An Introduction to Solid State Lighting: LED Selection and Optics

Wed, 03/16/2011 - 10:14am
Rob Rix, VP Lighting, TE Connectivity Ltd.
Unlike the incandescent/fluorescent lamp world of today’s electrical lighting fixtures, solid-state lighting is in the realm of electronics with multiple interdependent ancillary systems. The purpose of this series of articlesis to provide an informative summary of the elements that comprise these new electronic systems and to introduce a holistic new systems approach to the design and manufacturing of SSL fixtures. In this second installment, the basic issues of LED selection and configuration into light modules will be discussed.

LEDS - SSL’S Enabling Technology
Light-emitting diodes (LEDs) have been produced since the 1960’s. Early applications were primarily for indicator lamps, since the power output was quite low and colors were restricted to red, yellow, orange and green. White LEDs did not appear until the late 1990s. Recent advancements in much higher output LEDs have made LEDs useful in illumination. The term high brightness or HB LED is used frequently to describe these higher power LEDs however, it is not clearly defined and as such tends to create more confusion instead of increased clarity. Understanding the different types of LEDs can help to select the right LED for the application.

Figure 1 shows a comparison of an incandescent bulb and compact fluorescent lamp to a lighting-class, multi-chip LED in a surface mount package. The obvious visual distinction is just one of the major differences between these light sources. In contrast to the others, LEDs are essentially a unidirectional, point light source that serves to put the light where a user needs it rather than backscattered in a fixture. Further, LEDs are typically low voltage devices and require only a constant current source unlike fluorescent and high-intensity discharge (HID) lamps that require a high-voltage ignition source. As an added benefit, LEDs do not generate high inductive spikes or surges like magnetically ballasted HID lamps that often necessitate additional filtering devices to prevent wreaking havoc on AC power distribution systems. 

Figure 1. Physical comparison of typical 100W-equivalent incandescent, compact fluorescent and LED light sources

LED packaging and its semiconductor contents deserve a closer look. High power LEDs are currently offered in 1 to 5-W encapsulated packages as well as hybrid or chip-on-board / array packages that can exceed 50-W levels. As Figure 2 demonstrates, this packaging is not standard for high power LEDs. 

Figure 2. TheLumileds Rebel is an example of a discrete LED. The others shown are examples of LED array packages.

LED packaging has optical, electrical, thermal and mechanical design considerations. All these elements are accommodated within the very small space of the LED package. To start, the semiconductor chip or die needs to be mounted to a thermally conductive surface that allows the generated heat of the die to be efficiently transferred to a separate heat sink. Also within the package, electrical connections between contact pads on the package to the die are made with small gold wires called bond wires that need to be protected from mechanical damage. To protect the bond wires, the die and to focus the light, an optical lens or encapsulant is attached to the top side of the LED assembly.

LED Capabilities
Key performance metrics for LEDs include luminous efficacy (the amount of light provided in lumens, per watt of electricity consumed (lm/W), total power consumption, maximum current capability and the associated luminous output, and Correlated Color Temperature (CCT), Color Rendering Index (CRI) or Color Quality Scale (CQS). In this section, additional details on these metrics are provided.

The CIE (Commission Internationale de l’Eclairage) established a color rendering index (CRI) that rates how well a light source’s illumination of sample patches compares to the illumination from a reference source. The reference source is typically an incandescent lamp that approximates the ideal black body source and yields a CRI of 100 against which other light sources are measured. This is a somewhat subjective analysis that attempts to accommodate the tri-chromic nature of human vision.

A Color Quality Scale is being developed at the National Institute of Standards & Technology (NIST) to address the problems of the CIE Color Rendering Index for solid state light sources and to meet the new needs of the lighting industry and consumers for communicating color quality of lighting products. With the advent of LED sources, this new color reference measurement technique attempts to correct chromatic saturation deficiencies in the CRI method. As opposed to the single number CRI, the CQS results in a composite number that more accurately defines a lamp’s ability to render colors in a manner pleasing to most consumers.

Another important LED parameter is its correlated color temperature (CCT) specified in degrees Kelvin. The CCT provides a relative color appearance of a white light source when compared to a theoretical black body source. As shown in Figure 3, eight typical color classifications extend from 2,700K to 6,500K with the lower color temperatures often referred to as “warmer” colors and the higher color temperatures referred to as “cooler” colors. As a comparison, incandescent bulbs are approximately 2,700-3,000K, fluorescent bulbs are typically 2,700-5,500K and natural daylight often referenced as 5000K. White LEDs are therefore also identified as warm, neutral and cool based on CCT rating. 

Figure 3. The CIE 1931 chromaticity diagram depicts the eight nominal color correlated temperature

Improvements in these key metrics are occurring at a rapid pace. The industry is in a never-ending pursuit of ever-increasing performance requirements, yet one of the most confusing parts of this specsmanship is the broad chasm that often separates laboratory performance from the practical performance of available products.

Manufacturers continue to announce ever-increasing performance from their LEDs. In 2010, Cree announced its XLamp® XM LED that delivers 160 lumens/watt at 350 mA and 750 lumens at 2 A. The later rating is equivalent to the light output of a 60W incandescent light bulb but requires less than 7W. Recently, Philips Lumileds announced a Luxeon Rebel LED that delivers in excess of 300 lm with a 1A drive current. Bridgelux RS Array Series deliver between 3400 and 5000 lumens under normal operating conditions (Tc= 25°C). Based on the highly competitive environment and current market status, continued high-power LED improvements are expected in the future as the industry moves toward the maximum achievable range of 220 to 250 lm/W (versus the theoretical limit of 300 lm/w).

LED Binning Issues
White LEDs actually use phosphor to create white light from a blue LED. The color and output variations of both the die and the phosphors that occur during manufacturing cause suppliers to sell products using a binning approach. According to the DOE Report Solid State Lighting Research and Development: Manufacturing Roadmap, September 2009, “Understanding issues such as how much performance variability can be tolerated and which performance parameters are critical for the development of luminaires of consistent performance is crucial. Color consistency of the LEDs to be used in the luminaires was seen as the most important binning issue.” The National Electrical Manufacturers Association (NEMA) recently published High-Power White LED Binning for General Illumination that provides standardized categorization areas (bins) for the colors of “white” LEDs used for general lighting.

Even though solid-state lighting is produced in very sophisticated processing facilities there are many variations between the LEDs produced, even during the same run. Variations in forward voltage occur throughout the production simply due to normal statistical distributions inherent in all products. To provide repeatable and dependable systems, the LEDs must be electrically sorted based on this forward voltage. A similar sorting is done to accommodate variations in the color of the blue LED chips and phosphor chemistry and density. This sorting results in the “bins” in which all LEDs are offered.

The quality producer of LED systems will take all of these differences into account as the LEDs are matched for consistent color and performance. While the current ANSI binning standards allow variation of up to seven Macadam ellipses, most viewers are bothered by variation of more than four. A system producer must select the proper mix of LEDs to arrive at a consistent, comfortable, color rendition or with a supplier that makes this complex issue transparent to them.

Achieving Cost-effective LEDs
While performance is improving rapidly, the cost of LEDS is decreasingly rapidly as well. When will the cost of LED-enabled lighting decrease enough for wide spread general illumination adoption? Many think the time is imminent.

According to Strategies Unlimited 2010 report on High Brightness LEDs, the HB LED market is forecast to grow at average of 29.5% per year, reaching over $19 billion by 2014. The highest forecast growth rate is 60.6% for signs/displays. Illumination has the next-highest growth rate, with a projected CAGR of 45.4%. This growth is based on cost reductions. As shown in Figure 4, the DOE Report Solid State Lighting Research and Development: Manufacturing Roadmap, September 2009, projects packaged LED cost in 2015 to be almost 10% of 2009 levels. 

Figure 4. Packaged LED costs are projected to continue to decrease rapidly.

Several factors are in the LED’s favor to offset the initial cost difference when compared to other light sources. LEDs:
• Are inherently compliant to Restriction of Hazardous Substances (RoHS) as opposed to CFLs that have compliancy issues due to mercury content
• Provide significantly longer life (50,000 full-power hours or about 11 years @ 12 hours/day to 70% of rated lumens) than CFLs
• Operate at high efficacies (70-120 lumens/watt) rather than 10 lm/W or less exhibited by incandescent lights
• Are available in broader ranges of warm and cool white than CFLs
• Are easier to control than CFLs with better dimming performance and no flicker
• Start without the delay associated with CFLs

Some of these differences are subtle but extremely significant. Because LEDs are so much different than traditional incandescent or fluorescent bulbs, light fixtures can now be designed with a “clean sheet of paper” approach. This will provide a lot more design freedom for new form factors and thinner designs and should result in a considerable cost saving at the fixture level.

The Heat is On
Increasingly higher output LEDs mean increasingly higher power densities and tougher thermal challenges. In addition to decreased life with increasing LED temperature, LED light output decreases, dominant and peak wavelengths increase and color temperature shifts. Addressing the system issues of thermal challenges is of critical importance. This is specifically an area where Tyco Electronics plans on making a significant difference by introducing innovative tools to aid in fixture design. Section 8 will delve into more details regarding the thermal aspects of LEDs and SSL systems.

Optics – Controlling and Distributing LED Light Sources
Traditional light sources emit light in all directions. As a result, optical systems for these sources are typically less efficient because some light bounces within the optics components or luminaires’ bodies. In contrast, LEDs are mounted on a flat surface and emit light from the top and sides in a hemispherical pattern. In many applications, the LED’s inherent directional light pattern adds to its lighting efficiency.

In addition to the primary optics that protects the LED chip in its device-level package, secondary optics provide greater functionality at the system level. Without secondary optics, the LED’s Lambertian and other light distributions (Figure 5) make them less useful for lighting applications. Secondary optics optimize the distribution of the LED light for specific applications such as down lighting, broadly disbursed or focused lighting. Some system manufacturers offer easily interchanged optics to achieve different distributions within the same package that allow the user to adjust the system for the specific application. 

Figure 5. LEDs batwing (a) or Lambertian (b) distribution requires secondary optics to

Challenges of Secondary Optics
Secondary optics has a direct impact on several performance aspects of the SSL product. Improperly designed secondary optics can significantly reduce the light efficiency of the LED and lead to inferior performance. However, light efficiencies within the targeted illumination area can exceed 90% or better with properly designed optics.

There are several designs of corrective optics for LEDs. They range from reflectors to TIR (Total Internal Reflection) polymeric optics and to free-form polymeric optics. The selection of the correct optic is based on efficiency, color shift, application requirements and cost. Free-form optics for large array LEDs are difficult to create. The size and volume of such an optic requires a lot of material and a substantial time for it to set up in the mold. Even though a free-form optic may give superior beam control, cost, weight and stability, applications often require the selection of an alternate optic. Reflectors show high efficiency in conventional tests. However, because the light is often bounced several times within the reflector, the edges of the beam may lack definition and accuracy. The selection of optics must consider all of these factors.

In addition to the specific angle and lighting efficiency, other system design considerations for the secondary optics include, diffusers, lenses, prisms, multiple LEDs and the ability to accommodate unusual footprint LEDs as well as form factor. Since optics are the corrective lenses for LEDs, these different aspects all become an important part of the SSL design process for the supplier and decision process for the fixture designer. Figure 6 shows three different optics designs. 

Figure 6. LED lamp products for different levels of power dissipation and lumen output

This article is excerpted from the white paper, “A Systems Approach to the Design and Manufacture of Solid State Lighting Fixtures” available for download at

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