Over the past two decades, channel letters have been the most common type of illuminated signage in North America. The vast majority of these signs are illuminated with LED lighting.
According to recent studies, channel letters represent one in about three signs manufactured. Typically channel letters come essentially in four flavors:
-face lit
-reverse or halo lit
-front & back lit
-open face
The first and most common is face lit letters. These “standard” channel letters typically have aluminum returns (or sides) and backs and typically have a translucent white or colored plastic face and are illuminated with colored or white neon or LEDs. Reverse halo letters are illuminated in the same manner; however, they are somewhat the opposite of standard channel letters and typically have an aluminum face and sides and a clear polycarbonate back. The result is a “halo” effect around the letters at night by reflecting off of the surface of the structure. Front/Back lit letters are basically a combination of the previous two and are designed with a translucent front and back face and can emit the same or different color from the front and a halo on the back. Open face letters are the least common but are typically used when the customer wants to create a skeleton neon effect. In this case the light source is visible and typically has a clear plastic front or is completely “open.” In this article I will examine some of the materials that are used in channel letter faces and some new materials that are optimized for LED light sources.
Plastic Faces
Historically, face lit channel letters are made from acrylic or polycarbonate. Both acrylics and polycarbonates are thermoplastic polymers, which means they can be heated and shaped, as opposed to thermosetting polymers, which cure to a hard material and remain so even as new heat is applied. This allows both of these materials to be readily used for formed faces, although acrylic softens at a lower temperature. This is also why indoor signs typically require use of a polycarbonate face – as they typically considered “low flammability” materials. Both SABIC Innovative Plastics and Bayer Makrolon manufacture versions that are 5VA flame rated and useful for manufacturing indoor and enclosure rated signs. Polycarbonate is also much more impact resistant than standard acrylic (but more expensive), although modified or “impact resistant” sign-grade acrylics are readily available that have much improved impact strengths.
Both polycarbonate and acrylic can transmit light efficiently. However, over the past decade a slew of new specialized sign-optimized plastics have been released as sign makers transitioned from neon to LEDs for channel letter illumination. Fifteen years ago almost all channel letters were illuminated with neon. Neon glass has to be a certain distance from the face and had to “stand off” from the sign back with a tube support. Therefore, almost all standard neon channel letters were 5-8” deep depending upon the height of the sign. Neon was forgiving in the sense that it is a 360 degree emitter, throwing light all around the inside of the letter, allowing for even illumination at these depths in all four flavors of channel letters. As LEDs began to replace neon, these small, directional light sources could be placed on the back of the letters opening up the possibility of more shallow, thinner channel letters. However, traditional face material caused hot spots at lower depths, especially under 3” deep. Plaskolite’s Optix LD light-diffusing acrylic and Bayer Makrolon LD polycarbonate sheets are both examples of modified plastics that have been optimized to allow maximum light transmission with minimum hot spots. Understanding both the mechanical and optical properties of the face material is important to making sure the sign meets the customer’s expectations and helps minimize fabrication and total material cost.
It is generally understood that standard white translucent acrylic typically does a better job of “diffusing” the light versus polycarbonate. This feature and its lower overall cost has made acrylic the most common material of choice for outdoor channel letter signs. Sign grade acrylic is available in a range of colors and different types of white. The two most popular white acrylics are “2447” and “7328”. 2447 acrylic lets about 50 percent of the light pass through, while 7328 blocks about two-thirds of the light. This means that a sign with a 2447 face will be about 50 percent brighter using the same LEDs. However, 2447 can show more hot spots and looks more “dull” during the day. Therefore, 2447 is ideal when a colored vinyl film is applied over all or part of the sign face and when maximum light transmission is desired.
Films
This brings us to the subject of films. A number of manufacturers (Avery Dennison, Gerber, Orafol, 3M) produce vinyl films that can be applied to channel letters for a variety of purposes. First and foremost, vinyl films are a flexible way to create a large array of color options and multiple color matching options versus colored acrylic. These translucent films can be applied to a clear or white translucent face, allowing for a large pallet of color options. One application example is when translucent vinyl is applied over a white plastic letter leaving a border. In this case, the sign will have a colored center with a white outline trim. This white border creates a more readable sign at night (especially with blue and certain other colored letters).
Opaque vinyl films can be placed over the channel letter face to create a certain color during the day and a skeleton outline at night. However, the most common use of opaque vinyl films are modified with small holes - called “day/night” films. Often the daytime color of the sign must be different from the night color. For instance, light colored buildings may need a darker face color during the day (like black) but a white illuminated color at night. This is accomplished by the use of a day/night vinyl which is an opaque vinyl film with transparent “holes” that let a portion of the light pass through the film. Using this approach, one can use white LEDs with a white plastic and a day/night film on the outside and a translucent vinyl on the inside to essentially create any two combinations of different colors during the day and at night.
For shallower letters, a number of manufacturers also make diffuser films. These films can be applied to the inside of the plastic face to spread the light more evenly and reduce hot spots in LED illuminated signs. These films work by having suspended particles that change refractive index and redirect or scatter the light. Diffuser films are particularly useful in low-profile channel letters that are less than 3” deep.
Recently, 3M has released its Envision film line. This family of both diffuser and translucent vinyl films are specially formulated to work with LED light sources. According to 3M, they offer improved light transmission while still diffusing the LEDs. I was able to see a recent demonstration in which application of this film to the sign face required 40 percent fewer LEDs, while providing the same surface LUX and even illumination. This means more energy savings and less LED material costs. Better diffusion also opens up new thinner form factors. According to Jeff Fuhrmann, V.P. of Manufacturing at Kieffer & Co., “Channel letters are evolving right now,” he said. “We just did some prototyping on a low profile letter using 3M Envision film and some new LED technology. We’re now able to create more unique designs that we were never able to do with neon lighting. We just refurbished the sign on our own building using LEDs and 3M Envision film,” said Fuhrmann. “It looks fantastic, and we’re tracking about a 60 percent reduction in energy use.” Envision films are also come with a nine-year non-pro-rated 3M MCS warranty.
To wrap it up, it is important to understand that the material choice of a given sign project depends on a number of factors, including illumination levels, durability, cost and appearance. The evolution of plastic faces and light management films has evolved alongside LED lighting to open up a broader range of colors, configurations, and new profiles that can give you and your customer new options for their next set of channel letters.