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LED Insights: Diffuser Films for Backlit Signs

J. Bryan Vincent is an expert in the field of solid state lighting and electronic materials. He has dedicated the past 10 years to developing LED solutions specific to the sign industry and is a partner at Principal LED. He has a bachelor’s degree in Chemistry and a Ph.D. in Chemistry/Materials Science.

One thing that I learned from the electronics industry is that if you make something smaller or less expensive, there is usually a demand for it. This also holds true in the sign industry where people are looking for ways to save money with a trend toward customers wanting lower-profile backlit signage.

But customers often get sticker shock when they find out that their elegant, thinner sign actually costs much more than a deeper one, as many sign makers just add more LEDs to try and make the sign look evenly lit. In this article we will take a look at diffuser films and their applicability with some newer LED light sources designed to reduce cost and optimize performance in backlit sign applications.

Illumination Factors

No one wants an illuminated sign that has hot spots or stripes. So recently, a lot of effort has gone into developing various products (from the light source, to diffuser materials and even paints) that are designed to improve evenness of illumination. In backlit signs, the evenness of illumination on the sign face depends on a number of factors, including:

  • The spacing of the lighting
  • The optical distribution pattern of the light leaving the source
  • The distance of the lighting from the face
  • The optical characteristics of the substrate and films
  • The reflectivity of the interior of the sign

It turns out that light as a function of distance follows what is called an inverse square relationship (1/R2). Therefore, if you have a single LED module that is evenly lighting the face of a sign at 1,000 LUX or lumens/m2, and you begin increasing the depth of the sign, the light will drop according to the following relationship or pattern:

  • 1” from the sign face = 1,000 LUX
  • 2” =250 LUX
  • 3” = 111 Lux
  • 4” = 62.5 LUX

Simultaneously, you can cover exponentially more surface area by increasing the distance between the light source and the sign face (see Figure 1). Historically, most LEDs (and many other light sources) all had a Lambertian optical profile with typically a 120 degree beam angle and a large percentage of the light directed at the face of the sign (see Figure 2). Using these LEDs, the only option was to vary the spacing between each light source or increase the depth from the face of the sign.

Even today I bristle when I go into sign shop and see guys literally sticking LEDs one right next to another to “make the sign even.” Often I have this recommendation: "You need to quit thinking about cost per foot and start thinking about cost per use." There is definitely a better, more economical, and more energy efficient way to evenly light low-profile signs.

Batwing LED Optics

Today many LEDs are available with batwing optics profiles from a number of manufacturers, which serve to redirect the light laterally at a relatively high efficiency. This virtually increases the “depth” of the sign by forcing the light to travel a longer distance and resulting in mixing of the light between modules. This can serve to reduce installation time and overall costs. It can also allow for illumination of backlit signs as low as 1.5”. “There are a number of types of batwing optics available.

Standard batwing optics will create a more flat optical pattern. One of the more specialized optics is employed on our new Street Fighter Slim Mod. Its donut lens construction really pushes light laterally, making it the first module of its type that can evenly light signs as low as 1.5” in total depth on reasonable spacing,” says Nick Kadlacek, Engineering Tech. with Principal LED (see Figure 3).

The point is that a number of LED manufacturers' specialized modules can evenly light many surfaces at relatively shallow depths. By purchasing the right LED for the right job and optimizing the depth and spacing, you can avoid the need for diffuser vinyl in many cases.

However, all practicing sign makers know that there are ALWAYS exceptions. Many highly translucent acrylics and certain other substrates may be specified for a specific color (or other reasons), resulting in a sign that has hot spots even with batwing type optics. In addition, polycarbonate faces and some flexible face material do a poorer job diffusing light. In these cases, a diffuser vinyl is definitely necessary.

First and Second Surface

If you are using a diffuser film and a rigid substrate, the diffuser film can be applied directly to the opposite side of the substrate as the film is applied. For many constructions, this is an excellent way to avoid three consecutive film layers. For backlit signs with graphics, there are two general constructions:

  • First Surface Option—First surface graphics are those in which the graphic is printed right facing, where the ink is on the visual side of a translucent film (or substrate if printed directly). Often an over-laminate is applied to protect the ink (see Figure 4).
  • Second Surface Option—Second surface backlit graphics are printed as a mirror image, and in this case the clear film or substrate also serves to protect the image (see Figure 5).

It is important to check with the manufacturer for the correct diffuser film type, as certain films are suitable for second surface application only. In addition to creating a more even overall illumination, diffuser films are also useful when you need to create varying degrees of illumination on the face of the sign. For example, diffuser films can be applied as a second surface application behind a background color or secondary line of text to reduce its brightness, while a logo, for example, can be fully illuminated for maximum emphasis.

Light Control Films

3M carries a wide range of light-control films and substrates. The Envision family of both diffuser and translucent films are specially formulated to work with LED light sources and retain up to 60 percent light transmission. According to 3M, they offer this improved light transmission while still diffusing the LED light.

I was able to see a 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. I was also impressed with the coloration of the 3M material, which not only increased the light, but visually seemed to keep a “purer” color that was not a dull greyish blue as with the standard diffuser film. This may have been more of a brightness effect more than a true color shift, but regardless the visual benefits were clearly apparent.

“We utilize Envision diffuser film any time we are using a colored vinyl face with a clear polycarbonate substrate for channel letters,” according to Michelle Geyer, a design engineer at Kieffer & Co. “We have also been matching the 3M Envision film on shallow letters with some newer optics-based LED technology. This allows us to create new designs and reduce overall energy costs,” adds Geyer. Envision films are also come with a nine-year non-prorated 3M MCS warranty. Application notes and technical bulletins are also available on the 3M website.

An Important Tool

Diffuser graphics are an important tool for not only creating more even illumination or reduced light intensity, but can be used effectively to vary the light, highlighting certain text, images, or logos. Diffuser films can be also used effectively with the latest LED technology that utilize specialized optics to create new form factors and lower profiles in backlit signs.