Colour Science and Quality Control in Printing
This section covers Colour Science and Quality Control in Printing, the domain that tests colour mixing, measurement, dot gain, registration and the checks used to keep a print run consistent.
Core rules and concepts
- Additive colour uses red, green and blue light; subtractive colour uses cyan, magenta and yellow, with black added for depth and text.
- Colour has hue, saturation and lightness; Lab is a device-independent colour space used for communication and measurement.
- Densitometers measure ink density and dot area; spectrophotometers measure spectral reflectance and can calculate Lab and ΔE.
- Dot gain or TVI is the difference between requested and measured dot area; it includes mechanical and optical effects.
- Grey balance, trapping, register and print contrast are controlled with control strips and standard viewing conditions.
- Proofing must use the same substrate and ink standard as production; wet ink prints darker and may change on drying.
Worked example
A 50% tint is requested, but the densitometer reads 62% dot area on the sheet. TVI = 62 − 50 = 12%. If the standard allows 10%, the press is over-gaining by 2 percentage points. Check ink film, blanket pressure and dampening before changing the curve; do not correct by altering the original file alone.
Traps
Do not use a densitometer as a colorimeter; do not compare RGB screen colour with CMYK print; do not measure wet and dry sheets as if identical; do not confuse dot gain with ink trapping; do not ignore metamerism under different lights.