Line Screens Printing Knowledge
Line Screens

Full-color photographs are continuous tone images, or images without dot patterns. If an image is to be output on a printing press, a process must occur to enable the process inks to be transferred to the paper and reproduce the separated color components of the image effectively. In order to print the continuous tone image on a press, dot patterns must be placed into the image to create what is referred to as a halftone image. By using dots of different sizes, three levels of color values can be accurately reproduced:

  • Highlight values - whites: Dots 5% to 10% in size
  • Midtone values - grays: Dots 30% to 70% in size
  • Shadow values - black: Dots 90% to 95% in size

Line screen, line frequency, screen frequency, or screen ruling are terms applied to the lines of dots in the halftone screens which must be used to create the image that is being printed. A line screen contains a designated number of lines per inch (lpi) comprised of dots, which are used to create the print resolution of the desired image. "Dots" has become the generic term applied to all of the dot structures used to create the halftone, regardless of whether the dot shape is square or grain texture. When making an image into a halftone, only one type of dot structure is selected to be used for screen conversion.

Consider the following when producing halftone images:

  • If the image is to be scanned into a computer, then the scanner or computer can apply a screen to the digitally created image.
  • If the image is provided as a continuous tone original from a camera, then an acetate film (contact) screen is placed over a halftone negative. Light exposes the film as it passes through the dot pattern to create the halftone negative. The resulting film negative is then used to burn the image into a plate.

The dot structures that are used to make halftones can be defined as:

  • Square dot structure: conventional or traditional dot structure
  • Elliptical dot structure: also called chain dots which provide smoother appearing middle tones
  • Round dot structure: may cause loss of detail - best for high-speed web offset - allows for improved control over dot gain in lighter middle tones
  • Dual dot structure: most often used for work requiring output of fine detail, often used as a secondary ruling for the middle tones and shadows
  • Cross line structure: may be used as a special effect screen
  • Cut line structure: special effect screen providing jagged or rough edge effect
  • Brick structure: can be used as a special effect screen
  • Grain structure: a textured pattern for special image effects

 

 

When a selected image is to be output to print, it is scanned and converted to a halftone image with a line screen digitally embedding rows of dots into the image. However, before it can be scanned, the line screen that will be used to print the image on a press must be specified. Knowing the line screen (lpi) enables the resolution (dpi) or the pixels (ppi) to be calculated for the image. During the halftone conversion process, the individual color channels (CYMK) of the continuous tone image are each converted into separate halftone images, which contain a variety of dot sizes and spacing. After a halftone is digitally prepared for each ink color or channel, such as cyan, magenta, yellow and black, the image is then ready to be output to an imagesetter. The imagesetter will provide film, plates, or hard copies for use in the printing process.

When outputting to print, raster images are dependent on the resolution (ppi) or the pixels in order to form the component parts of the image, whether it is output in two colors or four colors. As already stated, to create the proper arrangement and intensity of dots or pixels, an image resolution (dpi) must be determined for the imagesetter, which will be based on the line screen (lpi) to be used for the print product or print process.

Line screens are available in rulings from 65 lines per inch to 200 lines per inch. Selecting the proper line screen to be used for a print job requires an understanding of the relationship between the size of the halftone cell in the line screen and the size of the halftone dot. Screens with fewer lines per inch, such as 65, provide larger halftone cells while screens with a higher amount of lines per inch provide smaller cells. It is the halftone cell which holds and controls the size of the halftone dot. Large cells provide larger dots, which means there are fewer dots per inch, resulting in a coarse image. Smaller cells provide smaller dots, which means there are more dots per inch, resulting in more detail in the tonal color values.

Traditional line screens are referred to as amplitude-modulated (AM) screens having dots which vary in size, according to the number of lines per inch, and are aligned in columns and rows. Larger dots provide darker shades while smaller dots produce lighter shades in the entire image. An alternative screen method is referred to as frequency-modulated (FM) or stochastic screening, which has dots all the same size but scattered randomly throughout the screen. The greater the number of dots, the darker the shading, while fewer dots result in lighter shading. It is the dot size or placement in halftone screening, which provides different shades of colors within the image. FM screens are valued for their ability to provide finer image detail through the use of smaller dots and the elimination of moiré patterns which can occur if traditional (AM) screens are used and misaligned with other screens. The disadvantage of FM screens, however, is the resulting fuzzy appearance of solid colors and some vector images when printed.

 

 

 

As the number of lines increases on the screen, the halftone dots are smaller and more numerous which will allow for more dots per inch. More halftone dots per inch adds greater detail to the image however, it will decrease the number of gray levels or color tones that can be provided. Continuous tone images contain unlimited levels of gray, which provide tonal resolution or depth to the image. Tonal resolution, also referred to as bit depth, measures the number of bits of color or grayscale information that can be recorded per pixel. The greater the bit depth, the greater the file size, since the computer records data on each pixel. Digital devices, such as cameras, scanners, or computers can only capture and display a limited number of gray shades. The digital devices record grayscale as an 8-bit mode, which allows the image to contain the maximum of 254 shades or levels of gray per channel with black and white added for a total of 256 different tones.

By determining the shades or levels of gray (grayscale) that will be displayed when using a selected line screen (lpi) at a designated output resolution (dpi) you will be able to determine the tonal range that will be displayed by the printed image. A grayscale is a strip of gray tones (from white to black) used as a gauge to be placed next to the original or reproduced image in order to compare the balance of the three process colors (CMY) all at the same time, measuring the tonal range and contrast of the image.

To increase the tone or shades of gray, use a line screen with fewer lines per inch, which increases the size of the halftone dots. With an increase in the lines per inch, which increases the number of smaller dots in the line screen, there is a corresponding increase in the detail within the image but a decrease in the shades of gray. The calculation below illustrates how to calculate the effect of increasing or decreasing the lines per inch. If there are only 50 to 100 shades of gray displayed, then the image will appear darker.

To calculate the total shades of gray to be output in the image:

Factors to consider when selecting the line screen will be:

  1. Quality objectives of the image to be reproduced:
    • Consider the need to display an image that makes a product look more appealing, of higher quality, or with finer detail.
    • Higher resolution will produce sharper detail with brighter and deeper color values.
  2. Resolution limits of the imagesetter:
    • If the imagesetter produces a resolution of 1200 x 1200 dpi, then the image cannot be created with 2400 x 2400 dpi unless another imagesetter is used.
    • 1200 dpi is low range, 2400 dpi is mid range, and 3600 dpi is considered high range output.
  3. Type of paper stock carrying the image:
    • Stocks with higher ink absorbency such as newsprint or other uncoated stocks may broaden the appearance of the dot.
    • Higher quality coated stocks will often provide finer and more stable dot reproduction.
  4. Printing process and press that will reproduce the image:
    • The type of press and the printing process determine how well the halftone dot can be "held" on the paper. Some press types hold the dot better than others. Check with your printer for advice on which line screens run best on their equipment.
    • If the press runs at higher speeds, such as web presses, the lay-down of the ink may occur too quickly to provide sufficient depth of colors and fine detail.
    • Sheet presses which run at slower speeds enable higher numbered line screens with smaller dot patterns to reproduce images with finer details.

Line Screen Resolution Calculations

When selecting an image that will be printed, the quality of the printed image must be considered and planned for accordingly. In order to achieve the desired quality for the printed image, it is necessary to match the pixel resolution of image with the line screen suggested for the printing press. If the image is to be in a newspaper, a magazine, or an annual report, a line screen should be selected that is within the suggested ranges to provide the quality desired for the image. Different printed products and print processes use different line screens to accomplish the best results for the printed piece. A general rule to determine the best scanning resolution for the line screen is to multiply the selected line screen (lpi) by 2. This simple calculation enables an image to have two dots per inch for each line of dots in the screen or two image pixels for each halftone dot. The following chart illustrates the different resolution to use for different line screens if you use the general rule of multipling the lpi by 2.

Print Product

Line Screen (lpi)

Image Resolution (dpi)

Laser Printed Image

60 to 70

120 to 140

Newspaper or Newsletters

70 to 133

150 to  266

Magazines and Periodicals

133 to 150

266 to 300

Promotional Print Materials

150 to 177

300 to 354

High Quality Print Materials

177 to 200

354 to 400

Generally, if the scanned image resolution is 1½ times greater than the line screen, it is sufficient for most purposes. However, the rule of multiplying by 2 is often used as a simple guide to insure the highest quality possible.

 


 

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