Tertiary optics deliver benefits in SSL product design (MAGAZINE)

March 11, 2014
LEDs are relatively directional light sources but require secondary and tertiary optics to meet the needs of lighting designers and specifiers. Suleyman Turgut explains how tertiary optics are often a necessity for proper lamp and luminaire performance and can be cost-justified in new SSL products.

LEDs are relatively directional light sources but require secondary and tertiary optics to meet the needs of lighting designers and specifiers. SULEYMAN TURGUT explains how tertiary optics are often a necessity for proper lamp and luminaire performance and can be cost-justified in new SSL products.

When the use of tertiary optics or diffusers is an after-thought in the design of lamps and fixtures, the designer ends up facing many integration and performance issues after project completion. Conversely, tertiary optics can deliver many benefits in LED-based lamp and luminaire design when conceived at the start of the product development. Let’s discuss the concept of "collimate then diffuse" for solid-state lighting products and review real-world examples. We will consider various formats of tertiary optics like film, rigid panels, and injection-molded parts along with a discussion on how high- and low-performance optics differ.

LEDs have introduced a greater level of complexity to fixture and lamp design. Lighting designers have had to learn new techniques to control the light from these new light sources, as well as new terminology associated with these techniques. While the radiation from a bare LED with a primary optic can be fairly wide (80–90°), it is considered to be directional when compared to a conventional source like an incandescent filament. In projection applications where the target plane of illumination is more than 0.5m distant, the directional nature of an LED is not alone sufficient for most lighting uses. Indeed, a secondary optic such as a total internal reflection (TIR) lens or reflector is required to collimate the beam to somewhere between 3.5–15°. A tertiary optic such as a diffuser can also be required to improve the color and/or spatial uniformity as well as shape the beam to suit the photometric requirements.

Tertiary optic examples

One example of a real-world product that has all of these optical components is GE’s LED PAR38 lamp (Fig. 1). The LEDs with the primary optic are positioned at the base of the lamp; a reflector surrounds the LEDs with a circular Fresnel lens positioned at the output aperture. Both the reflector and Fresnel lens are considered to be secondary optics. A diffuser is placed on top of the Fresnel lens to clean up non-uniformity caused by the Fresnel structure and adjust the beam to medium and flood angles.

FIG. 1. A GE Lighting PAR38 lamp with a tertiary optic.

Another example is an LED recessed downlight (Fig. 2). Here the secondary optic is the reflector that collimates the radiation from the LEDs and the tertiary optic is a diffuser that increases the beam angle as well performing a hiding function that prevents someone from seeing the LEDs under direct viewing conditions.

FIG. 2. An example of a typical LED-based recessed downlight from MaxLite.

In the MR16-lamp sector, Soraa relies on yet a different structure. Here the secondary optic is a TIR-based collimator. The tertiary optic is an interchangeable accessory that is easily replaced by the customer in the field. The accessory kit also includes a linear spread optic that provides for linear or elliptical illumination patterns at the target plane. The easy and flexible use of tertiary optics of different kinds was what the designers of the product had in mind from the outset. As a result, the design of the secondary optics and the heat sink were optimized for use with tertiary optics.

FIG. 3. An Elation Lighting architectural wall washer.

One last example is an LED architectural lighting fixture (Fig. 3). Here the secondary optic is again a TIR- or parabolic-reflector-based collimator. The tertiary optic is an interchangeable accessory that is installed in the field and serves two functions: to mix the red, green, and blue components of the illumination in air before hitting the target plane, and to shape the beam pattern. Architectural lighting installations are typically complex for two main reasons. First, the distance of the fixture from the illumination or target plane can be different for each project, and the target or object of illumination is always different in size and orientation (e.g., horizontal or vertical). Second, a variety of diffusion angles (symmetrical and asymmetrical) can be offered as options to help customize the fixtures to accommodate many of these variables in a lighting project.

Collimate then diffuse benefits

These four examples help illustrate the concept of collimate then diffuse that many LED lighting designers use in specification projects. There are four main benefits of using this design concept:

Single secondary optical design: If a diffuser is designed into the product from the beginning, there is no need to redesign the secondary optics for a different project or installation that requires a different beam angle.

Reduced total cost: When only one reflector or TIR secondary optic is required, there is only one mold to be manufactured, instead of multiple. Due to a single stock-keeping unit (SKU), the inventory costs are also reduced.

Flexibility: The same fixture can be used in multiple projects since the beam angle change is a simple as changing out a window. For instance, due to the long throw in architectural lighting applications these fixtures require very narrow beam angles (e.g., 1–3.5°) and the illumination target is usually larger than the spot size of the fixture and varies with each project. The diffuser accessory provides the flexibility for the same fixture to be used in multiple installations by allowing for multiple beam angles in the same fixture (Fig. 4).

FIG. 4. The same fixture with different tertiary optics delivers vastly different lighting effects (images courtesy of City Theatrical).

Cleaner target plane: Regardless of how well designed the secondary optic is, there will be always be some level of non-uniformity in the intensity or color at the target plane. In spot lamp applications with narrow beam angles, a small-angle tertiary optic or diffuser (e.g., 2° or 3.5°) works well to maintain the punch while removing striations in the illumination plane (Fig. 5).

FIG. 5. A diffuser producing the spot beam on the right eliminates the striations evident in the beam on the left.

Materials, formats, and options

Now let’s discuss the material and manufacturing aspects of tertiary optics. Materials for tertiary optics are available in both thin film around 0.25 mm and thick sheet options around 3 mm (Fig. 6). Rolls of thin film up to 600 mm in width are common, but for linear fixtures (e.g., wall grazers and washers) the mini-roll format allows for easier assembly and integration. For small fixtures or lamps less than 150 mm in size (e.g., recessed downlight, PAR lamps, etc.), where the volumes are higher and selling prices lower, the injection-molded format using thicker material is the best option. Molding allows for the diffuser to double as the window of the fixture or lamp and the tertiary optic.

FIG. 6.Tertiary optic materials on the left and finished products on the right.

Polycarbonate is the preferred plastic of choice for lighting applications. It’s available in UV-stable versions for outdoor applications and in UL-listed versions to meet flammability requirements that are typically dictated by the class of the LED driver.

Holographic versus volumetric diffusion

There are also different approaches to tertiary optics that must meet application requirements. Tertiary optics are available in low- and high-performance versions. The main difference lies in the diffusion mechanism. High-performance holographic diffusers incorporate a sub-micron surface relief pattern on top of a very clear substrate. Through a combination of diffraction and refraction, the diffusion (which occurs only on the surface of the substrate) is extremely efficient (up to 92%) — independent of thickness — and provides precise angular beam control.

Lower-performance diffusers, such as volumetric diffusers, incorporate scattering elements within the volume of the substrate. These diffusers typically are less efficient in the 30–70% transmission range. The optical properties of volumetric diffusers are very dependent on thickness and provide little or no control of the beam angle of a lamp or fixture.

Optical modeling

One of the most valuable tools used by optical engineers and designers in the product development phase is optical and lighting simulation software. It’s been difficult to model the behavior of tertiary optics in the past. Today packages like LightTools from Synopsys, TracePro from Lambda Research, various tools from Optis, and Zemax from Radiant Zemax are now compatible with BTDF (Bi-directional Transmittance Distribution Function) data. This data describes how a ray of light behaves after entering and exiting a diffuser. This data has been collected for a variety of off-axis incident angles; thus it is useful in simulating wide-angle sources like LEDs.

In conclusion, to avoid issues in fixture and lamp performance after project completion we recommend considering the use of tertiary optics in new fixture and lamp designs. The main benefits include single secondary optical design, reduced total cost, flexibility, and a cleaner target plane.

SULEYMAN TURGUT is director of sales, Luminit LLC.