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Maintaining Dot Predictability

(August 2002) posted on Fri Aug 30, 2002

Coudray discusses the concept of linearization and explains how to apply it in digital prepress.

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By Mark A. Coudray

To test the monitor, make a tone ramp in Photoshop starting with pure white and blend it to pure black. On your screen, move the cursor to the point where you can first see a tonal change in the image. Then use Photoshop's Info Pallet to find the digital values for this area. The transition area should be very, very close to the end of the scale. If not, your monitor probably needs to be recalibrated.


Alternately, you can pick and measure a pure white and pure black area of the image. If the white value is anything other than 255, 255, 255 and black is anything but 0, 0, 0, your monitor needs some work.


Imagesetting is the next concern. Here, you need to make a simple tonal test image in a program like Adobe Illustrator or CorelDRAW!. Your test image should depict tones in steps, with the first six representing 0, 1, 2, 3, 4, and 5% dots. Continue in 5% increments until you get to 95%, then continue with 1% increments until the image is 100% solid. Send this test pattern to the imagesetter, then measure the resulting film with a transmission densitometer.


The first thing to check is that the output film has a Dmax in excess of 4.0. Do this by measuring the solid area of the film. Then conduct dot-area measurements at each tonal step. If the dot areas are close to expected values, but drift a percent or two, you can correct the problem using a transfer-curve function attached to the EPS file you send to the imagesetter. This transfer-curve function is a routine that adjusts an image's tonal curve as defined by the user.


If you own your imagesetter, the transfer curve can reside at the RIP level and be applied to every job that is sent out for imaging. If the dot percentages vary by more than 2% from the target values, you will need to make adjustments to the imagesetting hardware.


For conventional chemical-based imagesetters, corrections are related to exposure characteristics, film developing, or both. Here is a checklist of items that cause exposure to vary:


* laser voltage levels are too high or low


* laser tube or diode is too old


* dust is present on the imaging mirror


* developer concentration is wrong


* developer needs replenishment


* developer is old or exhausted


* developer temperature is too low or high


* development cycle is too long or short


* film is incompatible with developer chemistry


If you use an imaging system other than a conventional, silver-based chemical imagesetter (e.g., laser printer, thermal imagesetter), the basic principles still apply. And the imaging level and accuracy on the film surface is still the area of interest. Each situation is slightly different, but the amount of area imaged onto the film must be controlled precisely. The image areas must be opaque to light (have a high Dmax), and must have halftone information that accurately represents the information contained in the image file. Again, you will need to use a transmission densitometer to check the values.


Most service bureaus have testing and monitoring procedures to make sure that the film they are delivering is correct. It is not uncommon for them to run a test-image file where the minimum and maximum values are 0.5, 1, and 1.5% at 200 lines/in. for the highlights and 98.5, 99, 99.5% for the shadows. Whenever possible, you should perform your tests at a resolution that is at least twice as detailed as what you will be printing. For instance, if you are generating film with 65-line halftones, do your linearization tests at 133 lines. This narrows the parameters, giving you even better end results.


Screen printers have a nasty habit of assuming that the film that they are imaging is correct. No matter how sophisticated the imaging device is, unless you calibrate it and periodically test it, the film coming off is suspect. Since we are making complicated, compound dot-gain corrections, film output must be completely predictable.





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