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Bringing Backlit Graphics to the Forefront

(November 2001) posted on Mon Nov 12, 2001

With pressure-sensitive films


By Carl Sittard

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If you feel you are in the dark about self-illuminated signage, you are not alone. While backlit display graphics have been around for years, material limitations and production challenges tended to keep all but the most determined and disciplined screen printers from entering this market. Today, however, the boom in promotional graphics has been accompanied by a sharp increase in demand for self-illuminated displays, and the number of screen shops that are beginning to produce backlit prints is on the rise.

Among the factors fueling this growing demand for self-illuminated graphics, an important development has been the recent introduction of backlightable pressure-sensitive films, particularly new calendered vinyls. Used in conjunction with modern lighting technology, these films are proving ideal for backlit displays in all the same locations where daylight displays are traditionally found. But the backlit versions provide twice the bang for the same signage buck by keeping graphics continuously visible, day or night.

Suitable applications for backlit pressure-sensitive films not only include bus kiosks, wall murals, and similar two-dimensional signs, but also complex and curved backlit areas. Additionally, the flexibility of pressure-sensitive vinyl opens the door to a broad range of applications, including taxi-top displays, thin bus-side displays, cylindrical graphics, and self-illuminated displays that can be used on almost any surface.

For screen printers, the ability to work on familiar vinyl films is making it easier than ever to achieve successful results with backlit applications. Those who are experienced with these substrates find it easier to control the ink and print characteristics that lead to attractive, highly visible, and color-accurate promotions under both daylit and backlit conditions.

An enlightening history

The phenomenon of transit advertising seems so omnipresent today that it may be hard to remember a time when buses, subways, and taxis were not marketing mediums. Not so long ago, rigid-plastic printed panels and glued-on paper graphics were the only options for most vehicle advertising and outdoor-display applications. But improved vinyl graphic films and better adhesive systems have displaced most of these materials and caused new applications to emerge, such as full bus wraps and similar large-format graphics. So it's really no surprise that these materials are finding their way to nighttime uses, too.

That said, the early days of backlit signage with film substrates were less than ideal. While the translucent nature of many films made them seem well suited for illuminated graphics, few films then available met the flexibility and durability requirements that these graphics demand.

Calendered vinyl films, which had worked so well for daytime advertisements, were the first option printers turned to for nighttime displays. But the shops working with these materials discovered that when it came to calendered vinyl, backlit and frontlit graphics are as different as night and day.

At the time, calendered vinyl did not possess the uniformity required to achieve even illumination across an entire graphic display. The calendered film was perfectly acceptable for daytime graphics because the lighting source was in front of the graphic and the images were viewed strictly with reflected light. When the light source was placed behind the substrate however, the light transmitted through the material would highlight inconsistencies in the substrate's thickness and opacity. In completed graphics, these opacity variances would manifest themselves as "hot spots," allowing more light to pass through and creating undesirable color variances in the images. Advertisers were not pleased, preferring a more uniform look.

Printers attempted to compensate by strategically increasing the ink deposit in areas where hot spots might appear. However, this approach was both time-consuming and costly, which led many shops to switch to more uniform--but more expensive--cast vinyl. While this solution did help overcome hotspots and color inconsistencies, the added expense limited its appeal to print buyers.

A brighter future

Calendered-vinyl manufacturers recognized the issue and began examining ways to ensure consistent translucency in their films. By adjusting raw materials and retooling to optimize the manufacturing process, calendered-vinyl producers have succeeded in developing new cost-effective calendered-vinyl substrates that provide equal light distribution in backlit applications. For printers, this uniformity means less trial and error on press, as well as less waste.

At the same time that calendered-vinyl manufacturers were wrestling with light-diffusion issues, lighting engineers were developing new ways to illuminate the graphics. Using traditional fluorescent and incandescent light boxes as their starting point, these engineers have developed a variety of sleeker, more versatile lighting techniques that include edge lighting, reflective lighting, and even electroluminescent (EL) technology. EL lighting has proven especially well matched for use with translucent vinyl films and is opening the door to many new and unique applications.

Relying on neither the filaments of light bulbs nor gas-filled flourescent tubes, an EL lighting fixture is composed of a phosphorous-coated metal sheet that glows when an electrical charge passes through it. The result is an ultra-thin, uniform light source. EL signage is very flexible, allowing it to be bent around curves. It eliminates the problem of burnt-out bulbs, helping to reduce overall costs and allowing for easier maintenance.

These concurrent developments have opened up a new world of possibilities for promotional-graphic designers. Light boxes that may once have been too obtrusive because of their size can now fit comfortably in significantly less space. Locations that might have been passed over for backlit graphics, such as large flat walls and vehicle sides, are now open territory. Similarly, curved surfaces can now be regarded as areas of backlit opportunity.

Working with the best and the brightest

If you are new to producing self-illuminated graphics with pressure-sensitive vinyls, selecting the right material, ink, and lighting technology can seem daunting. So it's useful to gain some familiarity with the components available today in each of these categories. Keep in mind that all three elements of the display must work together as a system. Any weak link and the system fails. That is why it is critical to work closely with suppliers who will answer your questions, are willing to listen to your specific needs, and can provide the products and information that allow you to create successful backlit graphics.

Media Not all calendered vinyls are created equal. You must verify from your pressure-sensitive film supplier that you are selecting a product designed for backlit applications. The material must offer uniform light diffusion and be compatible with the ink system that you plan to use to print the graphic. You may also want to query your supplier about the substrate's shelf life. Over time, plasticizers in vinyl tend to migrate toward its surface, which can affect the material's printability, particularly ink adhesion. Some vinyls, however, are engineered to provide extended shelf and printing life.

You must also match your vinyl to the particular application. A kiosk graphic in a subway station will not require the same level of cold-crack resistance or UV-light protection as a taxi-top graphic. Likewise, a nighttime bus-panel graphic will likely be flat, making conformability less of an issue than if the graphic were to be applied to a curved surface.

Ink Because new calendered vinyls deliver uniform light diffusion, you can focus more attention on printing the best possible image. When it comes to backlit signage, achieving correct and consistent color density is critical. This density relates to the thickness of the ink deposit, as well as its opacity (pigment concentration). Print technologies that do not provide an adequate deposit or pigment concentration can lead to graphics that appear faded or washed out when backlit. Screen printing is ideal for laying down a heavy ink layer and producing vividly colorful rear-illuminated graphics. Digital-printing devices may also be suitable, but printed color density varies among printer models and ink systems. Each job you intend to produce on an inkjet or similar electronic printer should be evaluated on a case-by-case basis.

Lighting You can select the right ink and films and print a colorful, high-quality image, but still be disappointed with the final illuminated display if you didn't consider the light source over which the image will be displayed. An incandescent light will not produce the same effects as a fluorescent or EL light. You should have samples of each of these technologies on hand to accurately proof prints in the same lighting conditions they will experience in actual use. The distance between the graphic and the light source will also impact its visibility when backlit, so it's important to know this distance and proof the image under similar conditions.

All backlit signage is not created equal. While outdoor applications abound, substantial indoor possibilities exist as well. In the following section, we'll review several of the most popular uses for backlit graphics produced on pressure-sensitive, calendered-vinyl film.

Taxi tops Taxi tops may be the granddaddy of outdoor backlit promotions. And although taxi-top signage has become almost ubiquitous in major metropolitan areas, this market continues to experience healthy growth.

In this application for promotional displays, ease of installation and removability is a vital component that the graphic must exhibit, because the advertising message constantly changes. The graphic must be quickly and cleanly applicable and removable in all possible weather conditions. As with most backlit signage, application takes place in the field. Understanding that the specific conditions may be less than ideal, the calendered vinyl must be backed with an adhesive that is reworkable, aggressive, and easy to remove at the end of the promotion. And while it is affixed to the taxi-top display, it must remain stable and cannot flag, dart, or otherwise become separated from the surface.

Bus panels Located on all sides of modern buses, panels were once dominated by glue-applied paper placards. Pressure-sensitive vinyls have experienced substantial growth in this field, due in part to their flexibility, ability to be directly applied, and ease of installation.

With nearly 120,000 buses on the streets in the United States alone, the potential for backlit bus signage is enormous, and the market is growing quickly. Pressure-sensitive calendered vinyl is meeting the demands of this market with many of the same characteristics it brings to backlit taxi-top signage: durability, an adhesive system that allows it to be easily applied and removed for promotions that frequently change, and uniform light diffusion for high-quality, highly visible graphics.

Airport/subway kiosks and bus shelters Most metropolitan airports, subway stations, and bus shelters feature extensive advertising in order to generate additional revenues. Wall murals and stand-alone displays are everywhere, but the ones that stand out most are the ones that are lit from within. Pressure-sensitive adhesives enable graphics to be applied without the need for mounting hardware associated with most rigid and non-adhesive media. When it's time to change the advertisement, one graphic can be peeled away and the next applied with minimum effort.

Wherever imagination leads Pressure-sensitive backlit graphics aren't just for transit and terminal advertising. The same advantages they bring to these applications can also be realized everywhere else that traditional graphics are currently used. With EL technology and other lighting innovations, flexible vinyl films can be used to produce back-lit counter-top displays, floor graphics, and more. The possibilities are virtually endless.

The pressure-sensitive package

Flexibility, durability, easy installation and removal, even light distribution, and attractive cost all have made pressure-sensitive vinyl a great contender for almost any backlit application. But to develop the highest-quality backlit displays, you have to consider more than just the nature of the substrate. You also have to consider the characteristics of the adhesive it employs, the type of release liner the material offers, and the possible overlaminating materials you'll need to enhance the durability and longevity of the display.

Film As noted previously, calendered vinyl has become the film of choice for backlit graphics. Beyond the fact that it is considerably less expensive than cast vinyl, it is also very flexible. It will conform well to a variety of surface contours, and minor surface irregularities and simple curves are no problem for this substrate.

At the same time, calendered vinyl is extremely durable. This is critical in the world of transit advertising where buses, cabs, and taxis are exposed to a wide range of harsh environmental conditions. Calendered vinyl will withstand the frigid cold of a New York winter or the blazing heat of a Florida summer.

At the end of the promotion, calendered vinyl removes cleanly, without cracking or disintegrating. This is particularly important during the winter months. Some new calendered vinyls are specially engineered to resist cold crack, allowing graphics to be removed intact. This leads to quicker removal/installation and reduces the amount of time that the bus, cab, taxi, kiosk, or display system needs to be out of service.

Calendered vinyl is also a very printable film. There is no need to use a topcoat to ensure excellent ink adhesion. Screen printers who also work with digital imaging will be pleased to learn that many untreated pressure-sensitive calendered vinyls are compatible with solvent-based wide-format inkjets and well suited to low-volume and quick-turnaround process-color jobs.

Overlaminate Outdoor graphics must have excellent resistance to harsh environmental conditions. Whether the stress comes from the elements (rain, snow, slush, sunlight, etc.) or from man (smog, soot, road tar, gasoline, etc.), advertisers expect their graphic to look as good the day it is removed as the day it was applied. Flexible overlaminates made from a variety of different plastics have been developed with these needs in mind and offer an enhanced degree of protection. Available in a broad range of thicknesses, overlaminates can extend the life of printed graphics and provide the right balance of clarity, texture, and durability.

Certain digital-imaging processes, such as electrostatic transfer, require an overlaminate to protect the graphic. Conventional printing methods may not necessarily require overlaminates, but most outdoor applications will benefit from their use. The overlaminate may also protect more than the graphic alone. For example, pressure-sensitive film manufacturers are working to develop overlaminates specifically designed to protect flexible EL panels.

Adhesive The adhesive system stands behind the success of every substrate and overlaminating film. Printers can choose materials with a broad selection of solvent- and water-based adhesives that can be custom-tailored to match virtually any surface challenge. From the panels of EL signage to the low-surface-energy plastic surface of an airport kiosk, pressure-sensitive adhesives can deliver the necessary adhesive bond. Similarly, adhesives allow the overlaminate to adhere cleanly to the substrate. Like the other components, adhesives must hold up under the most grueling of environmental conditions.

At the same time, pressure-sensitive film manufacturers understand that daytime and nighttime advertising opportunities are short-term applications. The adhesive must not only allow for repositioning and reworking during the application process, it must adhere without letting the substrate lift until the promotion ends and the graphic is removed. Finally, the adhesive must be quickly and cleanly removable, leaving no adhesive residue behind. Your supplier can point you to the adhesive system that best meets the specific requirements of your application.

Release liner The release liner is instrumental during printing and finishing processes. It carries the substrate and maintains good lay-flat characteristics in the material, which promotes better registration accuracy on press and greater efficiency in material handling. It also provides excellent stability to the final product as it is transported and handled before being applied.

Backside-printable liners are also available. Given that most daytime and nighttime signage is applied in the field, this allows producers the option of printing installation instructions or tips directly on the liner.

Seeing the light

The market for backlit signage is still in its infancy, but it is growing up quickly. The printers who will make the most of emerging opportunities are the ones who are ready to explore the materials now available and work closely with suppliers to develop the best graphic products. By correctly matching application requirements with the right backlit-signage components, printers can expect to see a very bright future...even at night.


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