If you want to be taken seriously by customers, you have to provide sharp, accurate images. Calibrating your imagesetter helps you do that.
Running an uncalibrated imagesetter is a sure way to lower the quality standards of your in-plant. With a little diligence and the right tools, though, you can be sure that when you ask your imagesetter to produce a 20-percent dot, it delivers exactly that—not a 16- or 23-percent dot. A calibrated imagesetter will prove your expertise and ensure your dependability.
In addition to providing accurate tints, a properly calibrated imagesetter helps avoid blown out highlight and clogged shadow dots in your halftone images. Accurate halftone screens become much more important as your shop moves from black-and-white work into duotones, tritones and process color, since PostScript imagesetters use the same halftone techniques to produce color separations as they do to produce grayscale output.
When you process film output from your imagesetting system, the exposed areas are black and the unexposed areas are clear (or white, in the case of RC paper). The word "density" is used to quantify the darkness of solid black areas. Density is expressed as a number derived from a logarithmic calculation; the higher the number, the darker the black. The maximum density of a medium is called its Dmax.
You'll need to use a densitometer to measure the opacity or darkness of the media. There are two basic types of densitometers: transmissive units for reading film and reflective units for reading RC paper. There are also hybrid densitometers that provide both capabilities. Your densitometer should always be calibrated before you attempt to calibrate your imagesetter.
The density of the final output depends largely on the image recorder exposure setting. Most image recorders have a laser beam intensity adjustment (called a density or exposure setting) that controls the exposure level. Higher exposure settings produce darker blacks (higher density readings) on your film or RC paper output.
To produce high-quality output, select an exposure level that yields both black text and accurate tints in your pictures. This process is difficult because image recorders can't reproduce continuous-tone images. They approximate tints and grayscales using halftone screens.
The exposure levels needed to produce truly black text (generally a logarithmic density measurement between 3.5 and 5.0) may overexpose your photomedia.
• If your Dmax is too high, fine detail is lost, as shown in Figure A. Text characters lose their serifs, and your grayscales are too dark.
• If the Dmax is too light, your film won't be black enough to use to burn plates.
Once you've found a Dmax that satisfies your text requirements, imagesetter calibration software allows you to create PostScript transfer functions (look-up tables) that ensure the gray values you specify in QuarkXPress, CorelDRAW! and Adobe Photoshop are what you receive on your imagesetter output.
You don't automatically get the tints you specify; many factors influence final output, including:
• Processor variables (developer strength, temperature and speed settings).
• Media type and batch variations.
• Halftone properties and image recorder resolution.
• Exposure and dot area.
Ensure Processor Stability
It's important that your film processor is running up to snuff before you attempt to calibrate your imagesetter. Fluctuations in your chemistry (concentration, agitation and oxidation) or washing time will affect the output. The following factors will increase overall density:
• Increasing developer strength.
• Slower processing time.
• Higher temperature.
Processor stability is an essential step in establishing repeatability; keeping your film processor consistent means regular maintenance. You should always recalibrate your imagesetter after changing the processor chemistry or replenishment rate.
The optimum exposure setting for an image recorder depends on the type of media you select. Some photographic emulsion is more sensitive to laser light than others and can be overexposed more easily. The media thickness can also affect your output. Perform an exposure test to find your optimum exposure setting. The measurements you use to create a transfer function from an unadjusted gray page, imaged at the optimum exposure setting, ensure accurate dot sizes.
Media can vary from batch to batch. Watch the batch numbers on your film and paper supplies. Use your media a batch at a time, and calibrate each time you start a new batch.
Imagesetters create an electronic version of the traditional halftone screen. Screening software in the imagesetter applies an electronic dot pattern to the electronic image. Halftone dots are formed by the combination of the image recorder resolution (dpi) and the screen frequency (lpi). Figure B shows their relationship.
The image recorder resolution reflects its ability to place recorder spots close together. A recorder spot is created by the recorder's laser beam when it's focused on a point of media. When the media is developed, the area exposed by the laser beam (recorder spot) is black. The closer a recorder laser can place spots, the higher the resolution and attainable density.
Once you've calibrated your densitometer and established an optimum density for your media, you need to create a set of Transfer Functions. Generate accurate grayscales by creating transfer functions specific to your image recorder settings and media. A transfer function is a look-up table used by the image recorder to ensure that the tint values you specify are the values you output. This is often referred to as linearization. You should create a transfer function for each combination of media type, resolution, line screen and spot function.
PrePRESS SOLUTIONS' imagesetter utility program, PrePRESS ToolBox, uses a repeatable, closed-loop process for calibration. First, an unadjusted test page containing tint percentages is imaged. The output is read with a densitometer. These numbers are entered into a dialog box and PrePRESS ToolBox automatically creates a PostScript transfer function to pre-compensate for any deviations from the desired result.
Finally, an adjusted test page is printed and measured to verify accuracy. At intervals in the production cycle, the adjusted page is reprinted and read again to ensure consistency.
PrePRESS ToolBox and the Panther RIP store functions in groups called gray sets. One gray set at a time is downloaded to the Panther RIP and may contain transfer functions for different media types, resolutions or screen frequencies. ToolBox activates one of the transfer functions in the gray set or downloads alternate gray sets as needed. A number of gray sets may be created and stored at the workstation running ToolBox. Other manufacturers' devices may or may not implement similar features.
Calibrate your imagesetter on a regular basis, whether it's once a day or once a shift. Keep a close eye on the variables, and you'll be rewarded with a stable, dependable process. IPG
Daniel Gray is the manager of marketing communications for PrePRESS SOLUTIONS and the author of a number of graphics- and Internet-related books. You can reach him at (908) 281-7217 or Dan.Gray@prepress.pps.com
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