Optical material selection remains key consideration in lighting

Martijn Beukema of Dow Silicones discussed how experience from other high-power LED sectors may offer insights for horticultural lighting. With a background in physics and more than two decades in the lighting industry, he has worked extensively on the interaction between light and materials, including applications in automotive, street and sports lighting.

Today, his work focuses on mouldable optical silicones used in secondary lenses for LED modules operating under high temperatures and high irradiance levels.

According to Martijn, the horticultural sector shares some characteristics with other demanding LED applications. “We believe that silicone optics can make a difference for LED modules used in the horticultural lighting business, especially when the secondary optics are exposed to the combination of high operating temperatures inside the luminaire and the irradiance of high-power LEDs”

© Arlette Sijmonsma | FloralDaily.com
Martijn Beukema, Dow Chemicals, talking about the effect of photothermal ageing of thermoplastic lens clusters and possible alternatives at Greentech Amsterdam this year

Optical materials under long-term exposure
During his visit to GreenTech Amsterdam, he observed that multiple horticultural luminaires rely on LED boards without secondary optics, while others incorporate secondary lenses, often manufactured from thermoplastic materials.

One of the topics he highlighted was the long-term behaviour of optical materials exposed to blue light and elevated temperatures. Dow recently published a technical white paper examining photothermal ageing in several thermoplastic materials commonly used in LED optics.

Over approximately eighteen months of continuous testing, the study tracked changes in material performance. Martijn noted that, under certain operating conditions, some thermoplastics became increasingly yellow. “This yellowing indicates degradation caused by combined exposure to blue light and heat, which can reduce light output, particularly in the blue region of the spectrum,” he explained.

In more severe cases, the degradation can accelerate into a thermal runaway. As the lens darkens, it absorbs more light, which raises its temperature further and can ultimately cause the lens to melt.

Silicone as an alternative
As a comparison, silicone optics were included in the study. The test results confirm stable performance, in line with expectations. He noted that silicone already has a long history in LED packaging, where it is used as an encapsulant that is directly exposed to high temperatures and intense light output. The silicone material used for secondary lenses is very similar to some of the encapsulants, and is known for long lifetime under harsh conditions.

Considerations for lighting manufacturers
While growers rely on the performance of installed lighting systems, Martijn sees luminaire manufacturers as the primary audience for these material considerations.

“Growers need to rely on what lighting manufacturers offer,” he said. Manufacturers can limit potential degradation through several design strategies, such as reducing operating temperatures, lowering drive currents, or increasing the distance between LEDs and optical components to reduce irradiance.

He noted that many horticultural luminaires already achieve long operating lifetimes. However, as installations provide increasing PPF and accumulate more hours, the long-term stability of optical materials becomes increasingly important for luminaire design, performance retention, and warranty exposure.

Download the whitepaper here.

For more information:
Dow
Martijn Beukema
[email protected]
www.dow.com