Choosing lenses and reflectors for LED lights

There is no single parameter that can guarantee the optical performance of a given lens. We need to understand several parameters and the relationships between them, before arriving at a good lighting solution. And critical to this decision making process is an understanding of how LED lenses and reflectors can be combined for the best results.

By Deepshikha Shukla

The light from an LED’s primary optics is too broad for most applications, and lacks intensity over a distance. To alter the beam of light coming from the LED source, lighting fixtures require at least some type of secondary optics. There are four types of secondary LED optics—lenses, reflectors, TIR (total internal reflector) optics and holder kits. The secondary LED optics takes all the light within a given bulb or fixture, and magnifies the intensity towards the target, based on how the optics is designed. Secondary optics is not only made to collimate the light, but is sometimes also used to improve colour uniformity and light distribution within the targeted area. Choosing the appropriate optics or lens depends on the application. Reflectors and TIR optics are used for this purpose in many different applications, and both choices have advantages and disadvantages.

Secondary optics
Lenses are designed for use with single or multiple LEDs like LED modules or strips. Various properties of LED lenses enable the desired lighting effect and precise control over the beams of light. They come in various shapes and sizes—for example, round, square and hexagonal. The lens directs light from the centre of the source to the reflector, which then sends out a controlled beam in whichever direction it is designed to. Apart from this, additional surface treatments can be applied to shape light distribution.

LED reflectors are smooth, multifaceted inside, and come in different shapes. This enables them to create a range of lighting effects. They collect and disperse the light depending on their shape. Some reflectors contain a sub-lens, for additional control of the light. A typical reflector comprises a polycarbonate moulding with a metallised reflective coating. The metallised surfaces can achieve high reflectance, although a lens ensures superior beam control. In terms of cost and ease of manufacturing, reflectors are hard to beat but they offer less control than LED lenses.

The downside to reflectors is that the vast majority of light rays coming from the central emitter pass through the LED light source without even hitting the reflector. This means that a good portion of the light will stray wide off the intended target, creating glare, and this is where TIR optics is required.

TIR optics was initially used for outdoor applications but nowadays, is also used with indoor lighting. It is best used with tight, narrow beams and does not work quite as well when the goal is wide diffused light. TIR lenses ensure more control, particularly with high lumen-density light sources. These lenses use a refractive lens inside a reflector to control LED light spread efficiently.

TIR lenses are typically cone-like and are usually rotationally symmetrical. They can be solo lenses, or produced in arrays for use with multi-die LED sources. They may incorporate built-in features for attaching them to the LED or a circuit board. They typically work such that the lens directs light from the centre of the emitter to the reflector, which then sends it out in a collimated and controlled beam, whether narrow or wide.

When compared to TIR, reflectors are easier to implement and cost much less to manufacture. With Chip-on-Board (COB) arrays or emitters, LEDs emit light over such a large area that the only real solution is to surround them with a reflector. LED optics and holder kits contain both the LED reflector and lenses with a holder. Lens holders can be used to enhance performance and they are also much easier to install than a standalone lens.

Factors to consider
In order to get the lighting effect that best matches the desired result, lighting designers need to consider the following points.

Beam angle: This is the angle over which the light is distributed. Lenses and reflectors can be used to create the desired beam angle. Narrow beam angles have a tight beam of light and are ideal for spotlights. Wide beam angles have a larger coverage for wide area lighting. Clear lenses provide a crisp-edged beam of light while a diffused lens has a softer edge. When multiple LEDs are used, diffusion can also offer a more uniform light output.

Number of LEDs per lens: The lens can be designed for a single LED or for an array of LEDs. Multiple LED lenses are ideal to illuminate a wide area as in the case of street lighting. Lenses are often designed for use with specific LEDs, so it’s better to check the manufacturer’s data for compatible LED options. Optics made from glass with temperature and UV resistance features provide stability and a long service life. The concept relies on a bulk optics approach, trying to capture all of the LED’s light emissions and controlling it.

Specially devised colour correction filters: These filters cover a wide range of colours. Different coating technologies paired with an excellent choice of filters, lenses, arrays and reflectors help to upgrade the LED lighting quality. Matrix LED headlights use collimator lenses for beam shaping and to focus the light. There is an additional surface over the assembly that provides more opportunities to modify the light. The surface treatments like ripple, frosted, polished, etc, are used to widen the beam, diffuse the light, and shape the distribution. Relatively large light-emitting surfaces and scalability are the important advantages of glass.
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 sizes, which is often a key factor in attracting designers to create LED based lighting. Some component sizes are popular among designers as they provide a strong combination of efficiency and beam control, and are reasonably priced and compact. 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.

For optimum efficiency, the lens 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. By understanding the key parameters used to specify optical components and their interdependencies, one can assess the choices available and select the option best suited to one’s requirements.

While pairing LEDs and optics for an application, it is important to be clear about what features you are looking for. As far as lenses are concerned, a wide variety of shapes and properties are currently available 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 factors such as heat, cold, sunlight or moisture.

By using high-efficiency optical materials and the best design practices, it is possible to ensure that lenses transmit a high percentage of the emitted lumens in a tightly-controlled beam. There is no single parameter that can fully describe the optical performance of a given lens. To specify an optimal lens for a given application, we need to understand several parameters and the relationships between them.