Prepress: imposition, RIP and computer-to-plate

In short

Prepress is everything done to a job between the approved design file and the imaging of a printing plate or cylinder: preflight and colour preparation, imposition of pages into press-ready sheet layouts, rasterisation and screening in the RIP, and exposure of the plate on a computer-to-plate (CTP) device. It decides whether the press runs at the first makeready or wastes thousands of sheets, so it is where most of a printing company's technical staff actually sit. In B.Tech Printing and Packaging Technology it is a full paper, examined with imposition layouts, screening and dot-area numericals, plate-type comparisons and workflow flowcharts. Employers who interview on it include Manipal Technologies, Thomson Press, Replika Press, Nutech Print Services, newspaper presses (Times of India, Dainik Bhaskar, Malayala Manorama) and packaging converters such as Parksons, TCPL, ITC PSPD and Huhtamaki India, plus the equipment and software side (Heidelberg, Kodak, Agfa, Fujifilm, Esko, Screen).

Explained

1. The prepress workflow and file preparation

• Sequence: receive artwork → preflight → correct and normalise → colour management → trapping → imposition → proof and approval → RIP → plate imaging → plate quality check → press.
• Preflight is an automated check of the incoming PDF: fonts embedded, image effective resolution, colour spaces, spot-colour naming, overprint settings, hairline widths, bleed, total area coverage, barcode presence. Done with Enfocus PitStop, callas pdfToolbox or the RIP's own preflight.
• PDF/X is the exchange standard family under ISO 15930. PDF/X-1a is CMYK plus spot only, all transparency flattened, no ICC profiles. PDF/X-4 allows live transparency, layers and ICC colour, and is the current default for both commercial and packaging work.
• Bleed is image extended beyond the trim line, normally 3 mm in Indian commercial practice (5 mm often asked on cartons); safety or type margin keeps text 3–5 mm inside the trim.
• Total area coverage (TAC) is the sum of C+M+Y+K in the heaviest shadow, limited by press and substrate — roughly 300–340 % for coated sheetfed offset, 240–260 % for newsprint. Controlled by the separation settings or an ink-optimisation DeviceLink with GCR (grey component replacement).
• Trapping is a small deliberate overlap between adjoining colours so a slight misregister does not show a paper-white gap. Spread enlarges the lighter object, choke shrinks the lighter background; typical trap width 0.05–0.15 mm. Rich black and small reversed type are kept out of traps.
• Colour management: input profile → PCS (Lab) → output profile, with a rendering intent (relative colorimetric with black point compensation for most print work). Indian presses commonly characterise to the ISO 12647-2 data sets (FOGRA51/52 for PSO coated and uncoated v3).
• Packaging artwork additionally carries the dieline, creases, varnish and white-underprint layers, step-and-repeat position, and the statutory declarations — net quantity, MRP, importer or manufacturer, consumer-care details under the Legal Metrology (Packaged Commodities) Rules, 2011; the FSSAI labelling and display regulations for food; the GS1 India barcode with its bar-width reduction and quiet zone. Prepress is where a wrong MRP block or a failed barcode is caught.

2. Imposition

• Definition: arranging pages on a press sheet in the position and orientation that gives correct page sequence after folding, cutting and binding.
• Signature (section): one folded press sheet. A sheet folded once gives 4 pages, twice 8, three times 16, four times 32. Folio = 4 pp, quarto = 8 pp, octavo = 16 pp.
• Printer's pair is the two pages that fall side by side on the plate; a reader's spread is the two pages the reader sees. For a 16-page saddle-stitched section the outer printer's pairs are 16–1 and 2–15.
• Imposition schemes:
  • Sheetwise (work and back): separate plates for front and back; two plate sets, best register, used for most books.
  • Work and turn: both halves of the form on one plate; the sheet is turned about its vertical axis, keeping the same gripper edge and side lay; one plate set, then cut in half. Economical and safe for register.
  • Work and tumble (work and flop, end over end): sheet flipped about the horizontal axis, so a new gripper edge is used; needs accurate sheet size and squareness, register is poorer.
  • Work and twist: sheet rotated 180° in the same plane, used for ruled forms and simple one-colour work.
  • Come and go (head to head) and cut and stack for labels, tickets and numbered work.
  • Step and repeat (one-up to n-up) for labels and cartons, laid out on the die layout.
• Marks and allowances placed at imposition: gripper margin (8–12 mm, no image), side lay and pull marks, register (cross) marks, colour control bar, trim and fold marks, collating or back marks on the spine fold, plate and job identification, slug line, and the plate control wedge for CTP.
• Binding-driven corrections:
  • Creep (push-out, shingling): in saddle stitching the inner sheets protrude at the fore edge; inner pages must be shifted progressively towards the spine so the trimmed margins stay equal.
  • Bottling (binder's creep in perfect binding): pages near the spine of thick sections appear rotated; corrected by a small angular shift.
  • Bind margin allowance for the glue or stitch, and grain direction parallel to the spine.
• Imposition tools: Kodak Preps, Heidelberg Signa, Agfa Apogee Impose, Fujifilm XMF, Esko Plato and Automation Engine; templates are driven by JDF so the folding and cutting data pass to the folder and guillotine.

3. The RIP: what it does and how

• RIP = Raster Image Processor. It converts a page-description language file into a one-bit (or multi-bit) raster the imaging engine can expose.
• Stages:
  1. Interpret the PDL — PostScript Level 3, PDF, PPML or TIFF-IT — building an internal display list.
  2. Render (rasterise) vectors, type and images into continuous-tone pixel values at the device resolution.
  3. Screen (halftone) each separation into dots, applying the calibration and dot-gain compensation curves.
  4. Output 1-bit TIFF, LEN, or a proprietary raster to the CTP, proofer or digital press.
• In-RIP functions: colour conversion and DeviceLink, in-RIP separation of composite files, in-RIP trapping, overprint and transparency handling, transparency flattening, OPI image replacement, imposition of pre-ripped pages.
• Interpreter generations: CPSI (Configurable PostScript Interpreter) flattened transparency; the Adobe PDF Print Engine (APPE) renders native PDF with live transparency and is what Prinect, Prinergy, Apogee, XMF and Esko Imaging Engine use. Harlequin (Global Graphics) is the other major engine.
• ROOM — "RIP Once, Output Many": rasterise a page once and reuse the raster for proof, plate and reprint, guaranteeing that the proof and the plate came from the same data.
• Resolution versus addressability: addressability is the spot grid (2400 or 2540 dpi typical for CTP); resolution is the usable detail, limited also by spot size and plate response.
• Screening types:
  • AM (amplitude modulated, conventional): dots on a fixed grid at fixed ruling and angle, size varies with tone. Simple, predictable, prone to moiré and to a visible tone jump at the 50 % dot join.
  • FM (frequency modulated, stochastic): fixed micro-dot size, dot count varies. No moiré, no angles, excellent detail; harder to control on press, higher dot gain, needs stable plate making.
  • Hybrid or XM (Sublima, Maxtone, Spekta; Staccato is Kodak's FM screening, not hybrid): AM in midtones, FM in highlights and shadows, so 250 lpi-class detail runs on a conventional press.
  • Dot shapes: round, square, elliptical (chain), Euclidean — elliptical delays the mid-tone join and is the usual default.
• Screen ruling (lpi or lines/cm) by substrate: newsprint 85–100 lpi (34–40 l/cm), uncoated 133–150, coated sheetfed 175–200, high-end coated and cartons 200–300 or FM.
• Screen angles and moiré: C 15°, M 75°, K 45°, Y 0° or 90°, giving a 30° separation between the strong colours and a rosette pattern. Yellow at 15° from another screen causes moiré; subject moiré arises when the artwork's own pattern beats with the screen.
• Calibration and linearisation: the plate is linearised so a 50 % file value images as 50 % dot area on the plate; press dot gain is then compensated by a transfer curve aimed at the ISO 12647-2 tone value increase curves.

4. Computer-to-plate

• Workflow history: camera and film assembly (manual stripping) → imagesetter film plus contact exposure (CTF) → direct laser imaging of the plate (CTP) → direct imaging on press (DI) and computer-to-cylinder for gravure.
• Imaging engine geometries:
  • External drum: plate clamped on the outside of a rotating drum, single head with a multi-beam laser array very close to the surface. Standard for thermal CTP; high power, high throughput.
  • Internal drum: plate held on the inside of a fixed cylinder, a spinning mirror writes the line. Used for violet CTP; simple optics, excellent geometric accuracy.
  • Flatbed: plate flat on a bed, scanned by a moving optical head, used for large-format, flexo and screen plate making.
• Plate types:
  • Thermal (830 nm infrared laser diode): aluminium base, anodised and grained, with an IR-sensitive coating. Negative-working plates generally need a pre-heat to complete crosslinking; positive-working do not. Daylight-safe, very robust image, run lengths of several hundred thousand, extendable by baking (post-bake). Exposure energy of the order of 100–200 mJ/cm².
  • Violet photopolymer (405 nm laser diode): needs very little energy, of the order of tens to about 100 µJ/cm², so imaging is fast and cheap; must be handled under yellow safelight. Widely used in newspaper plants.
  • Processless and chemistry-free: on-press development or a simple gum-only clean-out — no developer, no rinse water, no effluent. Growing fast in India because spent developer is a hazardous waste to be handled under the Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016, as amended.
  • Digital flexo (LAMS): a black laser-ablatable mask on the photopolymer plate is imaged at 2540 dpi, then UV main exposure through the mask; surface screening and flat-top dot technologies add the tonal range. Letterpress, dry-offset and gravure (direct electromechanical or laser engraving of the cylinder) have their own direct-imaging routes.
• Plate processing line: pre-heat (if required) → pre-wash → developer at controlled temperature and conductivity → rinse → gum and dry. Developer replenishment, bath temperature and brush pressure are the three variables that move the plate curve.
• Plate quality control: expose the Ugra/FOGRA plate control wedge or the vendor's test strip and measure with a plate reader (X-Rite, Techkon, Heidelberg). Checks are: 1×1 pixel resolution patches resolved, solid cleanly imaged, 50 % dot area within about ±1–2 % of aim, highlight 2 % holding and shadow 98 % open, no scumming in the non-image area. Re-linearise when the 50 % drifts.
• Why CTP replaced film: no film cost or chemistry, no dot loss or dirt from contact exposure, 1-bit accuracy, better register because all separations come from one geometry, faster makeready, and pre-set ink keys from the raster (CIP3/PPF or JDF) that cut press waste.
• Limitations: no physical film to archive or check by eye, an error goes straight to plate, plate cost is wasted on a mistake, and a soft or digital proof must be trusted.
• Workflow automation and standards: JDF and JMF from CIP4 carry the job ticket from MIS to prepress to press to finishing; ISO 15930 (PDF/X), ISO 12647-2 (offset process control), ISO 12647-6 (flexo), ISO 12647-7 (contract digital proofing with its control strip and Delta E tolerances), ISO 12647-8 (validation print), G7 grey-balance calibration, ISO 9001:2026 for the quality system. Indian paper and board trimmed sizes follow the ISO 216 A, B and C series, adopted as the Indian Standard for paper sizes.

Key points

• Prepress - all file, imposition, RIP and plate-making work between approved artwork and the press.
• Preflight - automated check of fonts, resolution, colour space, bleed, overprint and ink coverage before anything is imposed.
• PDF/X-4 (ISO 15930-7) - current print-exchange PDF; keeps live transparency and ICC colour, unlike PDF/X-1a.
• Signature - one folded press sheet; folds of 1, 2, 3, 4 give 4, 8, 16, 32 pages.
• Printer's pair - the page pair that sits together on the plate, not the pair the reader sees.
• Work and turn - one plate set, same gripper edge, sheet turned on its vertical axis; work and tumble uses a new gripper edge and registers worse.
• Creep (push-out) - fore-edge protrusion of inner saddle-stitched sheets, compensated by shingling the inner pages towards the spine.
• Gripper margin - 8–12 mm image-free edge the press grippers hold; nothing can be imposed there.
• Bleed - 3 mm image overrun past trim so a cutting variation never shows white.
• TAC - total of the four ink percentages in the darkest shadow; about 300–340 % coated, 240–260 % newsprint.
• RIP - interprets the PDL, renders to pixels, screens to dots, writes a 1-bit raster.
• ROOM - RIP once, output many: the same raster drives proof and plate.
• AM versus FM screening - dot size varies at fixed spacing, against fixed micro-dots at varying spacing; FM kills moiré but gains more on press.
• Screen angles - C 15°, M 75°, K 45°, Y 0°/90°; 30° apart gives a rosette instead of moiré.
• Grey levels = (dpi / lpi)² + 1 — the reason a 2400 dpi device cannot give a full 256 steps above about 150 lpi.
• Quality factor - image ppi should be about 1.5 to 2 times the screen ruling at final printed size.
• Thermal 830 nm versus violet 405 nm plates - robust, daylight-safe and energy-hungry, against fast, low-energy and safelight-bound.
• Plate control wedge - Ugra/FOGRA strip imaged on every plate; a 50 % reading within about ±1–2 % confirms the CTP is in calibration.

Formulas and examples

Example 1 — Book imposition, plates and paper. A 160-page textbook, trim 215 × 280 mm, 4-colour throughout, 3,000 copies, perfect bound in 16-page sections.

• Pages per signature = 16, so sections = 160 ÷ 16 = 10 signatures.
• A 16-page section is 8 pages per side, laid 4 across × 2 down: image needs 4 × 215 = 860 mm and 2 × 280 = 560 mm, plus 12 mm gripper and trims — a 940 × 640 mm sheet suits.
• Printed sheetwise: 10 × 2 sides × 4 colours = 80 plates. At ₹850 a plate that is ₹68,000 of plate cost.
• Paper: 10 × 3,000 = 30,000 sheets plus 3 % spoilage = 30,900 sheets.
• Run as work and turn instead, each section would need one plate set of 4 on a 940 × 1,280 mm sheet, halving plate count to 40 but doubling the sheet and the cutting.

Example 2 — Step and repeat for a carton. Die-cut blank 320 × 240 mm; gross sheet 1000 × 707 mm; 12 mm gripper plus 6 mm back trim and 10 mm side trims, so usable area = 982 × 697 mm.

• Orientation A: floor(982 ÷ 320) = 3 across, floor(697 ÷ 240) = 2 down → 6-up.
• Orientation B: floor(982 ÷ 240) = 4 across, floor(697 ÷ 320) = 2 down → 8-up.
• Choose B. Utilisation = 100 × 8 × (320 × 240) ÷ (1000 × 707) = 100 × 614,400 ÷ 707,000 = 86.9 %.
• Going from 6-up to 8-up cuts the sheets for 200,000 cartons from 33,334 to 25,000 — a 25 % paperboard saving decided purely in prepress. Grain direction and the position of creases must still be checked before accepting B.

Example 3 — Screening limits and image resolution. A CTP writes at 2400 dpi.

• At 175 lpi: cell = 2400 ÷ 175 = 13.71 spots per side; levels = 13.71² + 1 = 188.1 + 1 ≈ 189 grey levels — below 256, so smooth gradients can band.
• At 150 lpi: 2400 ÷ 150 = 16; 16² + 1 = 257, so the full 256 levels are available.
• At 2540 dpi and 200 lpi: 12.7² + 1 = 161.3 + 1 ≈ 162 levels — a case for hybrid or FM screening instead.
• A photograph of 1500 × 2100 pixels is placed 180 mm (7.0866 in) wide. Effective resolution = 1500 ÷ 7.0866 = 211.7 ppi, giving Q = 211.7 ÷ 175 = 1.21 — too soft. For Q = 2 the maximum width is 1500 ÷ 350 = 4.2857 in = 108.9 mm.

Example 4 — Creep on a saddle-stitched brochure. 40 pages, 100 gsm coated art, caliper 0.09 mm per sheet.

• Sheets in the section = 40 ÷ 4 = 10.
• Creep = (10 − 1) × 0.09 = 0.81 mm total push-out of the innermost sheet.
• Shingling: shift each successive inner sheet 0.09 mm towards the spine, so the innermost spread moves 0.81 mm; without it the inner fore-edge margin loses 0.81 mm after trimming and page numbers sit visibly off.

Example 5 — Dot gain from densities, and the plate check. Solid black density Ds = 1.45; the nominal 50 % tint reads Dt = 0.45 (both over paper).

• 10^−0.45 = 0.35481, so numerator = 1 − 0.35481 = 0.64519.
• 10^−1.45 = 0.03548, so denominator = 1 − 0.03548 = 0.96452.
• A = 100 × 0.64519 ÷ 0.96452 = 66.9 %, so tone value increase = 66.9 − 50 = 16.9 percentage points.
• ISO 12647-2 specifies TVI aim curves and tolerances by paper type and screen ruling; a 50 % gain near 17 % is at the upper end of normal coated sheetfed practice, so check the current edition's table for the exact aim and tolerance before condemning the press.
• The plate for the same job reads 48 % on the control wedge against a 50 % file value: a 2-point deficit, corrected by re-linearising the CTP, not by changing the press curve.

Where it is used

• Book and commercial printing: Manipal Technologies, Thomson Press, Replika Press, Nutech — prepress operators build impositions in Preps or Signa, run Prinergy or Prinect, and image thermal plates on Suprasetter, Magnus or Avalon devices.
• Newspapers: Times of India, Dainik Bhaskar, Malayala Manorama and similar plants use violet CTP with pagination systems, panorama imposition and very tight 20–30 minute edition deadlines.
• Folding carton and label converters: Parksons, TCPL, ITC PSPD, Huhtamaki India — Esko Automation Engine, ArtiosCAD dielines, Plato step and repeat, white-ink and varnish separations, barcode and statutory-declaration verification.
• Flexible packaging and gravure: digital flexo plate making with LAMS masks, and direct laser or electromechanical engraving of gravure cylinders for pouches and laminates.
• Job roles: prepress operator, DTP and imposition artist, CTP operator, colour and quality technician, workflow or Prinergy administrator, packaging artwork or structural designer, application engineer with Heidelberg, Kodak, Agfa, Fujifilm or Esko.
• Allied use: wide-format and digital press RIPs (ORIS, Caldera, Fiery) use the same colour-management, screening and imposition logic for signage, photo books and short-run packaging.

Interview questions

• Q: What is the difference between a printer's pair and a reader's spread? A: A reader's spread is two facing pages as the reader sees them; a printer's pair is the two pages that must sit next to each other on the plate so they fall correctly after folding — for a 16-page section, 16 and 1.
• Q: When would you choose work and turn over work and tumble? A: Almost always. Work and turn keeps the same gripper edge and side lay, so register stays good; work and tumble presents a new gripper edge and depends on the sheet being cut exactly square and to size.
• Q: What is creep and how do you correct it? A: In saddle stitching the nested inner sheets push out at the fore edge by about (N − 1) × caliper; prepress shingles the inner pages progressively towards the spine so all trimmed margins come out equal.
• Q: Why must a gripper margin be left on the imposition? A: The press grippers physically hold that edge, so no image can be there; typically 8–12 mm, and it is why the imposed image area is always smaller than the sheet.
• Q: What exactly does a RIP do? A: It interprets the PostScript or PDF page description, renders it to continuous-tone pixels at device resolution, screens each separation into halftone dots with the calibration curve applied, and outputs a 1-bit raster to the plate or proof device.
• Q: How many grey levels does a 2400 dpi device give at 175 lpi? A: (2400 ÷ 175)² + 1, which is about 189 — fewer than the 256 the file carries, so gradients may band and hybrid or FM screening becomes attractive.
• Q: Why are screen angles set 30° apart and yellow at 0°? A: A 30° separation produces a rosette rather than moiré; yellow is weakest visually so it is placed only 15° from cyan, where its moiré is least objectionable.
• Q: Give one advantage and one risk of FM screening. A: It removes moiré and reproduces fine highlight and shadow detail without a dot-size limit; but its micro-dots gain more on press and demand very stable plate making and inking.
• Q: How do you confirm the CTP is in calibration? A: Image the Ugra/FOGRA plate control wedge, check that the 1×1 pixel patches resolve and that the 50 % patch measures within about ±1–2 % of aim on a plate reader; re-linearise if it has drifted.
• Q: Difference between thermal and violet CTP plates? A: Thermal plates use an 830 nm IR laser, are daylight-safe and need roughly 100–200 mJ/cm² with long run life; violet plates use a 405 nm laser at microjoule energies for very fast imaging but must be handled under safelight.
• Q: What is PDF/X-4 and why prefer it to PDF/X-1a? A: PDF/X-4 (ISO 15930-7) keeps live transparency, layers and ICC colour, so the RIP flattens and converts at the last moment with full information; PDF/X-1a forces early flattening to CMYK and can change soft shadows and overprints.
• Q: Why is total area coverage limited? A: Too much ink in shadows will not dry, sets off onto the next sheet and loses shadow separation; so the separation or an ink-optimisation DeviceLink caps it, around 300–340 % on coated and lower on newsprint.

Common mistakes

• Treating a reader's spread as a printer's pair, so the imposition sheet folds into the wrong page order — the classic lost-mark in the exam.
• Forgetting the gripper margin, side lay and trim allowances when calculating the sheet size, and so claiming an n-up that will not run.
• Testing only one orientation in a step-and-repeat and reporting the lower n-up as the answer.
• Ignoring creep on a thick saddle-stitched section, or applying it in the wrong direction and pushing the inner pages away from the spine.
• Confusing addressability with screen ruling, and writing grey levels as dpi ÷ lpi instead of (dpi ÷ lpi)² + 1.
• Mixing units — lpi with lines/cm, mm with inches, mJ/cm² with µJ/cm² — in resolution and exposure numericals.
• Supplying RGB images, unembedded fonts or missing bleed and blaming the press for the result; and assuming CTP calibration fixes press dot gain, when plate linearisation and press transfer curves are two separate corrections.

Related topics

Design software: Adobe tools · Offset lithography and press operations · Colour science, densitometry and colour management · Flexography and gravure printing · Packaging design, dielines and die cutting · Print finishing and binding.