the processed image and the printed sheet.
Halftone dot shape: square dots tend to be best for retaining image definition.
1. Square =
2. Ellipse =
3. Round =
Screen range:
1. highlight dot = 0.4
2. shadow dot = 1.6
3. middle dot = 1.6 – 0.4 = 1.2
(1.6 & 0.4 is density of dot).
New screening developments: traditional screening method used on scanner are based on the Amplitude Modulation (AM).
AM = no. of dot variation and dot sizes same.
FM = no. of dot same and dot size variation.
FM screen break down continuous tone original into small ‘microdots’ resulting in much smaller file size and therefore faster processing and improve printing detail. Microdot sizes used in FM screening vary from around 14 to 20 microns, going down to seven microns: 20 micron FM dot equates to about the smaller highlight dot on a 150 lpi screen.
150 lpi screen = 20 micron dot
Image setter: illustrates the way ‘pixel squares’ built up the desired halftone do shape in loi, through the use of an image setter.
Conventional screen: the three dot used.
FM: illustration the scattered, irregular pattern of FM/ stochastic dot distribution.
Different AM and FM
AM: the both screen same density value is the distribution of the image recording spots within the dot formation grid. Consider, a 50%% halftone dot that is exposed within a 12X12 grid (144 recoding dots). 72 recoding dot will form a single square halftone dot.
FM: recoding dot will be distributed through the grid. The exact distribution of the recoding dot will vary according to a given manufacturer’s screening program.
Screen Ruling:
Ruling can be specified in lines per cm or lines per inch.
1. Coarse screen: 25, 34 and 40 ruling per cm. (for rougher, uncoated paper).
2. Fine screen: 48, 52 and 60 ruling per cm. (smooth, coated paper).
3. Very fine: 80 and 118 per cm are also available.
Lines per cm lines per inch
23 50
26 65
34 86
40 100
52 133
60 150
80 200
118 300
Screen resolution:
Screen resolution depend on fine image detail.
Digital halftone dot are formed by selective laser exposures within a 12X12 (or higher) grid structure. The overall 12X12 grid represents one halftone dot at a particular screen rulling. It may not be possible to expose a halftone grid fine than ‘say’ 8X8.
8X8 grid allows 64 tons steps
12X12 grid produces 144 tone step.
More tone step and printing produce finer or perfect colour matching.
Special effects:
Special photographic effects are produced to meet two general goals.
1. Printing presses can reproduce only a limited density range with a ‘double dot black duotone’. It is possible t overprint a second, specially produce halftone in register with the normal halftone on the printed sheet. The result is to increase the shadow density. It is a DUOTONE.
Duotone: duotone implies the use of two layer of tones or produce on final image.it also called fake duotones.
Screen angle: a complication is introduced whenever two different screen patterns (or dots) are over printed. If two screen are randomly positioned over each other, an objectional moire ‘moire ray’ pattern could form.
The first screen angle is45 degree
The second screen angle is 30 degree less.
From first screen is 15 or 75 degree.
2. The second goa of any special photographic effect is to be able to generalize an idea or an image rather than to produce exactly some specific picture or to create a pleasing visual impression. “posterization” can generalize idea and also produce an attractive image. It is a POSTERIZATION.
Photo-posterization: orthochromatic film was so limited that a ling range of tones could not be accurately recorded on a single sheet. The solution of this problem was to divided the tonal range of an original into several group and to deal with each group as a separate exposure on a new piece of film. So, the term is posterization. It is three tons.
The first exposure for highlight and second exposure for middle tone area.
Understanding halftone exposures:
Two exposures are generally used to produce halftone negative from a Vignetted contact screen.
1. Main exposure: the halftone exposure or detail exposure. It is simply an exposure on film through a contact screen using a process camera.
First the check (CDR) copy density range (CDR) = shadow area - highlight area
45 – 0.05 = 1.40 (CDR).
For exposure screen range is (BDR) basic density range.
CDR for original copy range
BDR for screen range
CDR is smaller than the BDR and will be problem so, because density range of 1.70.
The difference between BDR or CDR is called excess density.
2. Flash exposure: it is non-image exposure on the film through the contact screen.
Controlling halftone: the placement of the middle tone dots in a halftone photograph affects contrast.
a. By using a filter
b. By using a special camera exposure (bump or no-screen exposure).
1. Halftone Exposure
2. Special Features of Halftone Exposure
3. Factors Affecting Halftone Exposure
Several factors determine the quality of halftone images:
4. Basic Halftone Exposure Settings
a) On Ordinary Camera
b) On Computerized Camera
1. Contrast Control
Contrast control is the technique of managing tonal range and density during photographic reproduction. Proper contrast ensures good highlight, midtone, and shadow detail in the final printed image.
2. Contrast with Glass Screen
Brief Study: Glass screens allow control of dot gain and tonal range, particularly in black-and-white line or halftone reproduction.
3. Contrast Control with Contact Screens
4. Determining B.D.R. and Main Exposure
5. Use of CC (Color Compensating) Filters
1. Auxiliary or Supplementary Exposures
2. Contrast Control with Supplementary Exposures
3. Flash Exposure
4. Exposure Calculations
a) For Contact Screens
b) No-Screen Exposure
Purpose: Ensures proper image density and detail for solid areas or continuous-tone originals without using halftone screens.
UNIT-3
LIGHT AND COLOUR SEPARATION
Basic colour theory:
1. Light and colour
a. Light: light is radiant energy that is visible to the average human eye.
b. Colour: colour is a complex visual sensation.
2. Seeing and measuring colour:
3. Colour as a wave length: the visible wavelength between 380 and 760 nm (one nano-meter equals one millionth of a millimetre)
380nm = violet, 760nm = red, 570nm = green
The balanced all wavelength we see white or grey
Colour separation: colour printing, in its most expression, involves the overprinting of coloured dots at various densities to produce a wide range of secondary colours. Since each individual process colour needs to be printed separately, each colour needs its own plate. To make a plate, therefore, each colour needs its own negative. The conversion of a full colour continuous tone photograph to a series of individual colour negative or positives is called colour separation.
Traditional colour separation: traditional colour separation were performed either manually or, more often, photographically. Essentially, a full colour image (either reflection copy-such as a print or transmission copy-such as a transparency) was photographed three times, through a red filter (which produces the cyan film, a blue filter (which produced the yellow film, and a green filter (which produced the magenta film). An additional film-black was also needed to ad shading and contrast. These four films-called printers or process colour separated film could then be used to make plates.
Often additional manual colour correction (such as dot etching) was required to adjust any hue error generated by the colour separation process. The four process colour CMYK is the acronym of the three subtractive colour primaries plus ‘k’ for black. The ‘k’ stands for key, as it was the black printer that was printed first for registration.
Method of producing:
1. Direct method: in this method, the colour separation exposures are made through the halftone screen onto high-contrast film or plates so that halftone separation negatives are obtained in the first step. In other words the halftone and the colour separation are made at the same time. A glass cross line screen or a grey contact screen should be used in this method, but not the magenta contact screen.
2. Indirect method: in this indirect method the negatives or positives are not made directly from the original copy, but from intermediate continuous-tone separations. In other words first produce a continuous tone separation negative next, the continuous tone negative is screened to produce a halftone positive. The screened positive is the contact printed to make the final separation negative (it is oldest technique).
Digital colour separation: in the late 1970s and early 1980s, Scitex and other version began introducing colour electronic pre-prepress system (CEPS) which quickly rendered photographic colour separation processes virtually obsolete.
In addition, the prevalence of the PS device independent page description language has made digital colour separations of higher quality and greater each.
Digital colour separation typically functions by means of converting from one to the CMYK colour space.
All colour produce in process colour printing are combinations of CMY, black is added to increase the density range of the reproduction and reduce the amount of the more expensive ink.
The nature and quality of the colour separations include optical-mechanical electronic design of the image capture system, image recording distortions, image processing compromises and output recording system.
Steps in halftone reproduction using camera:
Electronic colour separation: scanner are primarily machine that separate colour copy into its components so that it can be reproduced on a printing press.
The colour scanning process involves four steps. They are scanning, analysis and modification, storage and image editing and exposure.
The most important element of the scanner is the photomultiplier tube (PMT). The PMT has the ability to change light into an electrical signal. The PMT can send a signal that varies in strength with variation in the light it receives.
In the operation of rotating-cycle scanner, original transparency or reflection copy is mounted on a transparent analysing cylinder, or drum while one or more sheet of unexposed films are mounted.
The original copy is scanned by the small spot light, which travel in the direction of the axis of the drum.
Generally the no. of lines to the inch (or cm) is varied to reproduce as much image detail as is desired.
Each spot on the copy is analysed as light passes from the copy through a small aperture to the scanner optical system. The optical system usually consists of some arrays of lenses, prisms, mirrors and interference filters.
The two most common light sources are high-pressure xenon or a tungsten-halogen lamp. The light is passed through a condenser lens and is deflected by a mirror. The mirror is set at a 90 degree angle to the drum surface. The narrow beam of light from the mirror passes through the colour transparency and is split into four light signal that passes through a colour separation filters (R, G, B) is focused on a PMT that converts the optical signal to electronic signal.
These three electronic signals which correspond to the M, Y and C printing inks are directed into the colour computer where the signals are modified to suited specific inks and are colour corrected. Next, the signal go to the tone and UCR computer, which into-duces the desired range compression, tone reproduction and neutral grey balance, and at the same time, computer a signal for the black printer.
Subsequently, the electronically generated and computer modified signals are sent to a digital scale computer that controls reproduction size.
Analysis and modification: the electronic signals from the photomultiplier tubes must pass through the computer control system. The most common areas of concern are colour correction and under-colour removal.
Storage and image editing: not all scanned images are stored within computer memory. After modification, the information is removed from computer memory, sent to the exposure unit to expose the film separations, and data from the second scan line is replaced in computer memory.
Exposure: colour scanner may also be classified by type of output. There are three basic method of electronic image output. 1. Continuous tone, 2. Contact screen halftone, 3. Dot generated halftones (laser scanning).
Achromatic colour: a new colour separation technique that is unique to the modern electronic colour scanner is achromatic colour. The technique is also known as grey component replacement.
GCR is an extension of UCR (under-colour removal) while UCR only removers Cyan, Magenta, Yellow in the darker neutral grey areas of the separation.
GCR: GCR replaces Cyan, Magenta, Yellow, wherever they overprint to produce a neutral grey, even in the highlights. Traditional electronic separation produce a black printer that prints from the mid-tone into shadows.
Screen angle:
The angle of the conventional halftone screen used In four colour separations must be different for each colour, to prevent the dots of successive colour become superimposed upon preceding ones, so forming an undesirable screen clash pattern or moire effect when printed.
Process for used screen angle:
1. Sheet-fed offset litho
a. Black = 45 degree
b. Magenta = 75 degree
c. Yellow = 90 degree
d. Cyan = 105 degree
2. Web-offset
a. Black = 15 degree
b. Magenta = 45 degree
c. Cyan = 75 degree
d. Yellow = 90 degree
30 degree of separation between the colours
Standard screen angle
d. Cyan = 105 (15 degree).
The extra colour for used angle, if red, green and blue are supplementary colour, they should be placed on the same angle as those inks that have the opposite hue. Green for example, because the opposite colour do not intract with each other.
Screen angle guidelines:
1. Three colour printing
Cyan = 45 degree
Magenta = 75 degree
Yellow = 105 degree
2. Four colour printing
Cyan = 105 degree
Yellow = 90 degree
Black = 45 degree
3. Six colour printing
L + C = 105 degree
L + M = 75 degree
4. Seven colour printing
Red = 105 degree
Green = 75 degree
Blue = 90 degree
When the same screen angle take to produce ROSETTE PATTERN, moire pattern.
Scanner programming: a scanner simple to operate and hat much more complicated.
The program include the following:-
1. Range compression
2. Grey balance
3. Tone reproduction
4. Black printer characteristics
5. Ink and paper densities
6. Plate making losses
7. Screen and photographic emulsion characteristics.
1. Colour and Appearance Measurement
2. Introduction to Colorimeter
3. Introduction to Spectrometer / Spectrophotometer
Key Concept:
UNIT-4
1. Introduction to Colour Separation Methods
2. Colour Separation and Analysis
a) Fake Colour Reproduction
b) Filters
c) Screen Angles
d) Study of Quality Control Aids
Importance: QC aids allow consistent print quality, accurate registration, and repeatable color reproduction.
1. Digital Photography: Introduction
2. Digital Cameras
3. Resolution
4. CCD Technologies
5. Implementation in Digital Photography
a) Lighting
b) Exposure
c) Color Balance & Consistency
d) Image Manipulation
6. Optics & Digital Photography