The basic principles of continuous-tone image reproduction

· 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:

1. AM Screening (Amplitude Modulation Screening) 

2. 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:

o Smaller halftone dots are generated.

o The surrounding non-image areas are relatively larger.

o The total dot area within a unit area is smaller.

o Ink coverage is lower.

o More light is reflected and less light is absorbed.

o Therefore, the printed area appears brighter.

· In the shadow areas of an image:

o Larger halftone dots are generated.

o The surrounding non-image areas become smaller.

o The total dot area within a unit area increases.

o Ink coverage becomes higher.

o Less light is reflected and more light is absorbed.

o Therefore, the printed area appears darker.

· Screen ruling, also known as halftone screen frequency, refers to the number of halftone lines per unit length.

· The screen ruling of printed materials has a direct influence on image quality:

o A higher screen ruling means more halftone dots can be accommodated within a unit area. This allows finer reproduction of image details, smoother tonal transitions, and improved tone reproduction capability.

o A lower screen ruling means fewer halftone dots exist within a unit area, resulting in rougher image details and poorer tonal reproduction.

· 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).