Half-tones, screen angles & morié

Eureka Printing Company, Inc. encourages you to contact us during the planning stages of your project so that we can help you with any questions or concerns you may have. We are happy to provide you with this basic information, but please be advised that these pages are only general guidelines. Your project may require different procedures.

Introduction to screens

Images can either be created within the computer with a vector based illustration application or may be nondigital and originate from a source external to the computer, like a scanner or digital camera.
The image is placed onto the page layout programs and combined with other objects to produce the final document. When the application sends the print job to the printer, it converts the color data in the image into the cyan, magenta, yellow and black (CMYK) color-separation files.

The separation files are sent to a raster image processor (RIP) that converts the information in the separated files into binary data and sends the data to an imagesetter. The imagesetter contains a laser that combines the laser spots into cells which form halftone dots on the film and create the image. The imagesetter will output four pieces of film, each containing one of the color separations.

Halftoning – A Definition

A halftone screen is a pattern of shapes that is used to simulate various intensities in the image (i.e., darker to lighter) with a single ink. Halftone screening is the process of reproducing a continuous-tone image with an imagesetter by using dots of various sizes and density to emulate shades of color. On laser printers that cannot print different sizes of dots, the halftone is produced by printing different numbers of dots in a given area.

The halftone-screen size corresponds to the size of the image, and the imagesetter places each screen of halftone cells in its location on the image by using a recorder grid. You could compare the recorder grid to a series of addresses that the imagesetter uses to determine where and how to place the halftone screens when it creates the image.

Halftone type refers to the type of dot that is being used to create the halftone. Typically, a halftone screen consists of rows of evenly spaced, round or diamond-shaped dots. However, it is possible to use halftone screens that have dots that are shaped differently. In fact, halftone screens can even use straight lines instead of dots to create an image. You can experiment with different halftone types to create interesting effects.

If you are printing a black-and-white image, the printer will recreate it with black ink only. Screening adjusts the concentration of black dots on the page to reproduce the shades of gray between black and white.

The halftone dots are grouped into grids of printer spots to form cells. If the screen frequency is set so that 100 spots (or points on the paper that the imagesetter can define) can be inked or not, then many shades of color can be produced within the cell. When the screen frequency is set to only 10 printer spots, fewer colors can be produced per cell. The capacity of an imagesetter to place printer dots on an image is defined as its resolution and is measured in dots per inch (dpi).

What Is Screening?

Originally, a halftone screen was an opaque screen with thousands of tiny holes. An image with shading was photographed through this screen using special photographic paper or film. The resulting image consisted entirely of dots. This image could then be used to create printing plates. Now, halftoned images are created with software to simulate the effect of a halftone screen. The halftone, used in conjunction with the color-separation process, is necessary to convert photographic images into material printed using process colors.

To the eye, a photographic image appears as a continuous-tone image because the chemicals used to produce the image blend smoothly at different intensities to produce shades of color. The printer creates the illusion of all those different colors and shades of color by overlaying patterns of tiny dots in only four colors—cyan, magenta, yellow and black. These colors, called process colors, are used to reproduce full-color images on a printing press. Although your monitor may be capable of producing millions of colors, a printer is capable of reproducing only a few thousand colors.

The screening process combines the technologies of halftone screening and color separation. The screening patterns form the shape and density of the dot patterns required for each of the four-color separations. When the four colors are combined in these patterns, the illusion of all the other colors and shades of color in the image are created.The screen technology should be set to match the type of imagesetter your service bureau will be using. Talk to your service bureau to determine the correct setting. If you are not using an imagesetter or you are unable to speak to your service bureau, use the standard default settings.

Screen Frequency

The halftone-screen frequency determines the number of dots used to create the image. The screen frequency is measured in lines per inch (lpi) or lines per centimeter (lpc). This measurement refers to the number of vertical rows of dots per inch (dpi) or centimeter.

When you choose a screen frequency remember that the higher the screen frequency, the sharper the image. However, there are limits to screen frequency that are determined by the type of printing press on which you are printing and the type of paper you are using. In general, a screen frequency of 85 lpi works on newsprint, and a frequency of 100 lpi or higher works on bond and glossy paper. Eureka Printing Company recommends that you use a frequency of 150 lpi for CMYK reproduction and 133 lpi for all other reproduction. If you have special needs, please contact us.

Imagesetters create screens using differing screen frequency values. When the screen frequency is high, the higher resolution screen produces an image with more detail. A lower screen frequency (the effect is exaggerated in the apple on the right) produces a screen with less detail and the image will look grainy.

Moiré Effect

Because each halftone screen consists of a regular pattern of shapes, it creates a pattern on the printed image. When the separations are combined, the patterns created by each separate halftone screen interact. This interaction can create an undesirable effect called a moiré pattern. These patterns appear when an image is printed from color separations with incorrect halftone screen angles. Therefore, the four screens used in the color-separation process must be properly aligned to avoid undesirable geometric patterns in an image.

Moiré patterns can be eliminated by changing the screen angle of each color separation. If you were using an actual screen and a camera, you would rotate the screen 15° by hand for each separation by hand. However, because you are using software to create halftone screens, you must change certain print options to change the screen angle. When you print color separations, the screen angles are set automatically. If you change these settings incorrectly, your image may not print properly.

Above is an example of moiré displaying a rosette pattern in the screening. Please consult us before you change any of these settings.

A moiré pattern can occur when you try to scan a picture that has already printed. This occurs because the printed photo has already been screened. A prepress service can usually remove this pattern, however fine detail may be lost.

Conventional Screen Angles
The image is converted to four grayscale separations based on four process inks used to print the image. Conventional screen angles for a CMYK image are demonstrated in the separations below.

Cyan: 15° Magenta: 75° Yellow: 0° Black: 45°

Early printing with CMY devices achieved best results when the screens were offset from each other at 45°, with the yellow screen at 0°. With the introduction of CMYK four-color printing, the screens could no longer be rotated in 30° increments because four rotations of 30° exceeded a total of 90° and a 0° screen would give the same results as the 90° screen. A compromise was achieved by offsetting three of the screens by 30° and the fourth by 15°.

Note: Most manufacturers use proprietary algorithms to generate screen frequencies and angles that may not conform exactly to traditional screening.

Screening Considerations

Screen angles are offset so that the moiré pattern can be avoided; however, when you combine the four-color separations, the slightest misalignment of the separations can lead to moiré. This can also result in color shifting because misalignment of the halftone dots can result in a change of dot density. To correct this problem, the screen frequency or angle must be modified. Some experienced operators will modify default screen angles, but this is probably something that should not be attempted from the desktop.

The screens may be rotated or angled, but the resolution grid of the imagesetter itself always remains stationary at 0° X – Y. This creates a problem when the screen-rotation angle does not allow each halftone dot to correspond to an absolute square on the resolution grid. Some screening compromises have been developed to help overcome the problem.