Optics for Tunable White Light Fixtures

By Anthony Silvestris

More and more light designers are striving for Human Centric Lighting (HCL) with a goal to produce natural daylight effects in indoor spaces. One way to achieve this is with tunable white light LED fixtures. The benefits of using such fixtures are well documented and range from improved overall health and increased productivity in classrooms and offices. Lighting manufacturers produce tunable white LED bulbs and strips with different color temperatures that can be selected by an external controller, but that’s not where the process ends. After specifying the dynamic controls and LEDs, optics are added to improve the light quality and shape the light pattern as well as the intensity profile.

Designing in the Right Optics First

If proper optics are not chosen for the design, many times there can be color shift and color over angle issues. Color shift or fading occurs in solid state lighting as well as traditional lighting but can be more pronounced in LEDs due in part to their long operating life. Color over angle is a byproduct of color variations in the light output and can make white light emitted from the LED surface appear bluish or light emitted parallel to a surface appear yellowish. Therefore, the material selections and the type of optics chosen at the design phase can be critical. After all, some materials absorb light at different wavelengths, while some structures diffract specific wavelengths. 

Many lighting OEMs spent time and money getting the LEDs acquired and tested to correct for such issues highlighting the two options that can be taken: 1.) The OEM can choose to go back to the LEDs and controls to try to compensate for the losses or 2.) design in the correct optics before the fixtures go to market. Obviously, the latter of the two options will be easier and lower cost to the OEM in the long run as there are readily available materials to chose from that will not negatively impact the LED system. 

diffusing tunable white LEDs
Left: Color over angle as phosphor converts the blue LED radiation into yellow and red.* Right: Color over angle when light is emitted parallel to a surface and appears yellowish (credit Photopia).

Materials First - Microstructures Next

Reviewing the LED wavelengths and comparing material choices such as PC, PET, PMMA are the first steps, then finding the correct pseudo-random microstructure to mold or replicate on the surface is next. The pseudo-random microstructure will allow the light to be mixed and simultaneously provide improved uniformity and beam angle control. An optic that works on the basis of refraction is preferred because it performs over a wide wavelength range (300-900nm). When designed into a tunable white LED fixture, Luminit Light Shaping Diffusers can prevent unwanted optical artifacts such as color shift and color over angle. Light Shaping Diffusers are micro-structures pseudo-randomly embedded on a substrate such as thin film, and when applied to a lighting structure can clean up and shape a light beam. Because our microstructures are random and non-periodic, Light Shaping Diffusers are not wavelength dependent and work well from 400nm to 700nm with superior optical transmission. These properties and more are why Luminit technologies are helping OEMs design tunable white LEDs that put humans first.

For more information on Luminit Light shaping Diffusers for tunable white LEDs, contact us at sales@luminitco.com


*Cited from Visibility of Color over Angle for Gaussian Luminance Profiles by Suvadeep Mukherjee, TU/e