Prepress and reproduction workflow: resolution chain, separation, RIP, CTP and imposition
What the exam tests
Prepress questions test the resolution chain from capture to plate, separation and RIP settings, the CTP workflow and imposition arithmetic. You must be able to trace a file from scan to printed sheet and justify every number in the chain.
Resolution and the chain
- Output ppi = screen ruling (lpi) × quality factor, normally 1.5–2. For 175 lpi, use 300–350 ppi at final size.
- Scan ppi = required output ppi × enlargement factor. Enlarging after scanning only interpolates; it cannot add detail.
- Interpolation invents pixels. Resampling down is safe; resampling up is cosmetic.
Separation and RIP
- RGB is captured, CMYK is printed. UCR removes black from neutral areas; GCR replaces the three-colour grey with black, saving ink and stabilising grey balance.
- The RIP rasterises the job and applies screening (AM, FM/stochastic, hybrid), the calibration curve and trapping. Trapping overlaps adjacent colours slightly to hide register variation; overprint lets the upper ink print onto the lower.
- Bleed is 3 mm beyond trim on every edge; crop marks sit outside the trim.
CTP
Computer-to-plate images the aluminium plate directly by laser (about 405 nm violet or 830 nm thermal). Compared with film it removes one generation and the contact frame: sharper dots, faster make-ready, less chemistry. Plates must be calibrated to the processor and the press, or dot gain runs uncontrolled.
Imposition
Imposition arranges pages so that after printing, folding and trimming they fall in reading order. Pages per sheet side equals the number of ups; a 16-page signature is two sides of 8 pages. Work-and-turn prints both sides from one gripper edge; work-and-tumble flips the sheet end for end.
Worked example
A customer supplies a 90 × 120 mm image at 72 ppi and wants it 180 mm wide, printed at 150 lpi.
- Required ppi at final size = 150 × 2 = 300 ppi.
- Enlargement = 180 ÷ 90 = 200 %.
- Scan ppi = 300 × 2 = 600 ppi.
- The supplied file would print at 72 ÷ 2 = 36 ppi effective - unusable, so it must be rescanned or replaced.
Traps that cost marks
- Treating lpi and ppi as interchangeable.
- Forgetting the enlargement factor in the scan calculation.
- Upsizing an image in the layout and calling that resolution.
- Confusing trapping with overprint, or UCR with GCR.
- Omitting bleed, or placing trim marks inside the trim.