Secondary Optics

Light rays are highly directional. The path followed is governed by three fundamental principles, namely reflection, refraction and diffraction. These principles are applied to define the photometric properties of luminaries in terms of lighting patterns. LEDs have a much smaller footprint and are directional compared to other light sources, such as incandescent lamps. They come with primary optics attached, but the light distribution pattern is excessively broad for most applications. The intensity also decreases over distance. Secondary optics are used to modify the output beam of the LED so that the finished lamp’s output beam will efficiently satisfy the desired photometric specification.

Secondary optics consist of

Lenses

Lenses come in different materials, like polymethyl methacrylate (PMMA), also known as acrylic, polycarbonate, silicone plastic, and polybutylene terephthalate (PBT). Most lenses use optical grade PMMA and achieve a lighting efficiency of 90%. Optical grade PMMA lens material is harder and more fragile compared to polycarbonate and allows for the use of high current and high-temperature conditions. Apart from PMMA, polycarbonate material also offers excellent optical characteristics. When lenses get exposed to heat and light, the materials used degrades over time. The lenses become yellow, leading to color shift, and due to this, the performance will vary between two luminaries.

Reflectors

Reflectors are typically metallic, cone-shaped and sit over the LED to alter the beam of light. However, they do not offer as much control as lenses. One significant advantage of lenses over reflectors is that the light source is covered, which reduces the harsh glare from LED light sources. Hyper-Reflective PC (HRPC) and aluminum material are used in reflectors.

Total Internal Reflection (TIR) optics (a lens and a reflector)

These lenses are typically cone-shaped, often called TIR lenses as a large part of the design relies on total internal reflection. TIR optics, which consist of a refractive lens inside a reflector, capture and redirect increased quantity of light emitted from an LED compared to a conventional optic. The fundamental working principle is same for both TIR lenses and reflectors, but the TIR lenses enjoy greater control over light. With reflectors, a large amount of light doesn’t touch the reflector, and this light can’t be controlled in any way. Reflectors can be easily implemented and cheaper to manufacture than TIR optics, but the reflectors’ efficiency is clearly lower, compared to TIR optics. A TIR lens manages both direct and reflected light, whereas a reflector manages reflected light but leaves the direct light unmanaged.

Lenses play an important role in secondary optics.

The viewing angle is a vital attribute of lenses. The LED radiation pattern plays an important role in LED applications, which offers lighting designers increased flexibility when it comes to controlling the light in their application.

Viewing angle (Beam angle) FWHM

If you want to know more than the intensity in the center of the light output, then you must know the width of the beam. Beam widths are usually specified as Full-Width Half Maximum (FWHM) or Half-Width Half Maximum (HWHM).

Beam angle describes the measurement of the radiation pattern of an optical output. The method employed is called Full Width Half-Maximum (FWHM) relating to 50% intensity. For example, a 30º optic with an on-axis output of 100 lux would measure 50 lux at 15º off-axis.

The following image shows the difference between two different viewing angles and the respective observed luminances for the same lumen source.

Radiation patterns

The following are three different radiation patterns found in most LEDs. Choose the one which suits your application. These radiation patterns describe the light’s relative strength in various directions from its source.

The following kinds of radiation pattern exists:

  • Batwing – Even illumination of a flat surface
  • Lambertian – Point source
  • Side-emitting - The side emitter is used to throw the light out of the side of the LED and scant light forwards

Secondary Optics Selection

The selection of secondary optics is based on the amount of illumination or light required. This is calculated by the lux level and illumination area. The number of LEDs required is also another determining factor in secondary optics. Designers must evaluate off-the-shelf components to check if these available options achieve the required performance. If not, a custom optical system or component may be required.

While selecting secondary optics, few more following parameters must be considered:

  1. How much Lux level of illumination is required in the area being illuminated?
  2. What is the height between the light source and the area being illuminated?
  3. What is the viewing angle required?
  4. Lenses can be of different types: Single, Multi, and Color Mixing.The designer should select from these different types based on the application need, as elaborated in the following text:
    • Single Lenses - Lenses designed for a single light source.
    • Multi-Lenses - Lens arrays in different shapes and sizes for quicker installations and higher lumen output from compact areas. The following are a few examples.
    • Color Mixing Lenses - Color mixing lenses are available for RGBW color mixing applications. These lenses are designed to be used with multi-chip RGB-LEDs and provide color-mixing to achieve full-color lighting with excellent color uniformity.
  5. Symmetrical and asymmetrical lens - Symmetrical lenses deliver maximum intensity at the center. They are a perfect fit for spotlight applications. Alternatively, asymmetrical lenses deliver oval or immensely wider beams. These lenses are symmetrical on the 0-180 axis and asymmetrical on the 90-270 axis. The asymmetric design helps to direct more light on the street side without the need for tilt on the lamp head. Elimination of tilted lamp heads contributes to less uplight and, often, less glare.
  6. Select beam ( narrow, medium, wide)
  7. Select radiation pattern ( Batwing, Lambertian, side emitting - Refer to the IES file to see light distribution pattern - Allows a user to simulate the different optical parameters of an LED without physical presence of the components. Light distribution data is available in the photometric files that are compiled in IES. Free viewers for these types of files are available online. The user, with the help of this IES viewer, can see the light distribution pattern and intensity of light emitted from the lens. An IES view is a photometric viewer. It is needed to see the light distribution pattern prior to adding it to your model and then rendering it.
  8. Read the photometric graph

The following is an example of how to read a photometric graph. This is vital in the lighting industry and it is imperative to know how to read them. This is an example of an asymmetrical lens with an oval beam pattern. The graph informs an engineer whether the LED has a “narrow” or “wide” distribution. It also tells them the light intensity at the given direction.

The point to remember is that the distance from the center of the diagram to one of the points on the “outline” corresponds to a luminous intensity value, expressed in candelas, in the given direction. These diagrams inform you immediately whether the flow of light goes up, down or sideways. In the diagram, all light flows in a downward direction. Point A in diagram informs us that the light intensity at 60° is 200 candela. In point B, the angle is 30° that yields a reading of 550 candela.

Optical Losses in secondary optics

The typical optical efficiency through each secondary optical element is between 85% and 90%. The losses contributed by various components are as follows:

  • Collection Efficiency: How much light from the LED streams into the optics
  • Absorption loss: Light which never exits the optics because it is absorbed by the material
  • Radiation Deviation: Light which falls outside of the intended target (wasted light)

Lens Holder

Lens holders provide a mechanical cut-off shield. They can also be used to enhance performance. An LED lens with a holder is much easier to install compared to a stand-alone lens. Optics will twist or snap on to an LED holder. Holders are generally straight forward and can be easily assembled. They can be fastened with positioning pins, clips or screws, glue, adhesive tape, snap clips or a holder.

Solderless LED Holder is a circular LED holder that provides a quick and easy solderless connection to the LED arrays.

Lens holders fasten easily to metal core printed circuit boards (MCPCBs) with the appropriate adhesive. These types of lens holders have legs that come down and will fit right into the holes of the MCPCB star boards.

Optics Design Tools

LED optics are usually modeled and designed with optics design software such as ASAP, Light Tools, TracePro, FRED, and Photopia, among others. A designer may download a ray set file for their high brightness LED and may perform the optics simulation with these softwares.

DIALux is free software developed by DIAL for professional light planning. This software is beneficial in designing an intuitive lighting systems for both indoor and outdoor use. If you use DIALux, you can plan the lighting you want to use in a room or a building.

Along with software simulation tools, there are many test and measurement tools to evaluate LED performance in the real world. A few examples include, Goniophotometer, Spectroradiometer, Spectrophotometer, Integrating Sphere, Photometer and Colorimeter.

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