Learn Sheeting: Chapter 3—Technologies II

As discussed in Chapter 2, there are two fundamental technologies for achieving retroreflection: glass beads and prisms. However, both glass beads and prisms can be manipulated to provide varying levels of retroreflectivity or performance.

Highlights:

There are at least four distinct retroreflective element structures: engineering grade (EG) beaded, High Intensity (HI) beaded, truncated cube prisms, and full cube prisms.
EG sheeting uses a space coating to provide some retroreflectivity, while HI sheeting uses an air cell structure to provide a more efficient performance.
HIP uses truncated prisms to provide a significant boost in retroreflectivity relative to beaded sheeting, while full cube prisms provide the highest level of retroreflective efficiency.

Glass Beads

Glass beads are used predominantly in two different product constructions: Engineering Grade (EG) beaded sheeting, and High Intensity (HI) beaded sheeting.

EG structures are the most simple and work similarly to what we discussed in Chapter 2.

This construction is typically used when some retroreflectivity is required, but it’s secondary to other factors like cost, or, in some cases, flexibility or conformability. Because of its simple construction, EG sheeting is fairly robust, especially for the cost. Typical applications could include license plates where there is plenty of light from headlights, utility applications, or parking signs and signs for pedestrians, where you may not want to distract drivers with a bright sign. In the US, the Federal Highway Administration (FHWA) does not allow EG sheeting to be used for traffic applications.

High Intensity Beaded

A step up from Engineering Grade sheeting is High Intensity (HI) beaded sheeting.

High Intensity beaded products provide increased efficiency in retroreflectivity through two main differences: one, the beads have a different composition, which means HI sheeting does not require the space coat found in EG structures. The space coat adds a gap between the bead and metalization layer to help the light focus. HI sheeting uses “bridges” to create an air cell in front of the glass beads. The air cell protects the beads, but also makes it easier to focus the light on the back of the beads. The second difference between EG and HI is that the beads in HI sheeting have a higher clarity. Because light gets to travel through air in High Intensity sheeting, and the beads used have a higher clarity, HI sheeting provides increased efficiency. HI is the minimum retroreflectivity level allowed by the FHWA for traffic applications.

Truncated Cube Prisms

Truncated cube prisms are the simpler and more common version of corner cube prisms introduced in Chapter 2.

High Intensity Prismatic (HIP) sheeting or simply "HIP" sheeting uses traditional triangular or truncated prisms, but the geometry of truncated cube prisms can be modified to provide a more specialized performance. While HIP uses a “balanced” design, the prism angles can be adjusted to produce a narrow cone of retroreflectivity, focusing light at longer distances, such as ASTM D4956 Type VIII products. Conversely, the prism angles can be adjusted to provide a wider cone of retroreflectivity ideal for shorter distances, such as ASTM Type IX products. ASTM D4956 retroreflectivity is discussed in more detail in Chapter 5. For now, it is important to note that truncated cube prisms generally provide the same level of efficiency. Adjusting the prism geometry simply narrows or widens the cone of retroreflectivity produced, shifting strength from long distance to short distance or vice versa; it generally does not increase the prism's ability to return more light energy. To increase overall efficiency “full cube” prisms are required.

Full Cube Prisms

Full cube prisms are designed to address the limitation of light only reflecting twice from the edges of a truncated prism, which was mentioned in Chapter 2. This is done by removing the ineffective edges off of a truncated cube prism to create either a hexagonally designed prism, or a more common rectangular design.

Because full cube prisms have more effective areas, they are able to return more of the light energy they receive. Manufacturing full cube prisms has some unique challenges relative to truncated cube prisms; however, the product structure and composition can be the same as other prismatic products.

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Learn Sheeting: Chapter 3—Technologies II

Glass Beads

High Intensity Beaded

Truncated Cube Prisms

Full Cube Prisms

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