From small trinkets found in cereal boxes to life-saving escape-route markings, applications for photoluminescent materials are everywhere. The ability of these products to absorb light and emit a steady glow in darkness makes them ideal for novelty as well as safety uses. And the flexibility of the screen-printing process means that graphic and garment printers alike are well positioned to take advantage of these materials and open the door to new business. However, before you begin adding photoluminescent graphics to your product mix, you need to understand how these materials perform, where you can use them, and in what forms they're available.

What is photoluminescence
The photoluminescent effect is achieved by using a non radioactive, nontoxic zinc sulfide pigment that absorbs daylight or artificial light (fluorescent, incandescent, mercury vapor, metal halide, etc.). The pigment stores the light photons as energy, which excites the zinc sulfide molecules and causes them to emit a greenish-yellow glow best observed in total darkness. The pigment will continue to glow until exhausting this energy, but it can be recharged repeatedly by re-exposure to light. Depending on the zinc sulfide concentration and exposure to environmental conditions, the material can be reused for years before its ability to recharge and emit light fails.

To become sufficiently charged, zinc sulfide pigment requires approximately 5-15 min of exposure to a light source that emits at least 30-50 lux. Natural sunlight and fluorescent tubes, which emit white light, work best to activate these materials. Under incandescent bulbs, which emit a "warmer", more yellow light at lower wavelengths, the pigment requires longer exposure times. Red light and yellow sodium-vapor light are not suitable for activating the pigment.

Another way to explain photoluminescence is to describe what it isn't. The following terms are often used mistakenly to describe a photoluminescent material:

Reflective This term refers to a material's ability to immediately return light waves from its surface. All materials reflect and absorb particular wavelengths of light - the color we "see" is determined only by the wavelengths that are reflected back to our eyes. So some external light must be present to reflect from surface. Examples of reflective products screen printers are likely to encounter include license plates and traffic signs. Photoluminescent materials, on the other hand, emit rather than reflect, light.

Fluorescent Materials that absorb light of a particular wavelength and immediately emit light at a different wavelength are called fluorescent. However, fluorescent materials also stop emitting light immediately after the incident light is no longer present. In a more contemporary definition, fluorescent is used to describe extremely bright colors that are only visible under external lighting. You'll often see such colors (e.g., green, orange, yellow) preceded with terms such as "day-glow" or "neon."

Self-luminous Frequently confused with photoluminescent materials, self-luminous products emit a glow in dark conditions without prior exposure to light. These materials, once commonly used to mark watch faces and other products require long-term light emission, contain radioactive elements as activators.

Phosphorescent This term implies a glow effect caused by the presence of phosphor. It is frequently used incorrectly in product literature for photoluminescent materials.

Novelty applications
For non-safety applications, including T-shirts, promotional items, children's stickers, and similar novelties, you can choose between premixed photoluminescent ink formulations or photoluminescent pigments that you can mix into a clear base. The pigments are available for water and solvent-based inks, including plastisols.

To achieve the best results when working with photoluminescent inks, you need to consider the following areas:

1. Pigment quality and base clarity
2. Pigment quantity
3. Color of printing surface
4. Ink-deposit thickness

Pigment quality and base clarity Ready-for-use inks for garment and graphics applications, as well as raw pigments for such ink systems, are available from many ink manufacturers and distributors. Check with your current supplier or refer to the Screen Printing Buyers' Guide for additional sources.

Whether you choose to create your own photoluminescent ink from pigments and base or buy the ink premixed, make sure the pigments are of the highest quality. Unfortunately, the only way to ensure pigment quality is to create sample prints and test the glow (see "Testing Photoluminescent Qualities")

Additionally, if you mix your own inks, make sure the base you use is optically clear as possible. The slightest presence of color pigments can severely reduce the glow effect of the final print.

Testing Photoluminescent Qualities Take several competitive products of the same dimensions into a room that can be completely darkened. Place the products on a table and space them 5-in. apart. Activate all markings with 5 min. exposure to fluorescent light. Turn off the lights. You will see all markings emit luminance right away. Stay in the dark for 10 minutes. During this time, some products may quickly turn grayish, indicating poor product quality. Others will show little or no reduction in luminance. Exit the room without turning on the lights. Leave the photoluminescent markings in the dark. Return after 1 hour (or longer, if your application requires extended glow duration). Do not turn on the lights. Initially, your light-adapted eyes may not see much in the dark. but as they adjust to the darkness, the photoluminescent products with the brightest luminance will be readily apparent. Other products may not be visible anymore, which is usually a sign of poor quality pigment and material construction.

EVACUATION PLANS Another ideal application for photoluminescent materials is evacuation plans. Note that text and graphic images printed on these materials with conventional inks appear as black foreground elements on the glowing sign face.

Pigment quantity When mixing your own inks, add about 30% photoluminescent pigment by volume to the base. And if you buy them ready for use, be aware that 5-10% is the likely pigment load. Higher pigment levels will overload the ink and decrease its printability. Lower levels may not provide sufficient glow strength or duration.

Substrate color and surface characteristics To maximize the glow effect, photoluminescent ink should only be printed on white surfaces. On dark surfaces, including black garments or posterboard, you first must print a white base coat or the photoluminescence may be drowned out by the substrate color.

As with any ink, test the adhesion of the photoluminescent ink on the stock you plan to print before you set up for a production run. Evaluate adhesion by waiting 24 hours, then performing standard washability tests (for garments) or a cross-hatch and tape test for prints on non-porous surfaces. Keep in mind that substrates such as polypropylene and polyethylene may require corona treatment to ensure good ink adhesion.

Ink-deposit thickness Another way to improve the brightness and duration of the glow your graphics provide is to print more than one layer of the photoluminescent ink. On average, an image screen printed with photoluminescent ink and fully charged under a light source will glow for approximately 20 min before it again requires exposure to light.

Because photoluminescent pigment particles are typically larger than ground color pigment particles, photoluminescent inks are generally printed with lower mesh counts ranging from 86-125 threads/in. By using these mesh counts in conjunction with high screen tension, moderate squeegee and floodbar speeds, and consistent squeegee and floodbar pressure, you can print a heavier ink deposit and reduce the number of strokes required to get a print that provides sufficient glow strength.

Do not subject the pigment to high shear forces or intense heat during printing. These conditions can damage the sensitive pigments and decrease their ability to store and release light energy. And because they are heat-sensitive, you'll either have to cure your prints slowly at the lowest temperature possible or use an ink formulation that can be air dried.

Safety Applications
Safety applications for photoluminescent materials generally involve markings that supplement emergency lighting, such as escape route signs, evacuation plans, and related markings. these applications often must incorporate luminance-certified materials that satisfy stringent safety codes and standards. Unlike photoluminescent garment and novelty prints, which have rather loose production and performance criteria, safety applications must provide maximum luminance for extended time periods.

While standards vary, some specify that a photoluminescent material should provide an acceptable luminance level in total darkness for longer than five hours (see "US and International Standards"). And customers may be even more demanding, requiring luminance for up to eight hours.

LOW-LOCATION PATH MARKERS Because of poor visibility caused by smoke from building fires, earthquake-prone California has adopted building code requirements that require luminous low -location markings along all emergency escape routes. As these photos show, such photoluminescent markings are clearly visible in darkness.

You might be wondering, "Why would a photoluminescent safety marking require eight-hour luminance? If you didn't escape a burning building within the first five minutes, you probably won't survive anyway." The fact is, emergencies can happen at any time, and regular lighting may have been turned off hours previously. Imagine you are a hotel guest and a fire has left your room without electricity at 4:00 a.m. If the evacuation plan on the inside of the door to your room is printed on a photoluminescent panel with long-luminance characteristics, you'll be able to review the escape route in darkness and exit the building quickly and safely.

Although emergency lighting is common in both public and private structures, it's not always reliable enough to safely guide people out of a building during a catastrophe. And photoluminescent safety markings, including evacuation plans and related signs, tape and paint, are becoming a popular second-line defense against total power outages in fires and similar disasters.

For example, during a disaster, power can be completely lost, forcing a facility to rely on backup generators that may also be damaged, or emergency light that can fail due to insufficient maintenance (e.g. , dead battery, burned-out bulb). With proper material selection, photoluminescent safety markings will glow visibly long after the light go out, helping lead people to safety during total blackout. And unlike the backup generators and lights, the markings are maintenance free.

Even when traditional backup systems don't fail, conditions may make them ineffective. Most building fires release dense smoke that rises toward the ceiling and fills interior areas. This smoke is toxic and individuals trapped in the building are forced to crawl beneath the smoke layer where it's easier to see and less hazardous to breath. However, emergency lighting and lighted exit signs are generally positioned high on walls and above doors. Consequently, smoke completely obscures the lights and signs, leaving interior areas pitch black and impossible to navigate.

To combat this problem, many states and communities now require low-location exit markers on doors and continuous low-location path markers along the entire escapee route. And increasingly, buildings such as New York's World Trade Center are turning to photoluminescent materials to provide these markings.

Material concerns
The main factor that determines luminance and duration is the pigment density of the material. As mentioned, screen-printable photoluminescent inks only contain about 30% pigment by volume, which provides insufficient glow strength for safety applications. Even if multiple layers of ink are printed, glow strength only improves marginally. For this reason, safety applications generally rely on photoluminescent substrates with dense pigment concentrations on which text and graphics are printed with conventional screen-printing inks. In darkness, characters or images appear as black elements on a glowing field of greenish-yellow and are easy to recognize.

Photoluminescent substrates are commonly produced from flexible vinyl and rigid PVC sheets. In come cases, the film is clear and incorporates photoluminescent pigment as a separate backing layer. In such cases, the pigment may have been added to an adhesive or another compound that was applied as a backing. it's important, however, that the pigments be dispersed evenly throughout the adhesive or backing compound; otherwise, the result could be an inconsistent and uneven glow from the final product.

Excellent glow effects are generally provided by clear films or rigid sheets to which the pigment was added during the manufacturing process. Materials that are composites of several layers that have been laminated together generally provide the best results.

Such composite materials might include three layers: a white backing film to reflect the glow from the photoluminescent layer and improve overall luminance, the photoluminescent film itself, and a clear film on top to protect the photoluminescent layer from abrasion and long-term exposure to UV light, which can reduce its light storing/emitting capabilities. In areas where the material may face simultaneous exposure to both direct sunlight and humidity - which will cause the pigment to turn gray and lose most of its photoluminescent qualities very rapidly - it's imperative that the material incorporate this UV-resistant coating.

Make sure you purchase photoluminescent substrates that are suitable for safety applications by carefully reading the manufacturer's product construction. Be critical if you hear a manufacturer claim that a product glows for long periods - the pure pigment itself may glow more than eight hours, but when it's incorporated into an ink or plastic, the glow effect can be significantly reduced.

Material suppliers should be able to provide evidence of luminance certification, a compliance requirement with several safety standards covering photoluminescence. Certification documents should represent the results of an independent test laboratory and state the intensity and duration of the light emitted by the photoluminescent material.

When screen printing on photoluminescent substrates, follow your ink manufacturer's guidelines for working on vinyl or PVC. Again, make sure to use inks that you can air dry or cure at low temperatures, or use a UV-curable formulation. Although photoluminescent materials can be damaged by UV wavelengths, the brief exposure they receive in a curing unit will have little noticeable effect on the material's glow characteristics.

If you use the materials to produce sign or evacuation plans that may require periodic updates or changes, print onto a clear film, such as Lexan or Mylar. Then place the film over an unprinted piece of the photoluminescent material and keep both pieces together using a frame. When your customer requires a change in the printed graphic, you can simply replace the clear film with a new image, rather than changing the more-expensive photoluminescent material.

The range of safety products you can offer with photoluminescence substrates not only includes exit signs, evacuation plans, and wall markings. These materials also can be diecut and embossed to produce products such as light switch covers, decals, safety tape, and even ADA-compliant signage.

Conclusion
Photoluminescent products can be a valuable addition to your product line. On the novelty side, photoluminescent inks can create new opportunities for your company with products such as Halloween garments and graphics, ad specialty products, and souvenirs. And photoluminescent substrates can open the door to new business in the architectural arena, allowing you to provide safety signage and related markings for facilities ranging from schools and hospitals to hotels and high-rises.

Whether your products are novelty or safety related, you'll need to take steps to make sure they provide the maximum luminance and duration possible. This includes selecting materials made with top-quality photoluminescent pigments, testing the materials for particular applications, and processing them according to manufacturer's guidelines. By following these steps, you'll earn a glowing reputation for your business.