Printing Technology

Xi’an University of Technology

Prof. Dr.Cao Congjun

Cp5. Color & color reproduction

Fig.Visible range in the spectrum of electromagnetic waves

For text and graphic

No problem

Why ？

为什么？

Cp5. Color & color reproduction

How to ？Multi-color printing

Autotypical multicolor printing (microscopic photo of a print image

section); scale: cell spacing about 167 µm.

a Image detail with homogeneous color (3-color print);

b Image detail with structured color (4-color print)

1. Color dispersion ：

White light

White light

prism

Convex mirror

R 700nm G 546.7nm B 435.8nm-Additive primary color

Color and Light

2. R / G / B basic optional colors ：

The building up of luminosity by means of the individual colors involved is known as additive color mixture.

3. Additive color mixing process

RR

G

Y

R

G B

Y M

C

W

Additive mixing of different luminosity

Light increasing?

4. Subtractive color ：

Subtractive color mixing ：

Absorb and Reflect Color

• Different chromatic value have a different absorptivity and reflectivity

The same chromatic

Different saturation

5. C / M / Y subtractive primary colors

fresco

photo

With subtractive color mixing, luminosity 发光度 decreases by the addition of individual colors.

C

M

C

B

YM

C

R

GBK

6. Subtractive color mixing processes

Light reducing?

Color mixing with different percentage of ink

The two basic condition of color reproduction:

Color separations and synthesization

Color original

Printing products

?

Color separations and synthesization

Color separations ：

Color original

C film M film

Y film

Color of original are separated to the basic ink color

Color synthesization

Printing products

Ink the three primary color on each plate and overprint.

Fig.Color separations and overprinting for reproduction in four-color offset printing

1. Color separations method ：

滤色片

R filter — C negative filmG filter — M negative filmB filter — Y negative film

According to Subtractive color mixing principle, process the originals with scanner or camera to get c/m/y color separations.

How to Separate?

Filter

Dye or Pigment

filter

Incident light

Transmission light

Y inked plate

originals

filter

Y negative film

Y positive film

Printing products

M C

返回：颜色合成 返回：总结加色法色环图 减色法色环图

2. Color synthesization methods

Dot overprinting

1. Copy separated color negative film to positive one 2. Platemaking3. Inking plate separately with C/M/Y basic ink4. Overprinting

Printing products

Dot position relationship when overprinting: parataxis superposition(overlap)

Did Superposition dot and parataxis dot have the same color result when overprinting?

A. Superposition dot: Subtractive color mixing principle

R G

B K

Note ： the color result is influenced by the chroma, transparence, ink thickness, and overprinting order.

bluepurple

Dot superposition can faithfully reproduce color

B. Parataxis dot ：

(W)R

(W)G

(W) B

(W)(W) gray

B. Parataxis dot ：

Note ： the color result is influence by the chroma

Dot parataxis can reproduce color

orange mauve

(W)R

IF considering the dot size and the reflection form white paper , then …

Note!

(W)R

The same color result

If considering the dot size the reflection from white paper , then…

Note

Whatever dot superposition or parataxis in a unit area, the reproduction result are the same in the theory.

Conclusion

(W)(W) Gray

(W) (W)GrayK W

Same color

But

In practice: we need to add a black plate because of material problem of ink and filter.

Ideal model: C ,M ,Y three primary ink color

Fig. Spectral distribution (degree of spectral reflection) of printing inks for multicolor printing -P102 Fig 5.6

Color Masking

Color masking has three distinct goals in color reproduction:

※To compress the density range of the color original

(called tone correction)

※ To compensate for color deficiencies in process inks (called color correction)

※ To enhance the detail of the final reproduction (called sharpness enhancement)

The Color BlackWhy black? Essentially, black is used in multicolor printing to

reduced the technology expense of three chromatic colors to create black or gray value by the direct use of black ink, to cut down the use of expensive high-quality chromatic inks and also, primarily, in order to stabilize the printing process, that is, to make it less sensitive to variations in the individual colors.

There are several methods for controlling the black color separation, that is, for supplement chromatic composition using the colors cyan, magenta, and yellow with the fourth color, black:

Chromatic composition with Under Color Removal (UCR)

Achromatic composition (or GCR: gray component reduction)

Black creation

Fig. Examples of determining the color separation for black using the multicolor printing of brown as an example. a Chromatic composition; b Chromatic composition with under color removal (UCR); c Achromatic composition (or GCR: gray component reduction); d Achromatic composition with chromatic color addition (UCA) Note: The chart serves to explain the principle and does not represent a metrologically accurate reproduction

Under Color Removal (UCR) is a variant of chromatic composition, in which a part of the example there is to be a 30%UCR of the used brown. To this end ,the achromatic portion composed from cyan, magenta, and yellow is reduced from 70% by 30% and replaced by the corresponding black portion. As a result of this the area coverage is no longer 240%, but only 180% with the same apparent hue. This greatly simplifies the printer’s task , because the danger of offsetting is reduced and the color balance is easier to maintain.

Chromatic Composition with Under Color Removal

Achromatic composition Unlike chromatic composition, in achromatic

composition in principle all achromatic portion are replaced by black (GCR: gray component replacement). Therefore, the blackening of chromatic color shades is no longer done by means of complementary color, but solely by black. The brown shown as example consists in achromatic composition of only magenta, yellow, and black. The area coverage is a total of only 100%. As a result of this, the color portion of cyan, magenta, and yellow can be noticeably reduced in all images and hues, the printing process becomes more stable, the ink acceptance (trapping) behavior noticeable improved.

Summary

Film Plate Color prints

Separation informationColor originals

Color reproduction principles:

Color separation and synthesization.

Homework

P105 Review questions 4-8. Describe the difference of UCR and GCR.