WHEN THE LIGHTS GO OUT …. Will your photoluminescent way-finding sign perform in a darkness emergency? By Marina Batzke, Screen Graphics, September/October 1997
| You’re
working late, hear a loud noise … and the light go out. Full
power failure: No window nearby. Phone’s out. Glass splinters.
Something (or someone) falls. You hear a scream. Suddenly the familiar
surroundings of your print shop have turned hostile.
During a total-darkness emergency, brightly glowing safety and way-finding markings have become a reliable aid to building occupants, helping them avoid panic and injury and – just as important – find a safe path out. While many screen printers serve "Safety" markets, misunderstanding persist about the nature of true photoluminescent safety products. Open a catalog from any signage, marking and identification products supplier, and you’ll inevitably see "glow-in-the-dark" signs. They may also be accompanied by performance claims, such as "eight-hour luminance," and by promises that product glow will last "all through the night." Unfortunately, you might be surprised at the remarkable variation in the actual performance of these products, if they were put to a practical test (see "Evaluating Photoluminescent Properties: A Quick Test"). |
Medium or message: Screen printers can render text or symbols dark on a light background (in lighted conditions or in darkness) or light letters on a dark background, simply by printing a positive or negative (reverse) opaque graphic image, respectively. Glow-in-the-dark inks, however, should be avoided. |
Not all photoluminescent products provide bright-enough luminance and/or luminance for sufficient time duration to qualify for use in true Safety Product applications. Indeed, among those exhibiting inadequate performance, we find many that were neither designed for nor intended for use in darkness emergencies.
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A Quick Test To determine relative acceptability/unacceptability of "safety-grade" photoluminescent products, perform the following simple test:
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Screen printers hoping for success in the safety-products field must understand photoluminescence and understand what factors contribute to varying levels of luminance from photoluminescent materials. This knowledge makes it possible to select the proper materials to avoid product failure in situations where it takes little imagination to visualize the catastrophic results such failure could magnify.
Light
on photoluminescence
Zinc-sulfide pigments are the most commonly used source or generators
of photoluminescence. They produce the effect by absorbing energy while
exposed to daylight (sunlight) and/or light from four types of artificial
lighting systems, specifically, fluorescent, incandescent, mercury-vapor
or metal-halide. Red light (such as that from safelights used in photographic
darkrooms) and yellow sodium-vapor light are not suitable for photoluminescent
pigment activation. (A new pigment type, however, is currently approaching
the market. These earth-borne pigments are in their initial stages of
exploration and development. They require a longer activation time, but
also feature a bright, long-lasting luminance. Contact your manufacturer
for the latest state-of-development information.)
| The
pigment particles store the received light energy. (These inorganic
materials are non-toxic and should not be confused with radioactive
light generators. See "Yes and No in Terms of
Glow,".) The particles than release that energy (much more
slowly than they absorb it) in wavelengths that include the yellow
portion of the visible spectrum. And this low-level glow can be
seen in total darkness.
Zinc-sulfide pigments may be re-activated again and again, for years, simply by exposing them to an activating light source. Sufficient activation requires minimum 5-15 minutes of exposure to 3-5 foot candles (30-50 lux) light. Fluorescent light (a cool, white light source) works best, while incandescent light (the soft, warm light from the standard bulb for a typical table lamp) has a lower wavelength, requiring a longer activation time. |
ADA signs: These safety glow signs also comply with Americans with Disabilities Act (ADA) requirements for the visually impaired. Raised lettering is applied over the photoluminescent rigid-plastic sheet, and Braille markings are made from imbedded glass beads. |
Safety
vs. novelty applications
Photoluminescent pigments are used in powder form, are added to screen-printing
inks, or are incorporated into finished materials, such a sign blanks,
paints or plastics. In either case, brightness and duration of the luminous
effect vary between manufacturers and – especially in the case of
screen inks – may also vary according to the proportion of pigment
to non-light active components of the material or ink. As we’ll see,
additional factors, such as method of manufacture or application and material
thickness, can affect both brightness and duration. Consequently, the
signmaker or screen printer cannot assume that every photoluminescent
product will prove effective in a darkness emergency. Glow-in-the-dark
inks, for example, are typically marketed for use on T-shirts, decals
and holiday items (such as Halloween decorations) and are not manufactured
with a life-safety function in mind. These and other novelty products
do not require either a guaranteed minimum brightness or minimum luminance
time. By contrast, photoluminescent safety products are intended as a
lifesaving supplement to mandatory building emergency lighting. Here,
brightness and duration are critical. Safety products are sold with the
expectation that they will be used in darkness emergencies and therefore
are often accompanied by brightness/duration claims.
| Are
dealers documented? A supplier’s claims, however, should never be accepted without proper documentation. Screen printers and signmakers should require suppliers to provide luminance-certified materials. Your supplier should be able to provide a Luminance Certificate. This document indicates that an approved testing laboratory has tested the materials and is issued by the laboratory for use by the supplier. In addition to Luminance Certificates and Type Approvals, the screen printer or signmaker should also conduct the quick test described on this page, and expect that customers may wish to conduct on-site luminance testing at the place of installation (in an office, plant, hospital, or on board a cruise ship) using samples you supply, prior to production. This is the customer’s guarantee that photoluminescent signage and markings will fulfill the minimum acceptable level of luminance under the lighting conditions present in their facilities. If markings fail the on-site tests due to insufficient luminance, they have to be removed. That can become a costly problem for those who risk use of a low-grade material for safety applications. |
Double duty, multiple construction: Evacuation plans that will perform properly in both lighted and total-darkness-emergency conditions can be screen printed with opaque inks directly onto a photoluminescent substrate, or on a clear film (second-surface, for added graphic protection) backed with a rigid photoluminescent sheet. (Typically, the latter are sandwiched together in a wallmounted frame. |
Ink
vs.
substrate
Our own testing has revealed great variation among products specifically
identified as "Safety Products." Generally, the group of sign
blanks manufactured with photoluminescent properties provides a greater
brightness and duration of effect than glow-in-the-dark inks, as a class.
Within each group, there is also wide variation between products from
different manufacturers. The luminance comparison charts (below) and (later
on this page) illustrate the decline curves of photoluminescent
sign materials from four competing manufacturers. Sign A is made
from a photoluminescent, calendar-laminated vinyl backing. Sign B
from a cast photoluminescent film. C is a clear film, backed with
a photoluminescent compound mixture, while D features a photoluminescent
pigment incorporated into its white-adhesive backing.
 As the chart demonstrates, the darkness effect of each begins at its peak (some higher, some lower), and luminance drops quite sharply during the first half-hour, then slowly declines over the hours that follow. All four suppliers list their materials as "visible for eight hours in the dark." However, signs B, C and D exhibit relatively lower luminance right from the point activation ceases. And despite the duration claims, B emits light for about two hours, while C and D have reached low luminance levels after just 60 minutes. None of the three perform as promised in the sales literature. |
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In similar fashion, the "glow in the dark" screen inks also vary in performance, from manufacturer to manufacturer. But the potential range of variance in brightness and luminance duration is far wider and impossible for the manufacturer to predict because both are influenced by four factors. Only the first one or two are under the ink manufacturers’ control:
- The quality of the pigment used (this varies, manufacturer to manufacturer).
- The quantity of pigment that either the ink manufacturer or the screen printer adds to the ink base (figured as a percentage of the total weight). This determination is made in-house, by the screen printer, if he buys the pigment separately.
- The color of the substrate surface.
- The thickness of the printed ink film (determined by mesh/stencil selection and other pre-press and press settings selected by the individual screen printer).
These four levels of uncertainty aside, our tests on screen inks indicate that even in a best-case scenario, screen inks are not suitable for safety applications. When we test-printed the highest-quality pigment powder available, mixed to maximum practical proportions (pigment-to-ink) and printed to a depth of three layers (maximum printable wet-ink deposits, wet-over-dry) on a white surface, the total-darkness luminance levels were comparable to sign material D. Moreover, the luminance of a single layer of photoluminescent ink does not last more than an hour in total darkness. In addition, the substrate affects the luminance. If the photoluminescent ink film has a dark (black) backing, the yellowish luminance would be considerably reduced. A bright white backing increases the luminance, acting as a reflector.
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Safety Product Q&A Q: Why do we need photoluminescent products when building codes require electrified systems, equipped with battery backup?* A: First, and most obviously, because natural disasters, such as hurricanes, thunderstorms, earthquakes (or such man-made disasters as terrorist bombings and accidental power-line cuts by construction crews) often eliminate electrified emergency lighting from the outset. Even in daytime, we can make no assumptions about the reliability of available light (such as that which might filter in through windows and door openings). Therefore, photoluminescent safety markings (signs, tape, paint or evacuation plans) are an important backup to mandatory emergency lighting. Second, and easily overlooked, is the fact that emergency lighting equipped with back-up batteries require regular maintenance checks of batteries and light bulbs. Since photoluminescent safety products are maintenance-free, the potential for failure attributable to human neglect is eliminated. The final - and arguably most compelling -justification? Code-required emergency lighting could be called "high -location lighting." That is , "EXIT" signs are typically suspended from the ceiling or mounted over doorways. In cases of fire, however, smoke can accumulate quickly and -- lighter than air - tends to rise. Building occupants are advised to get down on hands and knees and crawl beneath the smoke toward exits. Since high-location signage is now no longer visible, photoluminescent safety markings are installed as low-location lighting. (Earthquake prone California, for instance, requires low-location "EXIT" path markers in the bottom area of an exit door and continuous low-location path markings along the entire escape route.) Q: Why eight hours duration?* A. Emergencies don't come at our convenience. The 1994 Northridge quake struck Los Angeles at 4:17 am. When all the lights failed at one downtown hotel, the escape-route plan on each hotel guest-room door was still visible to darkness-adapted eyes six hours later, long enough to make them visible until daylight, even if the emergency had occurred much earlier. Q: Okay, but if I don't get out in the first few minutes, I'm probably dead or immobilized by an injury. While I'm waiting to be rescued, six-hour luminance won't be much help, will it?* A:
In a small building, that may be true. But what about an emergency
in a large hotel or office building? It took many occupants
of New York City's World Trade Center for to six hours to escape
from the darkened high-rise towers after the much-publicized
terrorist attack in February 1993. Many had to find their way
down smoke filled stairwells without any guiding lights. All
the code-required high location emergency lights were in place,
but the explosion knocked out both main and back-up power supplies.
Post -attack evaluations showed that photoluminescent markings
would have facilitated escape tremendously. |
Production
implications
If
the product application concerns true safety signage – such as signs
that read "NOT AN EXIT" or "FIRE EXTINGUISHER," or
any type of emergency way-finding signage, such as evacuation plan, or
ADA signage – These markings must meet standards for use in total
darkness (for a review of existing and proposed standards see "U.S.
and International Luminance Standards").
Given the above, screen-printed photoluminescent inks, even in multi-layered applications, are not suitable for safety applications. Brightness is simply too low, and duration is inadequate. Likewise, all but one sign-blank material failed to generate an acceptable level of brightness and sufficient luminance duration, for the following reasons:
- The brightness of sign D was too low owing to the fact that the white coloration of the adhesive partially obscures output of the photoluminescent pigment it contains. (In addition, it is practically impossible to mix the pigment powder into the adhesive evenly, resulting in visually uneven light output across the substrate surface.)
- Sign C, a clear film backed with a photoluminescent coating, and B, the cast photoluminescent flexible film, could not provide light for more than two hours. (the latter fails due to insufficient pigment load.)
Only sign a actually emits light for eight hours. The sign material features opaque (non-glowing) inks printed over a photoluminescent calendar-laminated substrate, consisting of three layers:
- White PVC backing (rigid or flexible)
- PVC film with incorporated photoluminescent pigment
- Clear, UV-blocking surface film
The yellow middle layer contains a high load of pigment. The intensity and duration of the luminance output are attributable to both the quantity and quality of the zinc-sulfide pigment. Eight-hour luminance, in fact, requires a 35-40 percent zinc-sulfide pigment load, in a material of proper minimum thickness.
Zinc-sulfide pigment does not lose its effectiveness when exposed to ultraviolet energy alone. UV and humidity in combination can affect performance negatively. Since moisture alone is likewise incapable of degrading the pigment, the application of UV-blocking layer lends the product sufficient protection. (The new earth-borne pigment that is currently under evaluation is UV-stable, and can be used outside.)
| Safety
first Screen printers and signmakers who serve safety markets have good reason not to compromise luminance standards. If photoluminescent safety markings they have supplied are found, in tragic circumstances, to be sub-standard, it is unlikely that they will escape being named in the product-liability lawsuits that follow when the finger-pointing begins. Photoluminescent products that comply with existing standards do cost more than those with toy glow qualities – but safety-product manufacturers must look beyond to the potential cost of defending themselves in court because the materials they used did not perform as promised in a colorful brochure. The only safe course? Provide signage and evacuation plans that have been made with top-quality, luminance-certified materials and have proven, in prototype, to perform to standard during on-site testing at the end-use location. |
One-stop shop?: Screen printers may offer unprinted safety items as value-added offerings, such as rolls of photoluminescent tape, used to highlight the frame of an exit door or the location of fire-fighting equipment and as warning indicators for dangerous protrusions. |