No matter how good your selection of LED lighting sources, your resulting design can only be as good as its optics. This article brings those considerations into focus.
The Bridgelux family of LED Array products delivers high performance, compact, and cost-effective solid-state lighting solutions to serve the general lighting market. These products combine the higher efficiency, lifetime, and reliability benefits of LEDs with the light output levels of many conventional lighting sources.
Bridgelux LED Arrays have been uniquely designed to produce a smooth, Lambertian light emission pattern suitable for a wide variety of lighting applications. Some applications, however, may require a different distribution of light. Secondary optics components such as lenses, reflectors, or collimators can be used to create a wide variety of optical effects in order to meet specific lamp and luminaire design requirements.
The purpose of this article is to provide a general understanding of basic optical concepts which can be used to modify the spatial light distribution produced by Bridgelux LED Arrays, and guidance as to how to change the light distribution to meet application driven requirements. Included in this article is an overview of geometric optics, a description of Bridgelux LED Array optical performance, and guidelines for specifying optical performance requirements.
Geometric optics and light propagation
The science of optics is the study of the properties and behavior of light. Although visible light is an electromagnetic wave, with both wave-like and particle-like properties, simplified models can be used to describe the general properties of light. One such model is that of geometric optics, which describes light as rays that travel in straight lines (see Figure 1) and bend as they pass through or reflect from surfaces in their path. In this model, variability that may be caused by the wavelength of the light is considered negligible compared to the dimensions of the optical components comprising the system.
Figure 1: Light source, light rays, and light wave fronts.
Some of the basic concepts used to describe light propagation in geometric optics include reflection, refraction, absorption, and transmission. These concepts are illustrated in Figure 2.
Figure 2: A ray of light moving between mediums may be reflected, refracted, and transmitted or absorbed.
Reflection
Reflection occurs when a ray of light impacts a surface, changing the direction of light travel. The law of reflection states that the direction of the incident ray and the angle of the reflected ray with respect to the normal are the same (see Figure 3). The normal is a line perpendicular to the surface at the location of impact of the light ray.
Figure 3: Graphical depiction of the law of reflection.
Commonly recognized reflective surfaces are mirrors, which are specular or polished. However, not all reflective surfaces have specular finishes; spread reflectors have rough surfaces and diffuse reflectors have matte surfaces. Figure 4 shows a close-up of a diffuse or scattering reflective surface, illustrating the variability of the path of reflection based on variability in the reflecting surface. Diffuse or scattering reflectors can be very efficient at reflecting light (high efficiency), but effectively mix the reflected rays to provide a smooth appearance. A diffuse reflector will not produce a hot spot in the beam pattern but may deliver very high beam uniformity and light quality, avoiding color separation that may occur with a specular reflector.
Figure 4: Diffuse reflective surfaces scatters light rays in multiple directions.
Reflectivity of materials can vary significantly. When designing an optical system that includes reflective components (such as using a reflector), select materials with high reflectivity in the visible spectrum. Table 1 indicates the variability in various reflective materials, both total reflectivity and diffuse reflectivity.
Reflective Material
Minimum Total Reflectivity (%)
Diffuse Reflectivity (%)
Anolux Stucco Miro 9
95
94
Labsphere 6080 Coating (white)
98
98
Dull Aluminum Foil
80
—
Bright Aluminum Foil
88
—
White Household Paint
60 to 85
60 to 85
Table 1: Reflectivity of various materials.
Reflection is responsible for narrowing or tightening of the light distribution. Reflectors with polished and continuous surfaces tend to have imaging effects, while reflectors with faceted surfaces break up the image and soften the light distribution.
Reflection is also responsible for some of the optical losses observed when a clear cover is placed over a light source, such as an exterior cover on a lamp or luminaire. Even with a highly transparent material, losses on the order of five percent per surface are common, due primarily to reflection. A typical cover will cause the light to travel through two surfaces, resulting in a ten percent loss of light.
Refraction
Refraction describes the bending (or changing direction) of light as it moves from one medium to another, such as a beam entering a lens from air. Refraction is caused by differences in material properties between the two mediums that cause a change to the speed of the light wave. Snell's Law is used to describe refraction, stating that the angle of incidence is related to the angle of refraction by the following equation:
Where:
v1 and v2 are the wave velocities through the respective media
θ1 and θ2 are the angles between the normal (to the interface) plane and the incident waves respectively
n1 and n2 are the refractive indices of the respective media
Equation 1: Snell's Law.
All angles shown are relative to the normal, a line perpendicular to the interface surface, indicated by a dashed line in Figure 5.
Figure 5: Illustration of refraction.
The severity to which the light ray bends depends on the difference in index of refraction between the two mediums through which the light travels. Table 2 includes the index of refraction values of several common optical materials.
Material
Index of Refraction
Air
1.00028
Crown Glass
1.50-1.62
PMMA
1.489
Water
1.33
Table 2: Index of refraction of various materials.
Refraction is responsible for the image formation that occurs when a lens is used. Figure 6 shows examples of two common lenses and their optical effects.
Figure 6: Examples of common lenses: a converging lens focuses parallel rays to a focal point (F), a diverging lens has the opposite effect.
The lens examples in Figure 6 either magnify or shrink the image. These types of optics are referred to as imaging optics. In general, it is not recommended to use imaging optics with Bridgelux LED Arrays. An imaging optic requires extremely precise design and positioning as there is only one focal point for the lens at which a good image will be maintained. At any distance other than the focal distance the light will be distorted, which may result in a non-uniform image causing undesired optical effects.
Total internal reflection
As mentioned previously, when a light ray strikes a surface, the light may be reflected, refracted, absorbed, or a combination of these three effects. Depending on the angle of incidence, however, it is possible for a light ray striking a medium with a lower index of refraction to be totally reflected, an effect known as Total Internal Reflection (TIR).
When a ray of light moves from a material with a higher index of refraction to a material with a lower index of refraction, the ray bends away from the normal. Using Snell's law, we may calculate the incident angle that will result in the light ray bending to 90 degrees from the normal, resulting in a light ray traveling along the surface of the interface of the two mediums. This incident angle is referred to as the critical angle, θc. Light that is incident at any angle less than the critical angle may be partially reflected within the first medium, and partially refracted and transmitted to the second medium. Light that is incident at any angle greater than θc is totally reflected within the first medium. Figure 7 illustrates this effect. The principle of TIR is used for fiber optics systems where the light reflects (TIRs) off the side wall of the fiber, allowing it to transmit through the length of the fiber rather than exiting at the circumference.
Figure 7: Critical angle and internal reflection.
This concept of TIR is commonly used in designing optics, such as collimators, for LEDs (see Figure 8). Collimators are typically molded from transparent materials and condense broad light distributions into tighter collimated beams. The side walls of the collimator redirect light upward using TIR, in a manner similar to a reflector.
Figure 8: Illustration of a TIR collimating optic.
Collimators for larger optical source sizes, such as that of some of the Bridgelux LED Arrays, can become quite large to capture and redirect the light with a high degree of efficiency. As these are optically molded components, this can result in long cycle times and high costs. Therefore, typically reflectors rather than collimators are used for optical management of Bridgelux LED Arrays.
Absorption
Rays of light passing through a medium attenuate. One reason for this is that light may be absorbed and transformed into other forms of energy, such as heat. The degree of absorption is described by the law of absorption, shown in Equation 2.
Where:
dx is the distance through the material that the light travels
dΦ is the change of power of the light when it exits the material
Φ0 is the power of the light entering the material
α is the absorption coefficient
Equation 2: Law of Absorption.
When designing or selecting an optical system, select materials that have low levels of visible light absorption. Minimizing the distance (dx) that the light travels through light absorbing materials will also reduce absorption.
Figure 9: Illustration of the variables effecting absorption.
Bridgelux LED Array optical performance
The light distribution emitted from Bridgelux LED Arrays may be described in different ways. Using geometric optics, the light may be described using rays. Rays of light near the LED Array are referred to as near field. Rays in the near field may cross, resulting in changing patterns of light in this region. Rays of light at a distance far from the source are referred to as far field. Figure 10 illustrates both near field and far field rays.
Figure 10: Near field and far field rays.
Near field
The near field is located in the area close to the emitting surface of the light source. Typically, the near field is described as a distance that is less than fifteen times the source or aperture size. Near field light rays may cross other light rays. In contrast, far field light rays diverge and never cross one another. For Bridgelux LED Arrays, the source size is the diameter of the resin portion of the LED Array. If optical elements (such as reflectors or other optical components) are used to change the trajectory of the light rays, then the source size becomes the aperture of the optical element from which the rays exit. Figure 11 illustrates the source size of Bridgelux LED Arrays with and without secondary optics.
Bridgelux LED Arrays are not point sources. Light is emitted from the resin area of the LED Array. Radiant Imaging Source files and raysets describe the near field light emission of Bridgelux LED Arrays, and may be used to design optical systems. These files may be acquired from the Bridgelux website. Multiple formats are available for use with many commonly used optical design software programs. Bridgelux recommends using near field ray sets containing a minimum of one million rays for optical modeling and design.
When physically evaluating the light distribution of the LED Array or optical system, it is important to ensure that the distance at which the light pattern is evaluated is identical to that of the final use distance. For example, if evaluating the performance of a street light, evaluating the light a few centimeters from the LED Array (in the near field) would not be relevant to the actual use condition.
Figure 11: Source size of Bridgelux LED Arrays without optics (7.0 mm for the LS Array series, 9.0 mm for the hexagonal star LED arrays, 16.5 mm for rectangular LED arrays and 31.0 mm for the RS Array series).
Far field
The far field describes rays far from the source, at a distance described as approximately fifteen times or greater than that of the source size. Rays in the far field do not cross and have stable patterns. Figure 12 shows the far field spatial distribution light produced by the Bridgelux LED Arrays.
Figure 12: Typical spatial radiation pattern for Bridgelux LED Arrays.
Specifying optical performance
When integrating Bridgelux LED Arrays into a lighting solution, it is important to define the light distribution requirements for their application. Once the desired optical performance is defined, the need for additional optical components can be determined. If additional optical components are required, designers may first wish to evaluate off-the-shelf components to see if these available options achieve the required performance. If not, a custom optical system or component may be required.
Specifying or selecting an optic for use with Bridgelux LED Arrays first requires careful consideration as to the requirements of the optic.
Defining optical performance requirements should include answering the following questions:
- What is the size of the area being illuminated?
- What is the distance between the light source and the area being illuminated?
- What level of illumination is required in the area being illuminated?
- What degree of uniformity is required in the area being illuminated?
- How much light is allowed outside of the area that is targeted to be illuminated?
- Should the transition from the illuminated area to the non-illuminated area be gradual or sharp?
- What degree of color uniformity is required over the area being illuminated?
A basic familiarity with common photometric measurements is useful in describing light or light distribution requirements. Table 3 lists common photometric measurements and Figure 13 illustrates how these measurements may be used to specify lighting systems.
Photometric Measurement
Unit
Illustration
Luminous Flux
Luminous flux is the rate of flow of visible light energy
lumen (lm)
Luminous Flux Total Power (lumen) "Light Power"
Luminous Intensity
Luminous intensity is the power of a light source, or illuminated surface, to emit light in a particular direction. Luminous intensity is flux per unit solid angle.
Candela (cd)
1 cd = 1 lumen per solid angle (in steradian)
Luminous Intensity Total Power (lumen) / Solid Angle "Candle Power"
Illuminance
Illuminance is the density of luminous flux reaching a surface, or the lumens per unit area.
lux (lx) or foot-candie
1 lux = 1 lumen/m2 1 foot-candle = 1 lumen/fit2
Illuminance Total Power/Unit Area "Illumination"
Luminance
Also known as photometric brightness, luminance is a measure of the flux emitted from, or reflected by, a relatively flat and uniform surface. Luminance may be thought of as luminous intensity per unit area.
candela per square meter (cd/m2), or nit
Luminance Total Power/Solid Angle/Projected Area "Brightness"
Table 3: Common photometric measurements used to specify optical performance.
Optic design and selection considerations
Bridgelux recommends considering the following elements when selecting or designing an optic for use with Bridgelux LED Arrays.
Figure 13a: Example of a light fixture specification. Center beam illuminance values at various distances are specified. Note how the 120° and 30° beam angles generate different effected diameters and different illuminance values.
Figure 13b: Example of spatial normalized luminous intensity performance of a luminaire. The intensity is normalized by dividing the intensity values by total flux.
Efficiency
The goal of an optical component is to take light whose trajectory is away from a targeted area, and redirect that light to a desired location. The optic should collect as much of the misdirected light as possible, while minimizing losses that result from redirecting the light. In other words, the system optical efficiency should be maximized by minimizing optical losses. This may be achieved by both minimizing losses when light is touched by optical elements, and by minimizing the number of times the light is touched or redirected.
When selecting commercially available optics, it is important to understand the optical efficiency of the optical component. This efficiency may be calculated by measuring the flux from the LED Array, both with and without the optical system, and comparing these values. The ratio of the light output with the optic to the light output without the optic is the efficiency of the optic. An efficient optical system should range from 85 to 95 percent.
Imaging versus non-imaging optics
Bridgelux recommends using optical components that do not image the light source. Imaging optics may result in undesired effects.
Optical center
Optics should be designed to be centered relative to the optical center of the LED Array. The optical center for the Bridgelux LED Arrays is defined as the mechanical center of the LED Array. The dimensions and tolerances to locate the mechanical and optical center of the respective LED Arrays relative to the mounting slots or holes can be found in the mechanical drawings section of the product data sheets. Note that the center of the resin area of the LED Array does not necessarily define the optical center.
Optic maximum wall thickness at base
If using screws with a 5 mm head diameter for mounting the LED Arrays to the heat sink, the space between the outer resin area of the LED Array and the screw head may be as low as 2.05 mm for the LS Array series, 0.65 mm for the hexagonal star LED Array, 0.46 mm for the rectangular LED Arrays and 4.4 mm for the RS Array Series based on a combination of dimensions and tolerances. The wall thickness of the optic near the mounting screw head should stay below these values in order to avoid mechanical interference.
Mechanical attachment of optic
Proper placement and alignment of optical components is necessary to achieve optimal and consistent performance. Tilting of an optic or not centering the optic properly can lead to undesired lighting effects. To prevent this from occurring, ensure that the attachment of the selected optic meets manufacturer specified placement tolerances in the x, y, and z directions.
There are many ways to attach optical components to Bridgelux LED Arrays. Some secondary optics require the use of a plastic holder. These holders may include an adhesive tape on the bottom of the holder to fix the holder in place. Mounting pins may also be included as part of the plastic holder, and these pins may be used to mechanically fix the holder (see Figure 14). These pins provide a tight and secure fit through the side notches of the LED Array, securing the holder and optic in position.
Figure 14: Mechanical attachment of an optic (or optic holder) using pins with a tight fit through the two side notches on the rectangular LED Array.
Reflectors may be attached directly to the LED Array without the need for a secondary holder using similar techniques. Reflectors can also be more permanently and rigidly fixed by clamping them in place using other mechanical components that comprise the lamp or luminaire assembly.
Mechanical clearance features
When designing or selecting an optic for Bridgelux LED Arrays, ensure that the physical dimensions of the optic or optic holder allow clearance for mounting screw heads and solder joints. Figures 15a and 15b illustrate options for providing additional clearance for the solder pads and mounting screws.
Figure 15a: Recesses on the outer wall of the reflector provide clearance for soldered wires above the LED Array solder pad (wire not shown).
Figure 15b: Recesses on the outer wall of the reflector provide clearance for mounting screws.
Size
The physical dimensions of an optic may dramatically impact its performance. For example, larger and taller reflectors may have higher collection efficiencies and may produce patterns of light with more refined features than smaller and shorter reflectors. The size of the luminaire and the size of the LED Array, including source size, may also dictate the size of the optic.
