Correcting colour shift

When a shopping centre in Bangalore, India specified that LED lighting should be neutral white (4000K) and that LEDs should be potted to protect them from adverse environmental conditions, LED lighting specialist Rockforest Technologies utilised Electrolube’s UR5634 — a semirigid, optically clear polyurethane resin which is commonly used to encapsulate LEDs.

UR5634 is generally an ideal solution as it has protective properties and is highly UV resistant, making it suitable for applications where there is a high degree of exposure to direct sunlight. In this case, however, the resin caused colour shift from neutral white to cool white (6500K), which was a deal-breaker for the client. Rockforest was given 24 hours to deliver a sample that met with the colour specification for retesting.

Rockforest turned to Electrolube’s UK technical team for help. Two different approaches were initially considered in order to solve the problem:

1.  Use an LED that provided a colour temperature below the specified colour temperature — in this case, one operating in the 2500–3000K range. By potting these with the UR5634 resin to the specified 5.5 mm depth, it potentially could bring the light back to the desired colour.
2. Assess the amount and type of resin being used to see if a thinner application over the top of the LED array would reduce the colour shift effect.

Ultimately, Rockforest opted to switch to a different resin — UR5635. As a semirigid polyurethane with a cloudy appearance, the resin is ideal for light diffusion, which also meant that an additional diffuser medium was no longer necessary. This allowed the customer to achieve the desired aesthetics and, through trials of various resin potting depths, also the correct colour temperature. The rapid gel time of UR5635 initially created added complications, but this was alleviated via a staged potting procedure.

There are several requirements for protection compounds within LED applications. Typical examples include optically clear compounds for protecting the LED itself and thermally conductive materials to help dissipate heat away from the LED array or LED lighting unit. Other necessary properties are UV resistance, colour stability of the resin and minimal colour temperature shift of the LED.