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ANSI evaluates revisions to SSL chromaticity standard (MAGAZINE)

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This article was published in the July/August 2011 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete July/August 2011 issue.

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In early 2008, the American National Standards Institute (ANSI) published its Solid State Lighting (SSL) color standard, ANSI C78.377-2008, entitled “Specifications for the Chromaticity of Solid State Lighting Products.” For manufacturers, this standard defines how to communicate the chromaticity of white-light SSL products to end users.

After two years of practice using this LED standard, the industry recognized the need for improvements in its accuracy and the need to make the standard more user-friendly. In fall 2010, the ANSI Technical Committee TC78, Working Group of SSL Light Source WG09, formed an ad hoc task force to define the appropriate revisions to the document, focusing primarily on making improvements without requiring major thematic changes.

Similar to other lamp-color standards, this specification will provide recommendations on the white-color-variation ranges when SSL products are used for indoor lighting applications. The white-light chromaticity specified in the standard may deviate from “perceived” white, but are generally acceptable to most users.

Variations of white

The variations of white are primarily described in four so-called directions. These are yellowish (or warm) white; blueish (or cool) white; greenish white, and pinkish white. The variation from yellowish to blueish white occurs in the direction of the correlated color temperature (CCT), where the CCT ranges from lower temperatures (warm white) to higher temperatures (cool white), and the change is measured in delta T. Meanwhile, the variation from greenish to pinkish occurs as a deviation from the Plankian locus or blackbody curve, and is measured in delta uv (Duv). The Duv value on the curve equals zero, while the direction towards greenish color has a positive Duv, and conversely the direction toward pinkish color has a negative Duv value.

The CCT range and Duv range together form white-color boundaries that are detailed in the ANSI SSL chromaticity standard. Using this specification, a manufacturer of SSL products and the users of these products can achieve a common understanding of how any given white color will appear on the chromaticity diagram where the color point of the product fits within (or outside of) these color boundaries.

The ANSI standard does not imply that the whites that fit within the color boundaries are the “good” whites, or that those outside of the color boundaries are inferior. This is because the perceived whiteness of a light source is not directly related to its color rendering capabilities, or to personal preferences. Interestingly, if products’ color variations or tolerance fall within these ranges, they will not necessarily be consistent for observers. In other words, the specified color tolerance in the ANSI standard is large enough that color inconsistency from the same CCT products can be detected.

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Thick-film technology with aluminum substrates optimizes LED assembly (MAGAZINE)

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This article was published in the July/August 2011 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete July/August 2011 issue.

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As the LED market grows, companies that assemble LED-based products are looking for methods to make them more efficient and reliable, thus making them more cost-effective for the consumer. To guarantee high-quality performance from an LED, efficient heat dissipation is crucial. LEDs convert only 20-30% of their electric power into visible light; the rest converts into heat that must be conducted from the LED to the atmosphere. This excess heat reduces the LED’s efficiency and reliability, resulting in a shorter lifespan. Thermal management, therefore, is essential for maximum performance.

FIG. 1.

In the standard method of LED assembly, the LED component is soldered to a metal-core printed circuit board (MC-PCB), a thermally-enhanced PCB or a ceramic substrate, which is then bonded to a heat sink. While such a configuration is currently popular in the industry, it does not manage heat conduction well and is expensive to produce. Consequently, many manufacturers are interested in mounting LED components directly on aluminum substrates. Although aluminum offers excellent thermal conductivity and is less expensive than ceramic or metal, it requires an insulation layer on the substrate.

Thick-film substrates

For one LED assembler, Norbitech AS, finding an insulation system for aluminum substrates was important in meeting their customers’ requirements. Norbitech, headquartered in Roros, Norway, is a supplier of electronic manufacturing services to the international market. The company executes SMT and high-level assembly, and is well-known in the industry for its thick-film production techniques.

“We use thick-film technology to manufacture hybrid circuits with inorganic substrate materials,” says Roar Sundt, Sales and Project Manager for Norbitech. “Typically, the circuit-manufacturing procedure entails the deposition of several successive layers (resistive, conductive or dielectric) onto an electrically-insulating substrate using a screen-printing process.”

FIG. 2.

Although the most common substrate material is ceramic plate, thick-film pastes can be printed on special steel alloy plate or aluminum alloy plate. Thick-film pastes offer a highly reliable solution for many purposes such as automotive, high-frequency applications, and high-power/high-voltage electronics.

Fig. 2. shows the stages in a typical thick-film process. Thick-film substrates can be assembled with all types of SMT electronic components by using soldering, gluing or wire-bonding processes. With these methods, thick-film technology has several benefits, which are also shown in Fig. 2.

Manufacturing challenges

Working with high-power LEDs and ceramic substrates can present challenges, says Sundt. “In 2005, we started working with high-power LEDs using standard thick-film material,” he says. “At that time, this was a good solution since thermal conductivity was acceptable (28 W/mK) with ceramic plates (alumina) compared to similar PCBs. However, the mechanical strength of ceramic substrates is fragile, making the ceramics prone to cracking. Mounting to the heatsink was becoming a challenge for our customers.”

Norbitech needed to find a solution to the mechanical problems, and they found their answer in Heraeus Materials Technology’s Insulated Aluminum Materials System (IAMS). Heraeus, a supplier of products for thermal-management applications, developed IAMS as a low-temperature firing (less than 600°C), thick-film insulating system that can be printed and fired on aluminum substrates. The IAMS material set consists of dielectric pastes, conductors, solder masks, and resistors.

According to Mitsuru Kondo, Global LED Project Manager for Heraeus’ Thick Film Division, IAMS was designed to be compatible with aluminum processing conditions. “The IAMS technology allows the LED circuit design to be screen-printed directly onto the aluminum substrate,” explains Kondo. “Because the IAMS pastes can be fired at less than 600°C, the problems of cracking and bowing are eliminated.”

Solving the problem

Heraeus’ paste system permitted Norbitech to print directly onto aluminum, allowing them to take advantage of all the benefits aluminum has to offer. “Aluminum is a less-expensive base material than ceramics,” notes Sundt. “Since a lot of today’s high-power LEDs are electrically insulated at the heat slug, IAMS allowed us to print directly between the LED and the aluminum. The heat transfer from the die to the substrate is excellent.”

Another advantage to using IAMS pastes is minimal material waste, resulting in lower production costs compared to etched MCPCBs and thermally-enhanced PCBs, where sheets of copper are chemically etched to create the circuit.

“IAMS is an additive process with selective deposition capability,” says Kondo. “The conductor paste is deposited only where the circuit is located. Thermal vias connect easily to the aluminum substrate.”

Norbitech used several of the IAMS products including IP6075 Lead-Free Dielectric Paste, an insulating paste that produces an extremely dense, grey, hermetic-fired film; C8829B Low-Temperature Silver Conductor, a low-firing, lead- and cadmium-free, silver-conductor paste; and PD5200 White Epoxy Insulator, a screen-printable, single-component, fast-curing, modified-epoxy coating for circuit protection.

Successful cooperation

In past experiences, Norbitech was not able to find a standard paste system that would exactly fit the application needs of its customers. By working closely with its customers and Heraeus, Norbitech was able to fine-tune the quality and the functionality of its customers’ products to fit the application.

“IAMS minimizes thermal resistance by reducing the number of interfaces or layers required in an LED module,” explains Kondo. “It allows low-cost design changes, offers the ability to use less expensive substrates, increases conductivity, and lengthens the LED’s life span.”

All IAMS pastes also meet RoHS requirements, which are especially important to Norbitech, since all the products they produce must meet these standards. Norbitech has held ISO 14001 environmental certification since 2004, and Heraeus has been in the forefront of developing RoHS paste systems.

To ensure that IAMS can provide the thermal management properties that are so crucial to LED manufacturers, the system has undergone extensive independent testing.

“The test results concluded that LEDs soldered with IAMS paste operated at cooler temperatures than the LEDs that were soldered on MC-PCBs,” notes Kondo. “The measured thermal resistance between the LED active junction and the board’s bottom was up to 10-percent lower with IAMS than in the MC-PCBs.”
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Wired and wireless interfaces convey dimming settings to luminaires (MAGAZINE)

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This article was published in the July/August 2011 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete July/August 2011 issue.

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LED-based solid-state lighting (SSL) is inherently controllable affording advantages ranging from lower energy consumption to more elaborate control scenarios that deliver the right amount of light when and where it is needed. But product designers face the dual problems of how to design dimming circuits, and how dimming information gets conveyed to the luminaire. In the June issue of LEDs Magazine (p.47), we covered basic dimming technologies and control scenarios that utilize existing AC wiring. Now let’s consider other methods of implementing dimming control through dedicated analog, digital or wireless interfaces, and the type of dimming technology that’s compatible with each.

As we discussed in the last issue, there are two basic alternatives that can be used to reduce the light output of the LEDs: analog dimming and pulse-width modulation (PWM). Whichever method is used, some form of signal is needed to convey the dimming information to the luminaire.

This signal can be carried through the AC wiring, an analog input, a digital input or through a wireless interface (the Table summarizes the scenarios). Each of these options has some advantages and some drawbacks, and different options are appropriate for different applications. In general the formats of the dimming signals are common to any lighting technology – they are not specific to LED lighting. The majority of LED luminaires will be retrofitted into existing installations and they must be able to interact with the existing lighting controls.

Analog (0-10V)

Analog dimming, often called 0-10V dimming, is defined in the IEC 60929 (Annexe E) standard, and uses a control wire separate from the AC input to provide the 0-10V dimming signal. The dimmer control – a dedicated 0-10V wall dimmer or a circuit in a control system – acts as a current sink, allowing one signal to control several luminaires/ballasts in parallel. When the dimming input is 10V the output is full brightness, with a linear reduction to zero as shown in Fig. 1. If the dimming input in a luminaire is not used it can simply be left open-circuit and is internally pulled up to 10V.

FIG. 1.

In a 0-10V scenario, a luminaire can use either a linear control of the output current or a variable PWM output to dim the LEDs. Fig. 2 shows a block diagram of a power supply and driver with linear output dimming. Note that the 0-10V signal controls the LED driver current, not the power supply output voltage which remains constant (24V in this example).

Fig. 3 shows a block diagram of a power supply with analog control that relies on a PWM output for dimming. In this case the 0-10V signal is input to a PWM controller in the power supply that provides a constant-voltage variable-pulse-width output via a MOSFET switch. An LED driver (not shown in Fig. 3) would convert the pulses to constant current and drive the LEDs. A PWM frequency of 200Hz avoids any noticeable flicker of the light output.

DALI digital controls

Moving on to digital interfaces, the digital addressable lighting interface (DALI) standard is also defined in IEC 60929, Annexe E. The main application for DALI is the control of multiple light fixtures in commercial applications such as conference rooms, offices and public buildings. One DALI interface can control up to 64 devices. All devices on a DALI network are controlled by an electronic control device (ECD), which can also include an Ethernet interface to allow the network to be administered from a PC.

FIG. 2.

DALI provides comprehensive dimming capabilities based on stored information for different brightness configurations, referred to as scenes. The DALI protocol allows control of brightness and of dimming speed within each luminaire, to coordinate all luminaires on a network and ensure they all react simultaneously. DALI also allows for status reporting from each device to the controller on request, including brightness setting as well as power status, fault information, etc.

For LED lighting in a DALI network, the dimming itself can be achieved through normal PWM control of the LED drive current. The same microcontroller that provides the DALI interface functionality can also provide the PWM signal to the LED driver.

DMX512

The DMX512 (digital multiplex) standard is defined in ANSI E1.11-2004. Its main use is to control theatre stage lighting, and it can support multiple functions such as pan, tilt, zoom, color and image effects in addition to dimming. A maximum of 32 devices can be connected on one DMX512 line. DMX data flow is unidirectional and does not include any provision for status reporting.

FIG. 3.

Dimming is normally implemented in one of two ways. The first method uses a specific DMX dimmer unit (called a dimmer pack) that provides several AC outputs into which lights are plugged. Each of the outputs has an internal phase-cut dimmer, controlled through the DMX signals. The second method uses a multi-channel DMX-to-analog converter that provides analog 0-10V outputs for each channel rather than AC outlets. These analog outputs are used to control lighting that can accept a 0-10V dimming input.

Control of LED lighting through DMX could use either method for dimming, but the 0-10V analog method is simpler and allows more complete control.

Zigbee wireless networks

Zigbee is a digital wireless mesh networking standard based on IEEE 802.15.4-2003. It is intended for applications such as building automation, smart energy and lighting control. One of the advantages of using Zigbee to control lighting is that no additional wiring is needed apart from the AC power. This can be particularly attractive for LED street lighting applications where the intention is to retrofit existing luminaires. A Zigbee network can support dimming during off-peak hours, multi-level dimming integrated with motion sensors, and other intelligent-lighting scenarios. It can also support status monitoring from the luminaire to provide warning of damage or failure, operating temperature, power consumption, indication of brightness level, and other operational characteristics. About the Author

Building blocks of intelligent lighting design help create successful LED products (MAGAZINE)

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This article was published in the July/August 2011 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete July/August 2011 issue.

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The long-awaited hockey-stick expansion of the LED-based lighting marketplace is beginning to take shape as more replacement fixtures are entering the consumer and industrial landscape. The market potential is leading more product-development teams to attempt their own solid-state-lighting (SSL) designs, whether the product is a small MR16 or a larger PAR lamp. Yet herein lies the issue. LEDs are powerful semiconductor devices, and so allow product developers to deliver a whole new world of differentiation with their designs, such as intelligent lights with sensors and dimming capabilities. Lamp and luminaire designers are no longer defining LED fixtures only in terms of basic lux and color requirements.

In this still-maturing technology segment, many product designers are still coming up to speed on the language necessary to understand how to build any of this potential intelligence into their lighting system. This puts them at a disadvantage when it comes to discussions with potential vendors and partners. The developers, trying to grasp a changing and maturing technology landscape, need an understanding of the basics – the building blocks of intelligent lighting design.

These essential questions can help a developer understand not only how to correctly define a project, but also how to choose appropriate partners, design-service providers and vendors.

Building Block #1:
Do you require dimming?

The dimming question is tougher than it appears at face value. A “yes” answer sets off a daisy chain of follow-up questions including three major issues: input voltage, the dimming scheme, and dimming quality/performance.

Let’s first consider the input voltage. Low-voltage fixtures such as MR16 lamps that have inputs of 12 VAC or 24 VAC make it more difficult to develop a driver that can operate with the majority of the TRIAC dimmers installed in the existing infrastructure. Companies such as Cypress and Zetex are creating such drivers at this time. For standard line-voltage applications, there are many more available drivers that support TRIAC dimming. At the high end, there are a small number of 277V dimmers that are available for high-bay lighting, although the requirement for this feature is trending upward.

The second issue is the type of dimming-control required (see page 49 for more information on dimming-control scenarios). TRIACs were not designed to interface with LED systems but are broadly installed. Your new favorite dimmable AC/DC LED driver may only work with half the TRIAC dimmers installed in typical application scenarios. A driver also may be unable to correctly read the low and high end of the TRIAC and so will only offer about a 20-40% dimming range without introducing flicker, especially on the low-voltage side of the range.

If the dimming control comes from a microcontroller, the power from the AC line needs to be appropriately managed. Standard AC/DC drivers from companies such as Advanced Transformer are not made to power a microcontroller that has a 5V input rail. The microcontroller will also require an input signal to control the modification of the output dimming waveform, which can even introduce the complexity of supporting a communication network to carry the dimming information.

The final issue is the quality of the dimming waveform itself, because all dimming circuits are not equal. Dimming is nominally done via a pulse-width modulation (PWM) signal, a digital waveform used to control power (usually current) to the load based on the PWM duty cycle (from 0-100%). But the PWM signal can introduce complications via EMI noise that can result in LED flicker and create obstacles in the regulatory approval process.

The details of PWM signal control are beyond our scope here, but product designers should look for low-noise implementations. Some drivers use pseudo-random control of the PWM signal to greatly reduce noise.

Dimming performance can also suffer in terms of how smoothly a light dims if the control comes from a digital output. An 8-bit PWM waveform only has 256 possible steps that can dim a string of white LEDs. Especially at the low-end of the dimming range, those individual step changes become visible to the user. However, a 16-bit PWM has over 65,000 steps, allowing for a much smoother dimming curve.

Building Block #2:
Do you require feedback?

The notion of actually being able to adjust the operation of a light engine on the fly, based on input from sensors and operating characteristics such as temperature, is an advantage that LEDs afford. Yet the concept is new both to product developers working on lighting and to lighting designers.

In the case of LED lighting, sensing and control can yield more robust products from a lifetime standpoint. In part this is due to proactively preventing potentially-damaging operating conditions. Any LED system should be able to appropriately track different conditions such as overvoltage, undervoltage, short circuit, open circuit, and thermal runaway. Let’s consider how monitoring these conditions can be leveraged using Fig. 1. While this is an example using a Cypress Semiconductor driver, other IC vendors support similar sensing in simple white-light applications.

FIG. 1.

The circuit relies on a transformer (in dotted lines) to create an isolated topology, which makes it easier to pass UL certification. However, the circuit itself is able to sense what is happening at the load through the tertiary winding of the transformer (at the bottom of the transformer), and as such is able to recreate the waveform internally and adjust how it drives the LEDs.

The circuit also includes a temperature sensor and will shut down if the temperature rises above a set threshold. Temperature is the bane of the existence of LED lighting design engineers, since LEDs conduct all their heat through the base. This puts the engineers into an uncomfortable position of having to work as much on thermal design as on electrical design. This ensures that temperatures do not rise beyond datasheet junction temperatures of the components on the PCB board and cause a failure. Also, it’s widely known that temperature dramatically affects the flux and color output of the LEDs themselves, which can make the visual appearance of a row of fixtures appear to be different in color or brightness.

A system with added intelligence and driven by a microcontroller can implement an improved temperature-compensation algorithm using a simple and cheap thermistor placed near the LEDs themselves. After reading the board temperature, a well-known equation is used to calculate the junction temperature of the LEDs. Junction temperature is equal to the temperature measured on the board plus the product of the thermal resistance of the board, the constant current of the LEDs, and the forward voltage of the LEDs. These are all easily-discoverable values. The calculated temperature can then be used to derive any adjustments to the drive current or voltage in order to keep the flux output (or the color output of an RGB series of LEDs) inside the visible limit.

Building Block #3:
How do you want to drive the LEDs?

This is another simple question that becomes more complex the moment you bring a power engineer to the table. When faced with the omnipresent cost question, most engineers will quickly turn to a linear implementation, which can cost half of the switching alternative. Unfortunately, the tradeoff for using a linear drive system is about a 50% hit in the overall system efficiency, and that tends to counteract the green energy-efficiency advantage of LED lighting.

Switching implementations typically use either a step-down buck or step-up boost topology. There is a wide range of suitable driver ICs on the market that support such topologies. But product developers should keep a critical eye on a few operational features that can crucially impact performance.

The first is switching frequency. For example, if a driver is able to switch at 1.5 MHz rather than 1.0 MHz it will reduce the size of the inductor needed for the circuit, which in turn helps solve the inevitable board-space crunch in most retrofit applications.

A second key specification is a resistance value called RDSon, which is associated with the high-voltage MOSFET that switches the output and in some cases is integrated in the driver IC. If that RDSon value is too high, over 1 ohm for example, then the power dissipation will suffer, again killing the efficiency of the system.
The final key concern is the driver efficiency specification. A decent switching regulator can get up to 95% efficiency, which can differentiate a lighting system effectively in this competitive marketplace.

Building Block #4:
What’s going to set your product apart from the competition?

To be frank, this final question is about the sum of the parts of a lighting-system design that truly differentiate a product – or the lack of differentiating features. There is a veritable crush of companies seeking to carve out a space in this burgeoning LED retrofit market. Many will simply decide to create a non-dimmable or TRIAC-dimmable LED fixture or lamp and try to win in the market based on low cost. These companies will rise and fall based on the commodity pricing of basic components, not on the quality of their overall system.

Companies who instead desire to push forward with a combination of simple differentiating techniques will carve out unique and stand-alone spaces for themselves. Many lighting engineers simply don’t know enough of what’s available in the semiconductor market to take advantage of simple solutions. These techniques include some features in LED retrofit bulbs and fixtures, and other features in complete lighting system designs. Fig. 2 depicts some examples.

FIG. 2.

The block diagram shows a potential retrofit bulb. It takes the AC/DC line voltage such as 120 VAC, and then drops the voltage to drive a microcontroller which handles the TRIAC dimming of the LEDs. This is a similar approach to creating the PWM signal that we discussed earlier. However, this example also interfaces with a motion sensor that is a relatively low-cost external device that might be implemented in a lighting system. The sensor detects the presence of an individual in a room, causing the intelligent light to turn on automatically. A sensor that can be in the sub-$0.10 range can result in a product that is easily differentiated from the competition.

Consider as a second example a table lamp. In the simplest fashion it takes an offline signal and drives a set of white LEDs with no dimming. Again, there are multiple vendors in the market designing this lamp. However, the development team can differentiate the product with the addition of a capacitive touch-sensitive slider on the lamp to both turn the light on and off and to adjust the dimming level. Adding a capacitive slider to a design that already includes a microcontroller can cost as little as a line of copper on a circuit board. In other words, it is not expensive but yet again provides a unique advantage.

As a final example, consider outdoor backlights, such as those used behind restaurant signs. To save energy, the restaurant will want to drive the sign at different levels in the day or at night. But the simplest version of an SSL design will not allow such control.

There are driver ICs that allow software control of the constant current used to drive the LEDs. For example, the level might be software-adjustable from 350 mA to 700 mA – significantly changing the brightness. Such an LED backlight design can offer even more energy savings to the potential customer, maximizing efficiency during the entire day.

There are far more examples of features the lighting-product developer will encounter as LED technology continues to mature. Examples include additional communication interfaces, control mechanisms or thermal platforms. Differentiating a product in this market is not an onerous process, does not have to be costly, and can ultimately help a company position itself effectively. The building blocks discussed here are obviously not the only questions necessary to create an intelligent lighting fixture, but they offer an excellent starting point to successful products.

About the Author

LED lighting and control systems evolve for optimal efficacy (MAGAZINE)

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This article was published in the July/August 2011 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete July/August 2011 issue.

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For multiple fiscal and environmental reasons, lighting efficacy – defined loosely as light only when, where, and how it is needed – should be given the utmost consideration when we deploy lighting systems. From an energy-consumption standpoint, LED-based lighting represents the most important advancement in lighting in decades. LEDs as light sources are inherently efficient and LEDs can be configured in systems that are much more intelligent in terms of both controllability and adaptability than traditional fluorescent and HID technologies. Indeed LED-based solid-state lighting (SSL) can provide an advantage in efficacy from many angles, but luminaire and control-system architectures must evolve to deliver truly optimal efficacy.

From a system perspective, lighting efficacy is comprised of several elements, all of which are of first-order importance. Several are outlined in Table 1. Light source efficacy is not enough. Truly efficient lighting also requires efficient electronics, fixtures that don’t waste light, and control systems that further reduce wasted light.

Table 1. Elements of lighting efficacy

Based on efficacy advantages, LED-based fixtures appear to be either in the lead or quickly approaching the lead in many applications such as high-bay lighting, street lighting, indoor downlighting and even fluorescent troffer replacement. Still, we need to rethink proper light levels, focus on lighting only where it is required, and push deployment of control schemes to maximize energy savings and eliminate light pollution in the environment.

The lighting industry still has work to do in determining proper light levels. For example, regulatory bodies in North America do not currently take into consideration the differences between photopic (day), mesopic (dusk), and scotopic (night) human visual systems. Our visual system has evolved to account for the differences in lighting between day and night. During bright sunlit days, our eyes are more excited by warmer CCTs (correlated color temperatures) than during dim nights when our eye sensitivity shifts toward the colder, more-bluish moonlight. Mesopic lumen output describes a situation in between photopic and scotopic and is generally considered the most appropriate measure for street lighting.

Efficacy and eye sensitivity

The differences in efficacy can be dramatic when considered relative to photopic, mesopic, and scotopic sensitivity. This is shown in Table 2, which compares a low-CCT high-pressure-sodium (HPS) source to a much-higher -CCT, metal-halide (MH) source. High-CCT sources such as MH and LED are not necessarily given proper credit for exciting the eye in an optimal way for given environmental conditions. Given the data in the table, it’s no surprise that many people involved with case studies report that LED street lights with a lower total lumen output appear brighter than higher-total-lumen HPS street lights. Note that this statement refers to the brightness of the target area and not the fixture itself which may (falsely) appear brighter due to glare effects. We need standards that ensure safety without wasting light and energy.

Table 2. Comparison of high-pressure sodium (HPS) and metal-halide (MH) source efficacies.

Likewise, some regulations and guidelines don’t consider the CRI (color rendering index) of a light source even though it has recently been proven to have an effect in some applications (again, like street lighting) where small-target visibility is critical. Both CCT and CRI are critical because the required lumen output of a lamp varies greatly based on these factors. That said, their importance is still being debated and as recently as 2007, CIE’s stated position in CIE 180:2007 is that, “Colour rendering is not highly important for roadway lighting, except in sensitive urban centres and/or areas with large numbers of pedestrians.”

Utilization factor

Now let’s discuss utilization factor. The first three efficacy elements in Table 1 are static, at least within a relatively short timeframe of days or weeks. This is not the case with the fourth element that addresses the difference between the light supplied relative to the light needed. Utilization factor is a combination of the percentage of time that the lights are on and, when lights are on, the intensity of the light compared to what’s required or being utilized. Optimized lighting controls are essential to improving utilization factor and thereby reducing energy costs. LED lights present a new opportunity for controls as they are easy to regulate using various dimming methods, sensor interfaces, and communication infrastructures that allow the light to be modulated based on environmental conditions.

Lighting systems can perform occupancy detection to control on and off states. Several technologies can detect occupancy including passive infrared (PIR) or ultrasonic motion sensors, capacitive- or MEMS-based microphones, and digital cameras that perform image processing. Motion sensors are relatively inexpensive and are used most often although a combination of a motion sensor and another occupancy-detection method can yield superior performance. Multi-technology sensors decrease the likelihood of erroneous behavior, thus maximizing precision and decreasing energy usage.

Controlling fixtures and dimming lights to produce the appropriate amount of artificial light based on ambient light conditions is critical to both energy efficiency and user experience. Dual-loop sensors are now able to differentiate between light provided by the sun and artificial lighting systems so that fixtures can maintain a consistent light level on a target area. LED-based lamps have the advantage that deep dimming is easy to do and actually increases lamp life, in contrast with competing technologies.

Leveraging lumen depreciation

SSL also affords the potential of further energy savings in luminaire designs that accurately account for lumen depreciation in regulating light output. Light-output regulation is very important to LED-based lighting because of the technology’s extremely long lifetime. If properly protected and driven, LEDs shouldn’t burn out. Instead, the LED light output decreases over time based on a phenomenon called lumen depreciation. L70 is a parameter that describes the point in time at which the light output has decreased 30% from its initial value, and is typically on the order of 35,000 to 100,000 hours for LED lamps, as shown in Fig. 1.

FIG. 1.

To maintain a minimum amount of light output over the lifetime of a fixture, say 750 lm for a 65W replacement lamp or 6,000 lm for a parking-lot light, many fixture designs initially output 30% more light than is required. This represents a significant waste of electricity in that the target area is being over-lit for virtually the entire lifetime of the fixture.

Intelligent fixtures can regulate the light output to a lower level initially and increase the output over the fixture life. Ancillary benefits include consistency of light intensity and color, lower overall energy expenditure, and lower total thermal load. Lowering the total thermal load is extremely beneficial as it leads to longer lifetimes for all electronic components, especially the LEDs and power electronics.

Though beneficial, light-output regulation provides a significant technical challenge. One could use a predictive algorithm that estimates LED efficacy or output based on hours of operation and temperature measurements. But LED performance over time and temperature may not be all that predictable. For several families of LEDs from various suppliers, the actual lumen-depreciation curves have been shown to be significantly shallower than those predicated by accelerated, high-temperature testing.

Alternatively, a fixture design could add a sensor to measure the lumen output during operation, but there are challenges here as well. First, achieving proper mechanical placement of the sensor to measure overall- or average-lumen output may be difficult or even impossible. Second, dirt can can prevent photons from getting out of the fixture and may even redirect them towards the sensor, thus corrupting the measurement. Third, sensor aging and temperature drift could complicate matters even further.

FIG. 2.

In lighting systems, external sensors could measure the light output and communicate the data to the fixture. Such a system could be cumbersome, costly, and have its own set of technical issues. The right answer is likely a combination of approaches, and light-output regulation appears to be one area that is ripe for innovation.

Microcontrollers and networks

Clearly the industry must move toward intelligent lighting platforms to maximize energy savings via sensors, programmatic controls, and communications links between fixtures. Such intelligent luminaires rely on driver modules that integrate a microcontroller for interfacing to sensors and for control of the dimming profile. The smart fixtures enable managed-lighting systems with wired- or wireless-communications capabilities.

The communications infrastructure allows lights to communicate with each other, with remote sensors, and with centralized control and data-collection points. Such control systems have existed for some time but have not been widely deployed, having an estimated market share at 2% to 4%. Cost and complexity have hampered deployments. Moreover, the lighting industry focused first on more efficient sources such as fluorescent and HID that weren’t inherently controllable.

With LED sources, it’s time for broader deployment of control networks although the technology landscape is fragmented. Wired communications options include 0-10V dimming, DALI (Digital Addressable Lighting Interface), DMX (Digital Multiplex) or power-line communications. Wireless personal area network (PAN) options include Zigbee, Z-Wave, 6LoWPAN, or even Google’s new Android lighting platform. All may find usage although the market will likely pick the winners.

New lighting system topology

The trend is clearly toward systems that integrate the control strategies and intelligence directly into the ballast or driver. But, the overall power-supply and control architectures of these systems will likely change to take full advantage of LED technology. For example, consider a space lit by four 25W downlights, as shown in Fig. 2.

The lamps are controlled by remote occupancy and ambient-light sensors over a wireless PAN. A wired configuration could just as easily have been shown. Regardless, each fixture operates from line voltage and includes significant intelligence and therefore requires:

• 25W AC/DC converter
• 25W DC/LED constant-current converter
• Radio for the wireless PAN
• A relatively expensive microcontroller including flash memory for the PAN protocol stack
• Energy meter
• Optional sensors (temperature, light output, or color).

FIG. 3.

As shown in Fig. 2, data gathered by the MCU could be backhauled to a central location that records energy usage. Such a system could also be under remote control in addition to being able to interface to local sensors. This system, while perfectly functional, is expensive to implement and does not take into account the simple but significant fact that we now have a light source that is easy to power remotely. An alternative approach is shown in Fig. 3.

In this case, the 100W power supply incorporates the room controller/coordinator and is therefore capable of communicating directly with the sensors and the remote-control/data-backhaul interface. In this case, each fixture contains:

• 25W DC/LED constant-current converter
• A relatively inexpensive microcontroller
• Optional sensors (temperature, light output, or color).

From a power-supply standpoint, one 100W AC/DC converter is both more electrically efficient and less expensive than four 25W AC/DC converters. Energy metering is performed at the centralized power supply instead of at each lamp. The lamps communicate with the 100W supply over an extremely simple and inexpensive wired interface and therefore contain a less-expensive microcontroller, lighter communications-protocol stack, and no radio. Finally, if the optional local sensors aren’t needed, then no electronics are required locally inside the lamp – the 100W power supply could send a constant current directly to the lamp.

Our proposed system lies somewhere in the spectrum between 100% local power supplies and intelligence and 100% remote power supplies and intelligence (something akin to Redwood Systems’ technology). The market must decide on the best solution.

Finally, artificial-intelligence or fuzzy-logic technology will enable these systems to become more efficient by enabling active learning – prediction of occupancy and even a user’s desired light level. Such systems could also greatly simplify and possibly even eliminate the commissioning process. This is obviously yet another area begging for innovative solutions.
About the Author

LFI report, part 1: Linear LED lighting, OLED and planar lighting (MAGAZINE)

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This article was published in the July/August 2011 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete July/August 2011 issue.

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LFI report, part 2: Retrofit lamps, modular SSL

LFI report, part 3: LED technology, outdoor lighting

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LEDs again stole the show at the annual Lightfair International (LFI) tradeshow. While you could find plenty of conventional lighting on the exhibit floor, it was solid-state lighting (SSL) products that were prominent in most booths, ranging from A-lamp retrofits to decorative and architectural lighting. Purpose-built LED-based linear lighting that might replace fluorescent fixtures was arguably the biggest story. There was little new on the OLED lighting front at LFI, but other planar technologies are coming to market. There were both new players and new looks in outdoor SSL. And adaptive-control technology for sensing and controlling light levels is headed into the mainstream – despite the lack of broadly-accepted industry standards.

LFI continues to surge in popularity and surely LED lighting is partially responsible. Despite some concern in the industry about moving LFI to Philadelphia due to construction issues at the New York venue, registered attendance hit 23,709 – up slightly from last year’s Las Vegas show.

Again this year SSL dominated the LFI Innovation Awards. The Most Innovative Product of the Year award went to the Revel OLED luminaire from Acuity Brands. The Design Excellence Award went to Tech-Generation Brands for a low-voltage LED-based wall washer. Philips Lumileds took the Technical Innovation Award for its Luxeon A LED that the company is hot-testing at typical operating temperatures of 85°C. LED-based products also dominated the product-category awards, with winners including Cooper Lighting, Visa Lighting, and Lumenpulse.

Ironically, LEDs were an afterthought in the keynote presentations this year. But the conference sessions included plenty of LED-centric content.

In the following pages, we’ll present what we saw as the most-compelling product announcements and demonstrations in OLED and planar lighting, linear LED lighting, LED retrofit lamps, modular SSL products, LED technology, outdoor lighting, and other areas.

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Section 1: Linear LED lighting

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At LFI a year ago, LED-based lamps designed to replace T8 linear fluorescent tubes were in the headlines as many companies sought to deliver an SSL retrofit for what is the largest installed base of office and industrial lighting. But as we reported after the show, LED tubes haven’t delivered equitable performance.

This year the focus was more on purpose-designed LED-based fixtures that can serve in place of fluorescent troffers. That’s not to say there weren’t LED tubes on display. In fact, Cree showed a T8 tube reference design that product marketing manager Paul Scheidt said “addresses all of the shortcomings that the US Department of Energy (DOE) has documented about LED T8s.” Still, the bigger fluorescent-replacement news in the Cree booth was the CR fixture that the company launched prior to the show.

RTLED from Lithonia Lighting

Lithonia Lighting (an Acuity Brand) was out in front of the purpose-built, LED, linear-fixture trend by announcing the RTLED product at LFI last year, and showcasing the family in its 2011 LFI exhibit.

The product also integrates support for Acuity’s lighting-control technology that relies on wired links between fixtures using Cat-5 (computer-network) cables. Moreover the products implement what the company calls lumen management where the LED driver produces less output early in the fixture life and increases the output over time to combat lumen depreciation.

Lithonia also demonstrated square surface fixtures called TLED, and square recessed ACLED coffer fixtures, both of which use an array of LEDs and feature integrated controls.

LED Distributed Array from Osram

Osram Sylvania introduced an LED module for linear fixtures that it will both sell to others and use in its own luminaires. The LED Distributed Array integrates 48 LEDs on a 2×9-in circuit board. Luminaire designers can utilize multiple modules to create fixtures of almost any size. The company states that the module design produces uniform diffused light with no apparent bright or dark areas associated with LED location.

Just after LFI, Sylvania’s sister business unit Osram Opto Semiconductors announced the Duris E3 LED designed with a wide beam angle to produce uniform light in linear fixtures.

ALM LED module from Cooper

Cooper Lighting launched an LED module called the ALM that the company will use as a technology base for linear lighting, and also unveiled 32 luminaires across Cooper brands that will utilize the new module.

The module design is based on a dense array of relatively low-power (0.25W) LEDs, and the design only drives the LEDs at half the rated power. The scheme optimizes efficacy, according to Cooper, and will yield products that last 50,000 hours. The company asserts that its linear products will match or exceed fluorescent systems in optical performance with a 15-20% reduction in power density.

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Section 2: OLED and planar lighting

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Acuity Brands took the top LFI Innovation Award with its Revel OLED luminaire, and actually announced two OLED products at LFI. The ceiling-mounted Revel (pictured) is more decorative in nature although the individual OLED modules can be positioned to direct light where it is needed. The Kindred is a stylish ambient light designed to be suspended from the ceiling. The Kindred integrates more OLED panels and produces more than 3000 lm in aggregate. Acuity termed the LFI announcement a commercial launch, but the products will not be available until the first quarter of 2012.

Oree’s LightCell planar LED-based technology

Oree and Future Lighting Solutions have partnered hoping to commercialize Oree’s LightCell planar LED-based technology. At LFI, the partners conducted private demonstrations of tunable white panels whereas much of Oree’s earlier efforts have been focused on color panels.

As shown in the picture, the panels are relatively small, but Oree believes they can be combined to construct much larger fixtures. Each small panel includes built-in LED emitters. Future plans to have a demonstration platform available for sale by the end of the summer, allowing product designers to experiment with the technology and start luminaire designs. Separately, Future announced an intelligent lighting platform based on a partnership with Synapse Wireless.

Rambus from GE Lighting

GE Lighting made LFI news with LED-based planar luminaires based on technology licensed from Rambus. Rambus’s edge-lit Pentelic technology relies on etching a substrate layer to control the ray angle of light to provide uniform distribution over a panel. The company has said that the technology delivers 92-95% optical efficiency. GE demonstrated the Pentelic-based Edge family of luminaires at LFI including a ceiling troffer, a circular suspended pendant and a suspended rectangular luminaire. GE plans to ship the troffer this year and the others in the first half of 2012. All of the products will support adaptive controls and dimming for maximum energy savings.

About the Author

The time for intelligent LED-based lighting systems is now (MAGAZINE)

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This article was published in the July/August 2011 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete July/August 2011 issue.

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We have the technology pieces that are required to broadly deploy intelligent LED-based lighting systems. Sure there are issues to be resolved such as the multiple wired or wireless interconnects that we might use to network a lighting system. But workable networks exist, mainstream LED-based luminaires support dimming and control, and intelligence is the key to really delivering the energy-saving potential of solid-state lighting (SSL).

You will find intelligent lighting as a recurring theme throughout this issue of LEDs Magazine, shared by contributed features on pages 25, 49, and 63.

Intelligent SSL technology was also a recurring theme in the Lightfair International (LFI) educational program. The “Incorporating lighting technologies of today with buildings of tomorrow” session yielded insight into intelligent lighting and perhaps afforded a view at the direction some industry leaders will take. Speakers included Osram Sylvania executives Makarand Chipalkatti and Karl Jessen; Mark Bauserman, executive director of engineering at Paramount Pictures; and Nadarajah Narendran, associate professor at the Lighting Research Center (LRC).

The crowd was sparse at the early-Sunday session and Chipalkatti used that fact to make a key point about the energy-saving potential of LEDs. Noting the two empty rows in front, Chipalkatti suggested that an efficient intelligent-lighting system would reduce the CRI of the lights that were directed at those empty rows and hence drop the energy those lights used by 25%, without affecting the attendee experience. It’s not just output level that’s controllable in SSL.

Still, first-level savings should come from supplying light only where it is required. Narendran stressed that efficient light sources alone aren’t sufficient saying, “Light source efficacy does not tell you whether you are going to save energy.” Narendran stressed the need for using sensors, and leveraging daylight to minimize the need for artificial light.

Bauserman provided insight from the user side of the equation. He said that when Paramount upgraded lighting with dimmable fluorescent lamps with a CRI of 85, he found that the lights could be set at lower output levels yet workers perceived an improvement in the lighting. And he believes that workers will save energy given the option. He said, “If you give an occupant the ability to control light in their space, normally they are going to save energy.”

Financial story is paramount

Bauserman is planning a major lighting retrofit across the 64-acre Paramount campus. His focus is both saving money and improving light quality. Discussing the pitch he will make to management, he said, “I have to tell the story financially, or there is no story to tell.” But he also added that the lighting must maximize worker productivity and mitigate any worker health impact.

For new lighting, Bauserman is looking for bidirectional communications so that he can automatically detect failures and monitor operations. Other goals include a lighting system that is easy to install and commission, as well as long life, and low total cost of ownership.

It turns out that reading between the lines there was a reason the four speakers teamed on the session. Osram Sylvania’s Jessen revealed that his company would be working with Paramount on a case study this fall involving 2×2-ft SSL luminaires with wireless connectivity. Moreover the test will use a DC grid and Class 2 cable to carry power, eliminating the need for an electrician to install the luminaires. Jessen did not say whether the installation will use the Emerge Alliance’s DC technology, but Osram is a member.

The speakers were careful not to provide too many details, but Narendran earlier had mentioned research that the LRC had done with a technology called Future Tiles in which the researchers used LED-based tiles in the walls and ceilings of a room. Jessen also mentioned “LEDs integrated into things like building materials” as a next phase in SSL. It appears we will have a compelling case study to cover later this year, although the speakers declined to provide more details at LFI.

Every retrofit or new lighting installation in commercial applications should include intelligence going forward. Not every case needs the type of technology that we may see from the Paramount installation, but sensing and controls should be universal requirements and LED sources deliver the best user experience and maximum energy savings.
About the Author

LFI outdoor sessions address best practices and MLO

The Lightfair International (LFI) conference program featured a broad set of classes and sessions, and several addressed outdoor lighting in general, whether the light source was LED-based solid-state lighting (SSL) or legacy lamps. Sessions on best practices and the new Model Lighting Ordinance (MLO) stood out, providing key information on glare, the human visual system, and lighting levels that balance visibility and minimal light pollution.

In the outdoor-lighting area, Ray Grenald and Mark Harris of lighting firm Grenald Waldron Associates presented “Street and area lighting around the world – reducing energy, carbon emissions and light pollution while maintaining quality of life.” The session proceeded informally but provided a series of lessons that could easily be the basis of a best-practices guide.

Inconsistent global regulations

Harris lamented the lack of consistency in lighting regulations around the world. He said that Ottawa, Canada city code prescribes brightness levels that are 50% of Illuminating Engineering Society (IES) recommendations whereas the nation of Qatar in the Middle East prescribes double the IES recommendation.

The pair are generally in favor of less light at night, and suggested that too much light can reduce safety. Grenald said, “You can inadvertently put more light on the pupil causing it to close and you get less light in the back of the eye.” In such cases visibility is reduced, and glare is often the issue.

Harris said, “The key for me is more fixtures and greater control.” But of course more fixtures would drive costs up. Still there are other ways to improve lighting with the knowledge of the human visual system. Harris said, “We see brightness not foot candles and we see more vertical brightness than horizontal brightness.” Again glare from high angles is a major concern.

Grenald also addressed the need for security lighting in applications such as college campuses. He said, “Don’t make it brighter like most people do, but light the bushes and trees at a low level.” His point was to light places where a predator might hide.

Model Lighting Ordinance

Nancy Clanton of lighting firm Clanton & Associates also addressed outdoor lighting and specifically the MLO that the IES and the International Dark-Sky Association (IDA) have jointly developed. Some have seen the IES and IDA as having dueling goals, but Clanton said, “There really was a cohesive common goal.” Clanton said the organizations were seeking to minimize excessive light to save energy, improve the enjoyment of the night sky, and minimize the impact on bio-cycles of people and animals.

MLO sample table

At LFI Clanton said the MLO release was imminent and it has in fact been released to the public as of June 14. The goal was an ordinance focused on lighting first rather than energy. It is meant to apply to lighting applications such as monuments, signs, water features, and seasonal, landscape, emergency, and temporary lighting. Generally the target is lighting on private property. The MLO exempts street lighting although it can be adopted by small communities that lack the engineering resources to create their own street light regulations.

The MLO is defined by a series of five zones that are designated LZ0 through LZ4. Clanton said LZ4 is for places like Las Vegas or Times Square and “is not recommended for most cities.” LZ0, conversely, prescribes no constant ambient lighting and the use of motion sensors. Ultimately the success of the MLO will depend on city planners that make the right decision in choosing the appropriate zones and enforcing the MLO. The easiest way to learn more is to visit the IDA web site. Clanton said that Plymouth, MN and Anchorage, AK would be the first cities to adopt the MLO.

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LFI shows LED skeptics remain and OLED still trails

While the show floor was brimming with LED-based luminaires and lamps, Lightfair International (LFI) sessions demonstrated that skepticism still lurks over solid-state lighting (SSL). OLEDs were also highlighted on the show floor, although an expert panel focused more on the roadblocks that limit the technology to decorative lighting for now.

There were numerous LED-centric sessions at LFI, but one entitled “LED performance: Myths and facts – an LED industry report card” served to remind that SSL is a relatively new technology. Indeed presenter John Curran of consulting firm LED Transformations took a skeptical approach in his session that seemed targeted at newcomers to SSL. And clearly despite the prevalence of LED technology at LFI, the technology is still new to some lighting designers, architects, building owners, and municipalities.

Component and luminaire life

Curran described known issues with LEDs in lighting such as component life and the need for thermal management. He presented a case study detailing LEDs that maintained L70 (70% of initial brightness) for 148,000 hours when the junction temperature was maintained at 55°C whereas the useful life dropped to 67,000 hours at 85°C.

Philips 75W-equivalent LED lamp

Curran also addressed LM-80 testing and misconceptions saying, “If someone tells you LM-80 is indicative of LED life, they are wrong.” He said LM-80 only tells you how to take the measurements. He expects the TM-21 standard for predicting lumen maintenance to be more valuable saying, “TM-21 will tell you how to connect the dots.”

Not all of Curran’s comments were skeptical. He told the audience that if you buy LED products from a reputable manufacturer that other components would likely fail before the LEDs. But Curran also warned the crowd about the realities of the market discussing retrofit lamps and the consumer market. He quipped that if he bought a $40 LED bulb, “I’m going to keep the receipt for a while.” His actual point was that residential customers are far more upfront-cost conscious than commercial customers, and residential customers look for much faster payback periods preferably inside one year. That could make residential a tough market to crack.

OLED lighting potential

While LEDs have become the workhorse star of LFI, OLEDs remain the technology of untapped potential. The “Creating a vision for OLED lighting session” allowed proponents to once again discuss that potential. The session featured Acuity Brand executives Jeannine Fisher and Peter Ngai, lighting designer Patricia Glasow, James Brodrick from the US Department of Energy (DOE), and consultant Paul Burrows of Reata Research.

Acuity Kindred OLED luminaire

Acuity clearly intended to use the panel to promote its investment in OLED technology and the introduction of the Kindred and Revel OLED fixtures at LFI. Burrow did the heavy lifting in the session providing a relatively complete explanation of how OLEDs work.

Burrows acknowledged the Acuity OLED announcements saying “Before today, there was no real OLED lighting product on the market.” What he didn’t say was that Acuity won’t ship the OLED products until next year and they are likely to be below the 100-lm/W efficacy threshold that most experts believe OLEDs must reach to serve in mainstream lighting. Acuity hasn’t released full specs for the OLED fixtures, but in the OLED session Fisher argued, “60 lm/W is the entry point for being able to mainstream OLED lighting.”

Burrows was probably more candid than Acuity would have preferred in his other remarks. He said that you can easily make decorative OLED fixtures today, “but that won’t save energy.” His point was that OLEDs will have to replace ambient and task lighting for there to be a compelling energy-efficiency story.

Panel efficiency research

In his role at the DOE, Brodrick is squarely focused on energy efficiency. And he said that efficient lighting needs to address “panels rather than pixels” – thus his continued interest in OLEDs as a planar source. Indeed OLED projects won more than $4.8 million of the $14.8 million in R&D funds recently awarded by the DOE.

Burrows did note several potential OLED advantages. He said that OLEDs need simpler driver electronics than do LEDs, and with OLEDs the device efficiency is the luminaire efficiency. He also said that OLEDs will better deliver warm white light.

Still Burrows noted numerous roadblocks. He said today only 20% of the light escapes an OLED source without out coupling. There are also dispersion-pattern and substrate issues. All of the roadblocks can be overcome but for now manufacturing costs that are too high. He said cost today is $50 per square meter and that needs to drop to $20 or less.

Glasow chose to focus purely on the aesthetic advantages that OLEDs offer. She noted that the technology offers better RGB color mixing than LEDs. And she said, “OLEDs let you get rid of those heat sinks.” But clearly energy efficiency and cost will gate wider deployment, and we will wait yet another LFI cycle or more to witness clearance of those hurdles.

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