The United Nations’ en.lighten initiative calculates that changing to energy-saving lighting such as LED-based lamps could save almost 600 million tons of CO2 emissions every year. As if this is not incentive enough, LED lamps can also last longer than conventional incandescent lamps, support new design ideas and imaginative lighting effects, and save space. The design of the optical system, which usually comprises a lens or reflector, can have a critical influence on a lamp’s success in any of these respects.
Lenses and reflectors
Although LEDs are far more efficient than incandescent lamps in terms of lumens emitted per watt supplied, the emitters can only handle a small number of watts. As a result, LED lumens must be carefully managed to reach their intended target. The optical components comprise an important part of the overall system. A lens or reflector is typically needed with any LED lighting system, to help make the most of precious LED lumens.
A typical reflector comprises a polycarbonate molding with a metallized reflective coating. The metallized surfaces can achieve high reflectance, although a lens ensures superior beam control. A lens is preferred in typical systems that use small light sources and one to four LED dies. Reflectors such as the LEDiL Lena family can offer advantages in applications that would require a prohibitively large or expensive lens, such as with a large light source or if the source contains several dies under a common phosphor layer.
As far as lenses are concerned, a large variety of shapes and properties are currently offered in the marketplace. In general, products that feature higher-quality optical design and materials can be relied on to provide superior illumination over a longer lifetime. Long-term performance can be extremely important, particularly in applications such as street lighting or automotive lighting that rely on LEDs to help reduce replacement costs. High-quality materials such as optical-grade polycarbonate or acrylics deliver high efficiency when new, and have superior resistance to age-related deterioration or environmental threats such as heat, cold, sunlight or moisture.
There is no single metric that can fully describe the optical performance of a given lens. To specify an optimal lens for a given application, lighting designers need to understand several parameters and the relationships between them. Optical efficiency quantifies the lens’ ability to transmit luminous flux, and can be over 90% for lenses made with high-quality optical-grade materials. However, the figure provides no information about how light emerging from the lens is directed. Lens performance can be described more completely using additional parameters including the Full Width Half Maximum (FWHM) angle and the candela-per-lumen (cd/lm) figure. Using this data, the designer is better able to assess the lighting effect that can be achieved in terms of its intensity and distribution.
For a symmetrical lens that delivers maximum intensity at the center of the light distribution, the FWHM is the angle at which the illumination intensity is 50% of the central maximum value. This provides an indication of the narrowness of the beam. To minimize ambiguity, a supplier may also publish the angle at which the intensity is 10% of the maximum. This information helps designers assess the amount of stray light outside the main beam. A small difference between the 10% and FWHM angles describes a lamp that produces a narrow, concentrated beam.
In conjunction with FWHM and 10% angle information, the cd/lm figure provides a measure of the peak intensity at the center of the beam. Together, these three parameters define a light distribution curve that closely resembles the classic “bell” curve and describes the beam intensity and width, and the sharpness of the cut-off as shown in Figure 1.
Figure 1: Distribution curve for a spot lens with tight cutoff: the 10% angle is very close to the FWHM.
Light distribution data is provided digitally in the photometric files that can be downloaded in addition to the conventional datasheet for individual lenses. The files are compiled in IES or EULUMDAT formats, and free viewers for these types of files are available online. Figure 2 shows one example, IESviewer, which features a rendering tool that can be used to picture the spread and intensity of light emitted from the lens. LEDiL has generated photometric data to show how each of its lenses performs with LEDs from leading manufacturers. The data is typically compiled from laboratory measurements or generated by simulation.
Figure 2: Photometric files provide enough information for a detailed assessment of lens performance (image: photometricviewer.com).
By using high-efficiency optical materials and best design practice, lenses can be created that transmit a high percentage of the emitted lumens in a tightly-controlled beam. Figure 3 illustrates the action of a total internal reflection (TIR) lens, which is the lens type commonly featured in the standard product ranges of major lens suppliers, such as LEDiL. The lens acts as a basic collimator, gathering light from a large number of incident angles as emitted from the LED chip and producing a concentrated directional beam. TIR lenses are typically cone-like lenses, and are usually rotationally symmetrical. They can be solo lenses, or produced in arrays for use with multi-die LED sources, and may incorporate built-in features for attaching to the LED or a circuit board.
Figure 3: TIR lenses deliver high performance in symmetrical spotlighting applications.
The collimating property of the TIR lens can be used to create pure spotlights that deliver intense illumination within a tightly defined area. Designers may, however, want to achieve a variety of effects for different applications and environments. A smoother distribution of intensity or diffused illumination may be desirable, while other effects can include asymmetrical beams such as an oval, or an extremely wide beam angle. To achieve effects such as these, the lens characteristics can be adjusted in a number of ways, including machining of the top surface.
LEDiL’s standard range of TIR lenses offers designers a variety of choices including smooth, diffused, medium-width and wide-angle versions, as well as lenses that produce an oval-shaped illumination. These effects are achieved by creating a “pillow” finish similar to the appearance of an insect’s compound eye, or a diffusive pattern in the top surface. The size and pitch of these finishes are adjusted to modify the cd/lm peak, FWHM and 10% angles of the plain collimating lens. An oval light distribution is achieved using parallel grooves.
It is worth noting here that simple patterns tend to allow higher optical efficiency, which can be greater than 90%. On the other hand, creating a complex beam shape by machining the top surface of a collimating lens can significantly reduce efficiency. However, lenses that are designed from the outset to produce a complex beam pattern and are not based on a machined collimator can overcome such limitations and achieve an efficiency above 90%. An example is the LEDiL Stella-A asymmetric series, which is available for single LEDs by manufacturers such as Bridgelux, Citizen, Cree or Philips, and can achieve an efficiency in the range of 87-93% depending on the type of LED.
Lens Size and Positioning
Ensuring the lens is optimally sized in relation to the LED also has an important bearing on performance. Generally speaking, larger lenses benefit from greater accuracy and so can be expected to ensure higher performance. However, this typically adds to the cost of the lamp, and fitting a larger lens can negate the advantage of small size that is often a key factor in attracting designers to create LED-based lighting.
Some component sizes have become well regarded by designers as providing a strong combination of efficiency and beam control with reasonable price and compact dimensions. The 21.6 mm circular lens is an example, which is popularly combined with small LED sources such as Luxeon Rebel, Cree XP, OSRAM Dragon or Golden Dragon, or Nichia N119 devices. The 21.6 mm size is regarded as optimal for these LEDs, although smaller round sizes such as 16 mm can be used while maintaining efficiency over 90%. However, as the size of the lens reduces relative to the light sources, the placement accuracy and the overall ability of the lens to capture the available light are impaired. Intensity, expressed as cd/lm, tends to drop as the lens size is reduced, and efficiency is likely to fall to 80-85% with a lens as small as 10 mm. On the other hand, using a lens significantly larger than 21.6 mm will not gain an appreciable increase in performance. Lens sizes of 26 mm to 30 mm or larger are usually only for special applications needing a very narrow distribution that requires a total FWHM in the region of 3-4 degrees, or where several different optical elements may be combined to create a complex distribution.
For optimum efficiency, the lens ideally needs to be able to capture all of the light emitted by the LED. In addition to ensuring that the size of the lens is adequate, the lens position relative to the emitter chip is critical.
A number of methods are available for holding the lens in the correct position. The Larisa-W-PIN lens designed for use with Cree XQ-E and OSRAM Oslon SSL 80 LEDs is available with molded locating pins on the underside of the square body. Other styles, such as the Lisa2-W-CLIP wide-angle or Lisa2-RS-CLIP real-spot lenses have integrated snap hooks to fasten the lens to the circuit board on which the LED is mounted. Snap hooks are fast and easy to use in a production setting, and can be effective and accurate provided the thickness of the PCB can be controlled within close tolerances.
Other means of fixing the lens include liquid adhesive or automotive-grade adhesive tape, which can provide high bond strength and allows the lens to be positioned as needed without relying on the accuracy of any pre-machined features such as holes for snap hooks. Lenses for multiple LEDs, such as the Stradella lens array may be fitted using screws, or as in the case of the Cute series of 3-lens arrays, by other means such as pins. Multi-lenses like the Cute series help reduce the cost of the optical system for a large LED array, as illustrated in Figure 4.
Figure 4: Multi-lenses can help save part count and streamline assembly.
As much as possible of the light emitted by an LED must be utilized for LED lighting to deliver its maximum potential. The design of the optical system is extremely important, but compromises must always be made in terms of light intensity and spread, as well as lens cost, size, and ease of assembly. By understanding the key parameters used to specify optical components and their interdependencies, designers can assess the choices available and select the option best suited to their requirements.