Facing Up To The Challenge of Printing Halftones: the next step in your evolution as a screenprinter.
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Facing Up To The Challenge of Printing Halftones: the next step in your evolution as a screenprinter.

When newcomers to screenprinting first set eyes on a screenprinted photograph, the damage is done. That's the kind of screenprinting they want to do. The wise ones put off tackling the job, at least until they've got a bit of experience under their belt. But sometimes the jobs don't wait on you.

By Bill Stephens

In fact, the next customer through your door may be holding artwork for a sign that includes a photograph. In this article and the ones that follow in this series, we’re going to look at the process and the rules that govern photographs. We'll also cover some of the pitfalls and point out safe paths around them. We'll begin by taking a look at what's involved in printing a simple black-and-white photograph.

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  • Make no mistake, printing photographic images places exceptional demands both on the skills of the printer and his or her equipment. If you find holding detail in fine line artwork a challenge, you may well be in over your head. Screenprinting a photograph means printing thousands upon thousands of tiny dots and printing them accurately. Unless virtually all of them print, the image that appears on your substrate won't bear much of a resemblance to the original photograph.

    Continuous Tone vs. Halftone
    Why do we have to print dots? Photographic prints, the kind that come from the photo lab, are called continuous tone images. A continuous tone image is so highly detailed it may be made up of hundreds of subtle, almost indistinguishable, shades. Even a black-and-white photograph may contain as many as 256 tints of gray.

    When you print something, however, you can only print one shade of ink at a time. (And this isn't just screenprinting; the same thing applies to all commercial printing processes.) Your only real choice is to apply ink to the substrate or leave the substrate blank. That's it. To apply a second color or even a tint of gray, you need to mix another batch of ink, make up another stencil, and put the substrate through the press a second time. And when you do that, you have to be very careful about registration, which means the two imprints have to line up exactly. To print a continuous tone image you would have to do that over two hundred times.

    But what if we could print something that almost identical to a continuous tone image and do it with one screen, one trip through the press, and one pull of the squeegee?

    Introducing the Halftone
    A halftone is a printable version of a photograph. Halftones convert the various tints or shades that make up the original image into very small dots. To get a close-up look at how the process works, take a magnifying glass and look closely at any of the photographs in a magazine or a newspaper. They're halftones.

    Under magnification the printed photograph breaks down into a bewildering conglomeration of dots. If you could reduce the magnification slightly, you would begin to see the bewildering field of dots transform itself into patterns that appear to mimic the light and dark areas of the photograph. If you continue to reduce the magnification, at some point you will no longer see even groupings of dots. What you will see is the original photograph staring back at you, all of its hundreds of subtlety blended tints apparently reproduced in perfect detail.

    It all depends on distance
    The truth is a halftone is nothing more than a kind of magic trick. Like other illusions, it takes advantage of a limitation of human vision, in this case our limited power to resolve images. Magnification equals distance. View any halftone from too close, and the tints begin to break down into printed dots. From the right distance, we don't see dots; we see only tints or shades. This is one of the most important things to remember about halftones: they depend on distance. The complex tints of gray that appeared in the black-and-white photograph have been transformed into printed dots of different sizes. In a traditional halftone, the only thing we control is the actual size of the dots. But this gives us a palette of such variety and range that we can reproduce most of the shades and delicate transitions of any continuous tone image.

    Clarke Systems Architectural Signage Systems Wayfinding ADA

    Since the halftone process was invented near the end of the 19th century, it has been nothing short of a runaway success. In fact, it is so successful that we now take it for granted. Halftones are all around us, staring down from every billboard, peering up at us from every magazine and newspaper.

    All of them, however, have one thing in common. They depend on distance. When making a halftone, its distance from the viewer must always be taken into account. It must be far enough away that the printed dots are seen as a color or tint.

    Measuring Halftones
    Viewing distance determines size of the dots used in the halftone. Halftones in a glossy magazine will be made up of very tiny dots, because the average reader will see them from 12 to 18 inches away. A billboard, on the other hand, will never be seen from closer than several yards away so the dots used in billboard halftones will be much larger. So, now we have different halftones used for different purposes.

    In a traditional halftone, which is the kind we are talking about here, the dots are arranged in ordered rows or lines. A single line of dots may contain dots of many different sizes, but the lines themselves will be the same size throughout the halftone.

    Because of this we can make a count of these lines, when we want to tell one halftone from another. Line counts are recorded as lines per inch or LPI (lines per centimeter or LPC in countries using the metric system). We also use the terms frequency or screen ruling, but all of them refer to the same thing: how many lines of dots appear in the halftone.

    As the number of lines per inch goes up, the smaller the dots within the line become. So, the higher the line count of the halftone, the closer it can be placed to the viewer without the image beginning to break up into dots

    To understand something about present day line counts, we need to make a brief detour in time back to the origins of the halftone process. Until about twenty years ago halftones were made on a graphic arts camera or process camera. The photograph was re-photographed through a transparent screen called a contact screen or levy screen. The screen was inscribed with a grid pattern that broke the image down into halftone dots.

    Different screens were placed in the camera depending on the requirements for the halftone. For example, a screen used to shoot a halftone for a magazine would be marked or ruled with a much finer grid pattern than the screen used to shoot a halftone for a T-shirt. Darkroom personnel started using screen ruling as a handy way of telling one contact screen from another.

    The screen used on a halftone for a glossy magazine might have a screen ruling of 133 lines per inch, or more. The screen used to make a halftone for a screenprinted sign might have a screen ruling of only 65 lines per inch or less.

    Today, most halftones are generated on a computer. The process camera has given way to a computer scanner, and the transparent ruled screen has been replaced by graphic arts software like Adobe PhotoShop or Corel Photopaint. However, one thing still survives from the camera era: we still retain the term screen rulings.

    Making Halftones today
    Think of a photograph, or any continuous tone image, as a collection of information, the seamless transition between tints as an extremely complex set of variables. This is information in analog form, a piece of music recorded on a vinyl record. When we make a halftone from a photograph we're converting analog information into digital form. It's the same thing that happens when music from a vinyl record is burned onto a CD.

    Clarke Systems Architectural Signage Systems Wayfinding ADA

    When we think of processing digital information, we automatically think of computers. Manipulating the vast amounts of data represented by a digitized photograph barely challenges its abilities. Little wonder then that in today's world, the computer all but holds a monopoly on the making of halftones. Yet, the conversion process itself hasn't really changed much since the days when halftones were made exclusively with cameras.

    To convert a photograph into a halftone we divide it into a finely ruled grid of invisible uniform squares. Think of them as containers. Some are empty; these form the white areas of the image. Every other square holds a dot placed in its exact center. The dots range in size from some so small they are nearly invisible to some so large they completely fill the square.

    Playing the percentages
    The amount of space a dot fills within its square varies according to the tint, or shade of gray, the dot represents. The actual size of the square is determined by the line count of the halftone, so the physical size of the dots representing the same tint will change from halftone to halftone.

    The squares may shrink or grow larger according to the line count of the halftone, and the dots within those squares will do the same. However, one thing will always remain the same: From halftone to halftone, dots representing a certain tint will always occupy the same percentage of the space available within the square. This stays the same for every screen ruling, every line count. A white square is 0%. The dot in a black square will always fill 100% of the space. A dot representing a medium gray will always be found somewhere in the vicinity of the 50% mark.

    By speaking of tints in terms of percentages, we can speak of tints without having to constantly refer to line counts. Imagine two halftones of the same photograph lying side by side. The first halftone has a screen ruling of 45 LPI (lines per inch); the second uses a screen ruling of 150 LPI. Yet the 80% dots in both will be found in the same place in the image, somewhere in the deep shadows.

    A 20% dot will fill always fill 20 percent of its square, whether the line count of the halftone is 50 LPI or 150 LPI. When you get into halftones, get used to thinking of tints in terms of percentages.

    Limitations of screenprinting
    In a traditional halftone, which is what we have been discussing, the tints that make up the image depend entirely on the size of the dots in the printed image. You can pretty well judge the ability of a printing process to print halftones by how closely in size the dots on the substrate match the dots in the halftone artwork. Screenprinters compare their prints to the positive they used to burn the stencil.

    The ability to produce a print that closely resembles a photographic print depends on the ability of the printer to print those dots accurately. If the dots on the printed page vary in size from the dots in the halftone, the tints will definitely not be the same. If the printed dots are smaller, the tint will be lightened. If the dots print larger, the tints will become darker.

    Unfortunately, for screenprinters, screen inks tend to spread out when they come into contact with substrate. This is one of the reasons why screenprinting really doesn't do a very good job printing halftones, at least compared to other printing methods. When dots increase in size they cause the tints in the image to grow darker. This is known as dot gain. In the print it's most noticeable in the shadows, where the dots tend to be larger. Dots of 85% or higher tend to swell to solid black, erasing detail.

    Dot gain is just one of the problems screenprinters encounter when printing halftones. Next time, we will look at some of the others, and we'll also consider what takes to make a good halftone positive.

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