Continuous-tone Image Reproduction of CTP Printing Plate

                                        Continuous-tone Image Reproduction of CTP Printing Plate

In the shadow areas of an image, the recessed cells are deeper and contain a thicker ink layer; in the highlight areas, the recessed cells are shallower and carry a thinner ink layer. The continuous variation in ink film thickness enables the reproduction of different tonal levels and densities of the original image.

By utilizing dot coverage (also known as ink coverage area ratio), the ink coverage on the substrate can be adjusted through the screening process, thereby reproducing continuous tones. This principle is known as tone reproduction through halftoning.

Screening technologies can generally be divided into two major categories:

AM Screening (Amplitude Modulation Screening)

FM Screening (Frequency Modulation Screening)

In printed images, the spacing between halftone dots remains constant, while the dot size is changed to control the ink coverage ratio within a unit area.

In the highlight areas of an image:

Smaller halftone dots are generated.

The surrounding non-image areas are relatively larger.

The total dot area within a unit area is smaller.

Ink coverage is lower.

The screen angle refers to the angle between the connecting line of adjacent halftone dots and the horizontal reference line.

Due to the periodic arrangement of AM halftone dots, AM screening has an inherent disadvantage: the unavoidable occurrence of moiré patterns.

Moiré patterns cannot be completely eliminated; therefore, the objective is to minimize their impact on image quality.

Common screen angle arrangements:

In optics, when two graphic patterns with slightly different spatial frequencies overlap, a new pattern with a larger spatial period is produced. This phenomenon is called a moire pattern (moire effect).