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What Are The Different Retroreflective
Sheeting’s Used for Sign Faces?
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Without retroreflective properties,
traffic control signs would be ineffective at night
Traffic signs provide an important means of communication to road users, and they need to be easily visible to be effective. At night, sign visibility is provided through the use of retroreflective sheeting materials. This material re-directs the light from vehicles headlamps back toward the vehicle. Without this retroreflective sheeting, signs would be largely ineffective…especially at night.
The sign face consists of film material, called sheeting, which is comprised of glass beads or microprisms to provide visibility at night. Highway signs, that are not illuminated by external lights, are visible at night because they have sheeting made of retroreflective material. Retro-reflection is a type of reflection that redirects incident light from the sign face back to the source—the vehicle headlights.
Retroreflection is achieved through either glass beads or microprisms embedded in the sheeting.
Over time manufacturers have developed different types of sheeting to accomplish retroreflection, initially using small glass beads, but now more commonly using microprisms with various angular designs.
Sheeting manufacturers have specific brand names for their materials. ASTM International has classified the different types by conformance to the retroreflectance properties, color, and durability. The types for rigid surface signs are as follows:
- Type I — commonly referred to as engineering grade and typically made as an enclosed lens, glass beads sheeting.
- Type II — commonly referred to as super-engineering grade and also typically made of glass beads.
- Type III — commonly referred to as high intensity and can be made of encapsulated glass beads or microprisms.
- Types IV, VIII, IX and XI — no common identifiers but referred to as prismatic with the differences related to the varying angles of the microprism.
* All the above types of retroreflective sheeting have a 10 year shelf life, except Type 1, which has a 7-year shelf life.
** If properly applied and sealed to a good surface, most reflective sheeting are quite water, ice, and salt resistant.
Lettering for retroreflective signs are produced by the following methods:
- Cut-out Letters and Symbols: directly applied to the sign face.
- Demountable Copy: This legend is made of reflective sheeting applied to thin aluminum, which is then cut out into the letter and legend shapes and then riveted to the sign face. This permits the sign legend to be changed or removed without having to replace the entire sign panel.
- Positive Silk-screen: Used for signs with legends darker than the background, such as most regulatory and warning signs. The legend is applied directly onto the colored sign face with opaque ink.
- Negative Silk-Screen: Used for signs with legends lighter than the background, such as STOP signs or Interstate shields. The process begins with a white sign face, and then a translucent ink is applied onto the sign face, with the exception of the legend (or regions of other colors. This produces a white legend on a colored background.
- Overlay Film: Also used for signs with legends lighter than the background. The process begins with a white sign face, and then the overlay film in the appropriate color is cut to remove the sections where the white is to show through. This overlay film is then applied onto the sign face.
Sign Backing
The retroreflective sheeting material is applied to the rigid sign backing (also known as the substrate), which can be aluminum, wood, or a composite plastic. Aluminum is by far the most commonly used material. The aluminum backing can be reused often, reducing the cost of signing and can be recycled easily to recoup a small portion of the cost.
Plywood is used occasionally for temporary signs because it can be cheaper and is lighter than aluminum. Fiberglass-reinforced plastic materials are used infrequently because of their handling and recycling issues.
