Maintaining Dot Predictability
Coudray discusses the concept of linearization and explains how to apply it in digital prepress.
In the past, I have devoted considerable time to discussing the causes and control of dot gain on press. My previous treatments of this subject focused mostly on the physical printing issues surrounding mesh selection, screen tension, press setup, ink, printing parameters, and substrate variances. But this month, I will focus on the fundamentals of linearizing digital prepress in order to achieve dot predictability on the final substrate.
Dot gain occurs in all printing processes, and each process and specific application is characterized with a unique dot-gain profile. In screen printing, to compensate for the inevitable gain on press, prepress functions such as film production and screenmaking must be adjusted and controlled.
The amount of prepress compensation you'll need to apply for a particular gain profile depends on how accurately you can maintain image characteristics throughout each stage of the printing process and repeat the procedure to achieve the same results. Without this control, it is not possible to make meaningful prepress adjustments that can be measured on press.
Linearization is another way of saying that the production process follows a linear, predictable flow from start to finish. It involves identifying variances throughout the process and correcting the variances to bring the image back into a linear progression. It means you are confident that the digital information you wish to reproduce is being faithfully and accurately moved along at each step of the prepress journey. In the end, you'll still experience dot gain or loss at each step, but the gain or loss will be consistent and predictable. And you'll be able to make corrections to the digital file to compensate for the dot gain or loss you expect to occur in screenmaking and on press.
Linearization is fundamental to achieving good halftone color reproduction. But optimizing your system for the best and most consistent results is somewhat of a mysterious process, especially since any correction you make influences another variable, which in turn affects another variable, and so on.
Two principles serve as the foundation for linearization. The first is that you can never assume the information on a film positive or screen is accurate. The second is that you must have some way to measure the process.
No matter how expensive your imagesetting device is, it must be measured and calibrated. Once you've done this, the daily output must be routinely monitored and corrected for drift from the ideal settings. This is a normal and expected part of any prepress operation. Even if you receive film from a service bureau or other provider, you still need some way of measuring to determine if the film is acceptable.
To measure halftone dot gain or loss, you need two different instruments. The first is a transmission densitometer. This device measures the amount of transmitted light that comes through your negatives or positives. It is most often used in "Dot Area" mode so that the reading you see will be the actual percentage of area covered by the halftone dot.
You also use the transmission densitometer to measure the Dmax (maximum density) of the black, imaged areas of the film, checking that the film is dark enough to avoid burn through during screen exposure. The reading the device provides is a logarithm of light transmittance. A reading of 4.0 means that 1/10,000 of the light hitting the film is being transmitted through the black background. Films with values below 3.0 run the risk of burn through during screen exposure, which can change the halftone dot size and tonal values.
The second instrument you need is a reflection densitometer, which measures the light reflected from the surface of the printed image. It also measures direct dot area, calculates absolute dot gain, and reports the Dmax of the ink you are printing. The reflection densitometer is filtered to give direct readings of yellow, magenta, cyan, and black ink. This helps you control the dot area and understand how the ink density affects dot appearance. (To learn more about densitometers, see "Densitometry: Your Guide to Print Quality," by Frank Basch, <I>Screen Printing</I>, Sept. 2000, page 18.)
You want each halftone dot to represent a tone between white and black. In other words, a 50% black dot should represent a visual gray. As simple as this sounds, many things can keep this from happening. That's why you need to measure the process.
Measurement begins by working backwards from the final print. This helps you get a mental picture of what is happening at each step. The following is a good generalization of a typical screen-printing image profile:
1. Final print: Dot loss may appear in highlight and quarter-tone areas; gain in mid tones, three-quarter tones, and shadows.
2. Screenmaking: Dot loss usually occurs in highlights, with less dot loss across the remaining tonal range. Loss is due to thread eclipsing and light undercutting during exposure.
3. Film output: Dots can exhibit loss or gain depending on a number of factors. Often, dots may appear consistent across most of the tonal range, but highlights drop and shadows plug.
4. Monitor: The image displayed on screen may appear as if it has experienced dot loss or gain. Factors that influence this virtual gain include gamma, white point, black point, and color temperature of the monitor, as well as ambient light levels in the design area.
5. Digital file: You may face loss or gain that is built into the original digital file, depending on how it was acquired or adjusted to compensate for its appearance on the computer monitor.
Compensating for dot gain or loss
In prepress, you must make sure that what you are doing is stable and predictable. If it is not, the next step will either cancel, or add, to the error. More importantly, you will have no confidence in the accuracy of the tonal reproduction at any later step in the process. It is difficult enough to get it right at the screenmaking and printing stages; if the other steps in the process are unpredictable, your chances of success are less than at a craps table in Vegas.
For the sake of this discussion, I will not spend any time discussing dot defects in the original digital file. Instead, I'll start with the monitor. First, make sure that no direct light falls on the monitor face. Working in subdued daylight is the best. Work areas with high levels of ambient light will cause the monitor to represent images inaccurately. Next, verify that the monitor's representation of white is pure, with an RGB value of 255, 255, 255, and that pure black gives a value of 0, 0, 0.
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.