Application Notes: Guidelines for Mounting Light Sheets
There are a variety of methods available for mounting Cooledge light sheets to various substrates for use in luminaires.
Light sheets are able to operate simply suspended in air with no substrate. However, due to their flexible nature, in most applications they will need to be affixed to a rigid substrate. There are several recommended mounting methods. The preferred solution will depend on the luminaire design, materials used, and required environmental operating conditions.
Recommended Mounting Methods
There are many possible methods of mounting light sheets to substrate materials used in luminaires. The list below demonstrates several of the more common methods. Alternative methods may be acceptable, but in all cases the mounting should be fully validated during the luminaire development cycle.
1. Double-sided adhesive tape
This is the mostly commonly used mounting method for Cooledge light sheets.
The selection of a suitable tape depends on the substrate material to which the light sheets will be mounted, and the environmental conditions under which the luminaire will operate.
The back side of the light sheets is PET (Polyethylene terephthalate). Many acrylic based adhesives give a good bond to this material. Rubber based adhesives are not recommended.
Note: due to the many possible combinations of adhesive and substrate, a prototype
luminaire should always be fully tested to confirm the adhesive selection.
Before adhering light sheet, ensure that the surface is clean and free from dust and debris. Some adhesives may require the use of a surface preparation or primer.
Narrow light sheets such as LSG1-P12-DRF-KXX-W36-LXX-SXX-C15-XX or LSG1-P07-CLF-KXX-W28-LXX-SXX-XX can usually be secured with a ½” (12.7mm) wide strip along each edge, as shown in figure 1 below:
For wider light sheets, differential thermal expansion between the light sheet and the substrate may cause the light sheet to bow up if it is only secured along the edges as shown in figure 2.
In this case additional strips of adhesive may be added in the center of the product as in figure 3 below.
Alternatively, a wide tape may be used to completely cover the back side of the light sheet as in figure 4, but this can make installation more difficult.
All Cooledge light sheets are supplied with a non-illuminated border on both edges approximately 0.4” (10mm) wide. This region can be used to mechanically clamp the light sheet to the substrate as shown
in figure 5.
Fig. 5. (cross section)
Care should be taken in the design to ensure that any electrically conductive clamping elements only contact the non-illuminated border region.
The mechanical grip of the clamp can be increased by adding a shallow “tongue-and-groove” feature into which the light sheet border is formed as shown in figure 6:
Fig 6: (cross section)
As noted earlier, differential thermal expansion between the light sheet and substrate may cause to the light sheet to bow, usually at elevated temperatures. Any clamping design should be tested over the desired operating range of the luminaire.
3. Mechanical Fasteners
The non-illuminated border of the light sheet described previously can also be punched at periodic intervals and the light sheet attached with mechanical fasteners (e.g. screws, rivets, brads) as shown in figure 7. Ensure that all fasteners are at least 0.2“ (5mm) away from any component.
Fig. 7. (cross section)
Washers should always be used in conjunction with screws/bolts in order to prevent the tightening action causing tearing of the light sheet.
As noted above in the clamping guidelines, care should always be taken to ensure that the fasteners only contact the non-illuminated border region and not any electrically active regions of the light sheet. For this reason nylon washers are recommended when using screws/bolts.
Differential Thermal Expansion
Differences in the coefficient of thermal expansion (CTE) between the light sheet and any substrate should be considered when designing your luminaire. Wherever possible, it is desirable to use substrate materials that have a Coefficient of Thermal Expansion (CTE) similar to light sheet.
Table 1 gives a list of CTEs for common materials as well as the light sheet base material.
|PET (light sheet substrate material)||60|
|Cold rolled steel||11~13|
|Polycarbonate (e.g. Lexan, Makrolon)||65~70|
|Acrylic (e.g. Plexiglas)||60~80|
|Woods, parallel to grain||3~5|
|Woods, perpendicular to grain||25~35|