Scanning photographs and generating halftones is a common task in the screen-printing art o department, but the results aren't always as good as you desire. It doesn't help any that scanner manufacturers tend to advertise the special features of their products, rather than the features that you actually need. The race is on to sell you scanners with resolutions of 2400 dots per inch (dpi), but it isn't likely that you will have any use for such high resolution. And you certainly won't enjoy paying for the disk space you'll need to store images scanned at this resolution.
The pursuit of higher resolution is unjustified for the vast majority of users. With scanned halftones, the only resolution you require will be determined by the halftone line count that you choose to print. If you will be printing a 65-line halftone screen, you need enough information in the scan to accurately generate the values needed to print the halftone.
The ideal scanning resolution for a screen-printing halftone is debatable, but the consensus is that you rarely need a resolution higher than twice the halftone line count you plan to print. Personally, I am convinced that the maximum beneficial scanning rate is 1.414 (the square root of two) times the line count of the halftone.
For example, if you want to print an 8 x 10-in. image at 65 lines from a same-size original, you will need a scanning resolution of 91-130 dpi, depending on where you stand on the overscanning controversy. If you want to get the 65-line screen from a 4 x 5-in. original, you will need twice that resolution in the scan. And if you want to try to fish this image out of a 2 x 2.5-in. original, you would need to scan at 364-520 dpi (or four times the resolution of the same-size original). However, we all know that most images that small aren't worthy of so much enlargement no matter how good your scanner is.
I assume that you already have a scanner, so I won't mention what you should look for in a new unit. But if you are thinking about adding a scanner or upgrading to a new model, you may want to read my Sep. 1994 column, "A Scanner Shopper's Survival Guide."
I've scanned thousands of halftones and some printed well while others didn't. But I've finally developed a reliable scanning procedure that is based loosely on work described in Real World Scanning & Halftones and Photoshop in Black and White. (I reviewed these books in my June 1994 column.)
To start the scanning process, you should evaluate your original image in much the same way that you would start a halftone project if you were using a traditional darkroom halftone. If the image isn't suited to screen printing, you'll either deliver a substandard job or get a better original to work from. Using an image-editing program such as Adobe Photoshop will give you more options for touching up the image, but such programs won't compensate for out-of-focus originals and can only go so far in dealing with bad exposures.
Next, you need to deal with the limits of your job, which may vary for different types of work. The real question is: What is the range of dots that you can actually print? This is no time to get "macho" and declare that you can print a 99% dot because there is very little likelihood that you can--most high-end litho printers can't do it. And you definitely aren't going to print a 1% dot either, particularly with evaporative solvent-based inks.
When your halftones exceed the range of the screen-printing process, two things happen. First, details in the shadow and highlight areas that may have been important parts of the image are lost, which can have a major impact on your satisfaction with the final product. And worse, where you gain or lose dots you get blotches or voids. In other words, colors print solid where your shadows were supposed to be and no ink at all prints where the highlights were supposed to be.
Assuming that you will be using Photoshop to manipulate the scanned image (and most people do), adjustments are made with the Image: Adjust: Levels controls. (Throughout this column, descriptions that use colons in this way refer to Photoshop menu items. In this case, the description means go to the "Image" menu, select the "Adjust" option, and finally select the "Levels" option.) With any scanned image loaded into Photoshop, go to the Levels dialog box (which can be accessed from the Photoshop menu as previously described or by using shortcut keys such as Control+L in Windows or Command+L on Macintosh systems).
Just below the "Auto" button in the Levels dialog box (Figure 1), are three eyedropper icons. The leftmost of these icons sets your "Black Target Level," and the rightmost sets the "White Target Level." Double click on each of these and set the CMYK values to match the maximum dot you can reliably reproduce. I'm assuming that you are working with grayscale images at this point, because if you were ready to run process color, you probably already have the target levels worked out. So, for the black target level you would probably set 0,0,0,85, (0% Cyan, 0% Magenta, 0% Yellow, and 85% black) and for the white target 0,0,0,15. Of course, you would replace the 85 and 15 with the percentages of your darkest and lightest reliable dot. These values are saved from session to session in Photoshop, so as long as you are working with scans to be printed under similar conditions, these values can be applied by using the "Auto" button.
If you have a scanner that is capable of producing "depths" of more than 8 bits per color, the software that came with the scanner is likely to have options to adjust the scanner to map all of the possible information in the image to a range that you set. For details on your scanner, consult your owner's manual. What you are looking for is a way to adjust the scanner so that all 256 shades of gray fall within the scan. Most desktop scanners can't be adjusted in this way, but high-end models, such as the Microtek Scanmaker III, do this very well.
Next, go into the File: Preferences: General dialog box in Photoshop and make sure that the Interpolation option is set to "bicubic." You'll get faster results with bilinear, but the tone transitions will not be as smooth. And you'll get even faster results with "nearest neighbor," but the results will be useless. Also go into the File: Preferences: Units dialog box and make sure that anything other than Pixels is selected.
With Photoshop set up to compress your scan into a screen-printable range, and possibly your scanning software configured to assist, you plan the actual scan. To get the best possible scan, you base the process on the size of the original, the size of the final print, the halftone line count, and your choice of overscan ratios. The formula is: scanner dpi = final size/original size x halftone line count x overscan ratio.
After starting your scanning software, you set the scanning dpi (again, consult the manual--this can be confusing) and ask for a preview. Most software supports a balancing feature that roughly compensates for the exposure of the original, and you should use it. If the auto-ranging feature supports custom black-and-white point settings, make sure the values are set and the option is turned on. Scan the image.
If your scanning software is of the standalone variety, save the scan and load it into Photoshop. If a Photoshop plug-in drives your scanner, now is the time to save the scan to disk for the first time.
I always end up with excess image area around the actual scan, so at this point I crop (possibly more than once) to make the file size as small as possible. There is no sense working on a 1.2-MB file if you are only going to use 800 KB. A smaller file allows Photoshop to work faster. Also, by removing the white or black borders, you avoid confusing the software in subsequent steps. Whatever retouching you intend to do to get rid of dust or scratches should also be done now.
The real key to successful halftone printing begins here. Go to the Levels dialog box, and look at the "histogram" (shown in Figure 1), which charts the gray values between black and white. Look at the bottom of the histogram to make sure that all of the values are represented by at least a few points. If the histogram has breaks in it, cancel the Levels dialog box immediately. Breaks mean that your scanner has mapped fewer than 256 gray tones into the scan and many operations that you might perform, especially level adjustments, will posterize the image.
If your scanner didn't map all the gray tones, go to the Image: Image Size dialog box and with the "File Size" option deselected (no x in the box), lower the resolution by one. For example, if you need to scan at 15O dpi, change the resolution to 149. This will force Photoshop to resample the image and will fill out the tonal range smoothly. (If you left your Interpolation setting at "nearest neighbor," you will damage the image slightly and still not fill in the histogram. And if you left units at "pixels," you will accomplish nothing at all.)
Back in the Levels dialog box, you can now press the Auto button to map the scan into your production range. This makes the image look somewhat "flat" on screen, but it can still be used for printing. Depending on your screen, stencil, substrate, and ink, you probably will need to compensate for "dot gain." This is done by entering a "gamma" value in the middle of the three numerical boxes at the top of the Levels dialog box. A value of 1.3 might be appropriate for your first test scan.
After making the Levels adjustment, most Photoshop users would apply a sharpening filter. The best of these is called Unsharp Mask, which can be accessed in the Filters: Sharpen: Unsharp Mask dialog box. This function looks for tone transitions in your image and boosts the contrast slightly at the image edges. My standard settings in this dialog box are Amount: 50%, Radius: 1.0, and Threshold: 3, and I apply the filter twice to most images. You will need to play with the Unsharpen Mask to develop your own standard and to train your eyes so that you can quickly adjust the values on different images for different presses and substrates.
If you follow these steps with a test image, you will find that it is a lot easier to do than to explain. But these steps are enough to get you off to a good start. Real Photoshop pros will follow a slightly different approach. Instead of using the auto-balancing and the gamma-setting features, they will generate test prints and use a densitometer to define custom curves for different work. Then they'll apply these curves instead of using the automatic approach. But this is either the subject of a future column or a good reason to pick up one of the books mentioned previously.
Should you buy your next computer from your local dealer? What can he offer? And will it be worth the price? Check out next month's "Computer Connections" to find out how to approach your next computer purchase.