How to control the dot gain in color image copy

The technical indicators such as tone level, color change, and relative contrast (K value) of the color image copying technology are all achieved by dot reproduction. Dot gains can result in reduced sharpness and subtle levels of loss, color changes and contrast, ink hue, ink density, and ink overprinting. The size of the dots and their position relative to each other determine the print color and print quality. Therefore, in the process of copying a color image, it is necessary to effectively control the dot gain.

The dots that we see on proofs or printed samples are the sum of "optical dot gain" and "mechanical dot gain." "Optical dot gain" is a visual phenomenon formed by the light absorption properties of the ink and the light scattering properties of the substrate. When light shines on a blank area, the light diffuses, and a part of the light near the net is suppressed. This light is considered to be absorbed when it cannot be reflected back into the observer's eyes. The dots appear darker and larger than the actual density and size, and it seems that a dot gain has occurred. "Machine dot gain" is the actual physical increase in dots during color separation, indexing, copying, printing, proofing, and printing. The pre-pressing factors that make up the dot gain include the shape of the dot, the thickness of the cable, the process method of the plate making and the environmental conditions, the exposure and processing methods, and the performance of the device.

Diamond mesh points are more likely to produce dot gain than other shape mesh points because the perimeter of the diamond mesh points is longer, so the diamond mesh points are suitable for images with a large number of significant midtones. In order to reduce dot gains, most printing companies prefer to use circular dots, because the circle maximizes the closed area and has the shortest circumference.

The number of mesh lines is selected based on the clarity and subtle levels of the original. The higher the number of mesh lines, the better the fine-grained effect of copying, and the better the image texture and color, but the higher the dot size during printing. In general, lower dot lines result in fewer dot gains. In printing, the control of high mesh lines is difficult. The number of lines used depends on what type of product is printed. The higher the product is, the higher the mesh line number.

Random Screening, also known as FM Screening, produces an illusion of hue by changing the number (frequency) of tiny size dots in a small area. Its advantage is that there is no intuitive dot pattern, no moire, no hue jump, and without losing the quality, the resolution and gray scale can be adjusted at the same time, and there is no stencil phenomenon. The random dot-screen increase is about 2 times that of normal screening. About 20% of the sites in the general general screening network increase, and 25%-'40% in the random screening.

The main reason for camera screening is to select the correct exposure, but also to consider the dot gain generated when compensating for printing. The dot on the screen image of the screen film will increase when printing, which is caused by the edge effect of the dot on the negative picture. The edge of the network refers to the fine part of the outer edge of each screen on the screen. The large screen of the screen is commonly known as the “virtual bank”. The old craft plate will have virtual outlets. When the virtual dot is severe, the developer and developer processes are mainly checked, and in particular, the developer temperature is checked. You can also use the copy method to change the virtual network point to a real network point.

When making plates, the exposure is critical. Minor changes affect the size of the outlets, light source, lamp distance, voltage conditions, vacuum suction time, film flatness, thickness, and manufacturing methods, materials and equipment, environment, and operator quality. Both will affect the reproduction accuracy of the outlets and must be strictly controlled.

In the process of plate making, it is required to have clear dots, accurate angles, no ghosting, and to control the increase value, K value, and field density of 50% dots. The clear point of the network refers to the density of the printed ink dot and the smoothness of the dot. Do: high-profile small outlets to complete, the middle transfer point to be clean, bright and clean network does not paste dead, solid ink film, density meets the requirements, the correct angle, no excessive increase in outlets, ghosting, etc., the clarity of the outlets Directly related to the image tone level, clarity, texture, three-dimensional and perspective and so on. The regularity of dot gain is outwardly extended from the edge of the dot (60% area or more extends to the inner circle). Since the dot gain value is proportional to the dot length, the 50% dot gain is the largest. Requires 50% outlets to increase the value of fine products for 8% -20%, general products for 10% -25%. When the proofs or prints display a basic color that is not full, the middle level is grayed out, and there is a lack of three-dimensionality, it can be considered as a dot gain.

The K value is the ratio of the difference between the density of the field and the halftone density to the density of the field. It is a measure of whether the actual ink volume and the contrast of the picture meet the requirements. At the same time, the K value can also be used to determine the increase of the dot. With a large K value, the picture appears gray and light, and the effect of thick and contrasting colors is lost. The K value is small, the overall tone of the picture is darker, and the middle tone is flat and gray. When printing, the relative contrast of fine prints: yellow 0.25-0.35, magenta and cyan 0.35-0.45, black 0.5-0.50; general print relative contrast: yellow 0.20- 0.30, magenta and cyan 0.30-0.40, black 0.30-0.45.

At the same time, in the process of plate making, it is also necessary to control the density of various colors in the field. Do: The density of fine prints is 0.85-1.15 yellow, magenta 1.25-1.55, green 1.30-1.60, and black 1.40-1.80. : Yellow 0.80-1.10, Crystal Red 1.15-1.45, Cyan 1.25-1.55, Black 1.20-1.60.

Reprinted from; Print World

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