“Develop success from failures. Discouragement and failure are two of the surest stepping stones to success.”
Reproduction photography
LENSES AND CAMERAS
Graphic arts cameras are now only retained to handle the odd price of flat artwork in conjunction with existing analogue film. When an original is exposed to light in front of a graphic art camera, the light is absorb in the black area of the original and reflected back by the white areas, through the lens onto the photosensitive material (photographic film) held in the camera. After development of the film material, a negative is obtained on which the white or clear area of the original appear dense and black area transparent.
Two sides of film:-
1. Right reading
2. Wrong reading
The negative is produce line negative. The negative used for litho plate and the wrong side up and emulsion to emulsion contact in film and plate for exposing.
PROCESS CAMERA: Produce high contrast film image from other 2- dimensional images such as line art, text. Black and white photograph and full colour.
Two types of process camera: (1) horizontal camera, (2) vertical camera.
a. Horizontal camera: the lens of a horizontal camera faces parallel with the floor and component are built in a horizontal line.
1. Galley camera: it are made in different sizes for larger size film.
It called light room camera.
2. Darkroom camera: two room camera. It installed through a wall between a darkroom and adjacent room with normal lighting.
Darkroom: Dark room that hold the film for exposing.
Normal light room: major portion like (artwork and copy, lens adjustment and exposure setting).
After exposing process doing in darkroom.
b. Vertical camera: it small room occupying less floor space. All elements (lens, copy-board, bellows) are arranged vertically with the copy-board being close to the floor. The component are built in vertical line and lens faces the floor. It installed in darkroom and daylight models that have special film holders that allow the camera to be used outside of the darkroom.
It have not allow for enlargement or reduction.
Operational steps:
1. Inspecting the scaling copy:
(a) The quality of the copy received,
(b) The reproduction size or scale required for the copy.
(c) Quality of copy: check colour, background and line quality. Ortho film not produce all colour and use of certain filers and background for use special filter.
(d) Setting copy: enlargement and reduction.
FORMULA= REPRODUCTION = IMAGE SIZE/ ORIGIANL SIZE * 100
2. Placing copy on copy-board: exact position for give marking are follows:-
a. Rectangles
b. Diagonals
c. Centrelines
Copy board hold the copy while it is being photographed. The copy-board usually consists of flat cushioned board while the hinged glass cover. Copy that is placed between the board and cover is held in position by pressure when the glass is closed.
Copy-board illumination lamp:
The light serves to illuminate the copy during exposure.
a. Incandescent lamp: light produced by in candescent light sources. It depends upon the temperature attained when the light sources are operating.
b. Tungsten halogen: regular light bulbs. The light emitted is high in red wavelengths and low in the blue-violet end of the spectrum.
c. Quartz-halogen: a quartz-iodine lamp used a tungsten filament surrounded by iodine and insert gases enclosed in a quartz bulb.
d. Pulsed Xenon: it electronic flash used in photography. The light output is close to that of daylight.
e. Mercury vapour: an electrical current passes through gaseous mercury with the lamp in order to emit light. The light is high—U.V radiation and useful when longer periods of exposure are required.
f. Metal Helide: it good source of blue-violet wavelength, metal helide lamps are mercury lamps with a metal helide.
45 degree angle require.
3. Setting the camera: the lens aperture, use of filters, lighting and camera setting for proper reproduction size.
a. Lens board: it is made up of several carefully sharped polished pieces of optical glass that are all held together in a barrel. The barrel is attached to the lens board.
b. Lens: it controls the amount of light reaching the film and the overall quantity of the photographed image.
When the lens is rotated, the metal blades of the iris inside the barrel of the lens, open or close creating a larger or smaller opening through which light can pass.
As the diameter of the aperature increase f-number decrease. So bigger the f. no. smaller the opening. Smaller the f. no. bigger the opening.
For line work, the most common lens opening (f-stops) used at same size are f-16 and f/22 as process lenses have their definition and resolution at these aperaturs. The aperature is varied according to the enlargement or reduction while the exposure time remains constant. In modern types of process cameras, the lens is equipped with a diaphragm chart mounted on the lens board.
c. Lens stop: it is an auxiliary device that will give a smaller or different shaped aperture than that produced by the diaphragm.
d. Shutter: it used to control the exposure time in process photography. It positioned behind the lens.
e. Ground glass: mounted on a hinged frame on the back of camera case, the ground glass is swung into place in order to check the positioning and focus of the copy.
f. Bellow: the bellows is an according like structure that extends from the lens assembly to the camera back because it is flexible.
g. Screen holder and filter holder: the screen required a screen positioning mechanism. The contact screen is made on a flexible film base, designed to be in contact with the film during exposure. The contact screen is held by vacuum camera back. The vacuum cause the contact screen to stay in place.
h. Filter: filter is necessary for holding the filter. Filter used for black and white reproduction serve two purpose: to increase the contrast of the original and to reproduce certain colour monochromatic. Contrast filter used for poor copy, pencil drawing and copy with a grayed or yellowed background. A filter is essentially a transparent foil of film or sheet of glass which transmits certain rays of the white light, while absorbing others.
i. Light angle: the lighting angle of 45 degree at a distance of 3 foot from copy-board is considered normal for process camera.
j. Setting for reproduction percentage: they require the use of a percentage scale for obtain the reproduction size and reference to the camera scale for proper setting number.
We might consider another step in the camera setting, namely the focusing of the image on the ground glass. Here the check sharpness of the image and adjust the positioning.
k. Colour temperature: a body which reflects no light falling on it and which on the other hand completely absorb all the radiation falling on it, is called a black body.
4. Loading film: after inspecting the setting the camera, the next step is the actual insertion of film in the camera.
5. Exposing film: during exposure, the photographic film receives the light reflected from the copy; the result of exposure is the formation of a latent image on the film.
6. Removing exposed film form camera: after exposing, the film is removed from the camera for further processing and store the film for developing.
7. Processing exposed film: during processing the latent image is converted into a visible image through the process of reduction in a solution called a developer. The developing agent reduce the exposed silver helide to black metallic silver, and fixer those unexposed and under developed area of the film.
a. Powder-type developers and fixer: these powders, which are packaged in a box, must be diluted to working strength using water. The temperature is between 90 and 100 degree F.
b. Liquid concentrate developer and fixers: it quite popular. It is so easy to mix them. Those developers supplied in powder form. It stored at temperature above 40 degree F.
EMULSIONS FOR PHOTOGRAPHY
Photographic films:
1. Lith film: Lith film is a high contrast, high quality film, usually with orthochromatic dye emulsion. It is one of the oldest of the current film technologies and its use is declining due to complex bath processing required in the process, especially in controlling the correct strength of the developer.
Most traditional photographic material is based on chemical compound of silver-producing light sensitive material in two main type paper based, known as bromide, and film based, as negative and positive.
It became transparent in the unexposed area, whereas paper remains opaque.
Films are made-up of light-sensitive material emulsion, consisting of silver helide salts and gelatine coating on a stable base. Silver-base film material are colour-sensitive, reacting normally to the ultra-violet, blue-violet and between regions of the spectrum. In order to extend this colour sensitive, colour dyes are added. The two most common films.
a. Orthochromatic films: orthochromatic films has an extended color spectrum, going from between blue-voilet, blue, green, yellow, through to orange, but not include red.
b. Panchromatic films: panchromatic films is sensitive to the whole visible spectrum, form blue-voilet through to red, it can be operated in total darkness.
It stored in dark-green light.
The newer generation of color scanner and laser image setter use are red sensitive film which require a cyan safelight.
This lith films used in conventional camera, which produce the whole tonal range.
2. Rapid access film: rapid access film has become much more popular than lith, due to the fact that keeping chemicals in balance is less critical in rapid access than in lith processing.
3. Third generation (daylight operated film): it insensitive to all area of the color spectrum other than blue.
It stored in red, orange and yellow safelight.
4. Fourth-generation films: it is good quality film and sharpness.
It is available as a conventional camera based option or for image setter and scanner depending on the exposure source used -.g- NeNe, IR laser etc.
Types of graphic arts films/ photographic films:
1. Color sensitive: orthochromatic, panchromatic, blue sensitive films.
2. Contrast gradient: high contrast (lith films), low contrast (continuous tone) films.
3. Effect of light on silver emulsion: more darkness (conventional emulsion) or less density (direct positive emulsion).
4. Base material and thickness:
Color sensitivity:
Color sensitive, meaning that they are sensitive to specific colour or colours of light. Normally, the sensitive of silver helide emulsion is limited to the ultraviolet, blue violet, and regions of the spectrum. Extent the colour sensitive of the emulsion, dyes must be added.
Colour sensitivity become three emulsion types:
1. Blue-sensitive films: it blue sensitive films and it does not record red or green. It (colour-blind) film and it store easily. It suitable for making both halftone and line image from black and white copy, gravour positive.
2. Orthochromatic film: film is used to make halftone and line for image from black and white reflection copy, black and white continuous tone film. It can be used for any purpose other than making colour separation from full-colour copy.
It most widely used photomechanical film, for both line and halftone work.
3. Panchromatic film: it used for making both continuous tone and direct screened colour separation.
Used for separation coloured original.
Contrast:
The tonal different between highlight and shadow areas of an image represent its contrast. Film emulsion records the tones of a grey scale.
For film contacts the grey scale is placed on the film in the contact frame and is used to check the film contact for proper exposure and development.
1. High-contrast graphic arts films: high-contrast graphic arts films t as lith type film.
2. Continuous-tone graphic arts film: lower contrast than lith film.
FILM BASE:
Film base coated equally and stable according job requirement different thickness available.
1. Anti-stress layer: this thin top coating or super coat reinforced the emulsion layer helping to maximize scratches.
2. Emulsion layer: this is the gelatin layer holding the light sensitive layer of silver helide grains.
3. Base material: it prepared plastic layers are few material.
4. Substratum layer: it weak mixture of gelatin and base material and it adhesion is for required.
5. Anti-helation backing: selected dyes are dissolved in gelatin and coated onto the back of the base material, this dyes are absorb any light and increased by tinting the base material with grey colour. It protect film curling in during exposure.
FILM SPEED:
Film is term describing the time response to exposure.
Blue sensitive film will be slower than panchromatic film.
FILM EMULSION: The silver emulsion used on sensitized material consists of a colloid such as gelation, silver halides, and additives.
1. Gelation: gelatine is a protein obtained from the tissues, hides, cartilage and bones of animals. The important specification for photographic use of gelatine are jelly strength, PH, moisture content and metal content of iron, lead, copper and alumina together with limits of ash and sulfer dioxide.
2. Silver helides: the important salts of silver chloride, bromide and iodide.
a. Silver bromide (AgBr): it most used of the silver paper and film products. It is usually the major portion of the halide salt in film and high speed or low fog action emulsion.
b. Silver iodide (AgI): it used generally in combination with other halides. It is for slow speed.
c. Silver chloride (AgCL): it is pure white powder and used on paper type emulsion.
The halides of silver are widely used the oxalates, nitrates, artrates and citrates of silver.
3. Additives of emulsion: the silver salts are colour sensitive inherently to the blue portion of the spectrum only. To increase this sensitivity to red and green, colours sensitizing dyes are added to the photographic emulsion. These sensitizing dyes are absorbed by the silver helide.
Preparation of the emulsion:
a. Emulsification:
1. Soaking and dissolving a portion of the gelation.
2. Addition of bromide and iodides used in precipitation of the silver helide and forming silver helide crystals.
3. Addition of silver salts (nitrate) can be added either rapidly or slowly. If it is a added very slowly, the size of the silver helide crystals will be considerably larger than when the silver nitrate solution is poured in at once.
b. Physical ripening:
1. Recrystallization of emulsion by heating.
2. Addition of the remaining gelatine and chilling.
c. Washing of the emulsion:
1. Washing to remove the emulsion of unwanted salts.
2. The formation of ‘noodles’ by forcing the chilled gelatine through a wire screen.
d. Second reopening:
1. Reheating, which referred to as after-reopening or second-reopening to recrystallize the silver salts.
2. This do not effect size but increase the sensitivity.
e. Doctoring: addition of special chemicals such as optical sensitizing dyes, preservatives, stabilizers, spreading agents, anti-fog agents, hardening agent.
LINE AND HALFTONE 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.
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.
Printing process
screen rulling
Sheet-fed (commercial colour)
40X60 cm
Sheet-fed (very fine screen / artwork)
80-118 cm
Cold-set web-offset (newspaper)
34-52 inch
Heat-set web offset (magazine)
40-80 cm
Screen printing
20-52 cm
Flexography letterpress printing
26-52 cm
Gravure printing
34-60 cm
For substrate:
Screen ruling
suitable for
45-55 lpi
low grade of newspaper
65-85 lpi
best newspaper
100 lpi
super calender, art paper
120 lpi
normal art paper
133 lpi
good art paper
150 lpi
finest art paper, chromo
300-400 lpi
Higher grade of plastic coated Surface.
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.
Resolution recording
Smallest recordable (value)
Tonal steps
Conventional offset
3386 dpi
8 micron
256
Direct imaging offset
1270 dpi
20 micron
100
Indigo Xeikon, chroma press,
812 dpi
31 micron
64
IBM, docucolour 70
600 dpi
42 micron
49
CLC, GCE and docucolor 40
400 dpi
64 micron
36
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 is 45 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).
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.
FILM PROCESSING & QUALITY CONTROL
For density range and detail rendering of a continuous-tone material negative or positive depend equally on the developer.
1. Action and composition of the developer:
a. Water
b. Developing agent (Glycin, hydroquinone, Metol, Phenidone, Pyrocatechin).
c. Activator (reducing action in the presence of alkalis (accelerator of alkali).
Like (Borax, sodium carbonate, potassium, carbonate, sodium hydroxide= caustic soda, potassium hydroxide = caustic potash.
d. Preservatives:- (sodium sulphite, potassium Metabisulphite, sodium bisulphite)
e. Anti-fog agent (Re-strainer):- (potassium bromide)
f. Special additions: sodium hexametaphosphate, wetting agent, hardening agent (sodium sulphate).
2. Developing time: 2 ¼ to 2 ¾ minutes.
3. Temperature of developer: 68 F (20 degree C) for normal, tolerance 65 degree to 71 degree F (18 and 22 degree).
4. Agitation of the developer: uniform flow patter of the liquid (plain level).
5. Exhaustion of developer: by frequently discarding the developer solution and replacing it by fresh solution.
6. Dilution of the developer: the contrast of the image can be decreased by diluting the developer or more parts of water.
7. Method of development:
a. Tray development.
b. Tank development: pipes is placed in a horizontal position, close to the bottom of the development tank. Each pipe has a number a small holes drilled according to a certain patter. When nitrogen has blown into the fork under sufficient pressure, the gas will escape through the little holes, from bubbles which rise through the liquid to the surface and finally escape into the air.
8. Inspection processed film: after development, the negative is inspected.
9. Stop bath: after a negative or print has been developed and it is used to immediately prevent further reaction of the developer with the exposed silver salts when after a certain time, the desired material density of the light-sensitive material has been obtained.
A solution of 2-5 % acid or citric acid or citric acid, or potassium meta-bisulphate is commonly used for this purpose. It neutralizes the alkalinity.
It is classified into three types of rinse bath:-
1. Water rein bat: it helps to slow down the action and remove the excess developer.
2. Hardening rinse bah: it is used to harden the emulsion when processing at high temperature.
3. Acid rinse bath: it used after high speed developer.
a. Minimize the dichloric fog.
b. Remove the scum.
c. Preserve the acidity.
d. Prevents excess swelling of gelatine.
10. Fixer solution: it is necessary to remove the residual unexposed and undeveloped silver helide crystal from the emulsion.
The most widely used fixer are based on
a. Sodium thiosulphate (hypo) current fixing.
b. Ammonium thiosulphater for (rapid) fixing.
11. Washing: it removal of water soluble salts now present in the unexposed areas of the emulsion and water change in every 5 minutes.
12. Drying: take place in cool air. (Internal temperature – 40 degree C (104F) and 55% humidity.
Film and film processing:
Typically, the separation film is loaded into cassettes in the darkroom, or the scanner is located entirely within a darkroom. In the former case, a light tight cassette is locked into position on the scanner. Its content are then automatically attached to the exposure drum. After exposure, the contents are unloaded and removed to the darkroom for processing. All four separation should be processed is the most desirable.
Before the film can be exposed, however the scanner must be calliberated to the film and film processing method. Four grey scales, each ate corresponding to a particular separation colour are scanned or electronically generated and exposed to the film. After processing, the exposed steps are measured on a densitometer.
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.
After film making, film assembled for plate making.
Film assembly: assembling all of these film image in their exact places on the flats for plate making is often referred to as film assembly.
Factories for film assembly:
1. Design, folding, trimming, binding.
2. The type of machines and size of machines before pasting.
3. On which sizes of paper, type and grain direction.
Flat: working with positive is called flat.
Window: negative assembled.
We use golden rod paper for window in industries. These paper are of orange and red colour.
For quality we use vinyl orange sheet.
For layout: standard grid sheet is used.
After the flat is prepared it is generally tested on inexpensive photosensitive material to check the image position and to ensure. So, it is called proofing.
The stripping artist uses a variety of tools like T-square, triangle, cutting tools, blades, exacto knives to trim films.
For positive stripping masking sheet and film must pass light in all but the image area. Opaque is a liquid material used to cover pin holes and other unwanted details on film negatives.
Masking sheet: the position of the image on printing plate is determined by the film position on the masking sheet. Care must be taken in placing the film image on the masking sheet to ensure that they are in correct position.
The cylinder line: it identify the area of the masking sheet that covers the part of the litho plate clamped to the plate cylinder.
The gripper margin: it is the area of the paper hold by the mechanical finger that pull the sheet for printing.
The plate centre line: it is centre of the plate cylinder.
Processing accessories:
1. Processing trays: it is open top container that holds one of the solution used to process photographic material.
2. Processing tank: process several sheet of exposed photographic material at one time.
Two types of processing:
1. Manual film processing: involves following steps.
a. Developing
b. Stop bathing
c. Fixing
d. Washing
e. Drying
2. Automatic film processing: the general construction of an automatic processor is around a frame work either of stainless steel or metal work sprayed with a chemically resistant paint.
This tank house frame work are fabricated of stainless steel or of a special rigid PVC.
These are arranged into three section.
1. Developing
2. Fixing
3. Washing
Apart from these section there is a dryer section at the rear end.
Film transport through the processor two system are in use.
1. Roller transport: this system consists of stainless steel, phenolic rubber, foam or plastic roller driven by chain.
2. Belt transport:
Working of an automatic processor:
1. It photographic material carried by rollers. The roller are arranged in racks for easy removal and cleaning.
2. Individual racks are movement unit to next unit.
3. Developing
4. Stop bath, fixing
5. Washing
6. Drying
It delivery in four to five minutes and it provide accurate time, temperature and agitation control and reduces cost.
Types of automatic processing:
1. Litho-type: process time 4 to 6 minutes.
2. Contact type: process time less than 2 minutes.
3. Rapid: processor time 90 second.
SHORT NOTES:
1. Dual tone
2. Orthochromatic film
3. Moire pattern
4. Panchromatic film
5. Aperture
6. Actinic light
7. Illuminants
8. Arc lamp
9. Developer
10. Fixer
11. Stop bath
12. Colour chart
13. Registration
14. Colour filter
15. Filter factor
16. Colour temperature
17. Panchromatic
18. Intensifier
19. Exposure
20. Screen distance
21. Latent image
22. Emulsion
23. Halftone
24. Middle-tone
25. Continuous tone
26. VIBGYOR
27. Lateral reversal
28. Duplicating film
29. Anti-halogen lamp
30. Densitometer.