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ILLUMINATION WITH SOLID STATE LIGHTING

SOMAK MISRA

Electrical and Electronics Engineering

Registration ID: 130906290

Section: C   Roll no: 9

[email protected]

Introduction

Since lighting counts for a large fraction of electrical usage in the industrial sector, the energy efficiency of lamp systems is of major interest. One promising technology which has the potential for use in specialized energy-efficient lighting application: is solid-state lamp technology. Recent developments in this field indicate various prospects for specialized lighting applications, such as in exit signs, where LEDs are now an energy-efficient, and low maintenance alternative for incandescent lamps. There are many reasons solid-state lamps may be especially useful for new applications. They exhibit long lifetimes, on the order of 100,000 hours. Coupled with the ruggedness inherent in solid-state devices, this indicates usefulness for low maintenance applications. Fast response times also make them ideal for some applications; one recent utilization is in high-speed automobile brake lights. There have been several reviews of solid-state lamp applications in the telecommunications and display fields.  LEDs have gained broad recognition as the ubiquitous little lights that tell us our monitors are on, the phone is off the hook or the oven is hot. Still the preponderance of applications requires that the viewer look directly at the LED. In this sense, even the high brightness or high efficiency LED applications are dominated by indicator LEDs. This is NOT “Solid State Lighting”.  Artificial lighting sources are fluorescent tubes, 60-plus Watt incandescent light bulbs, and high intensity discharge lamps etc. which all share three key characteristics differentiating them on the evolutionary tree as a species apart from the indicator lamp. First, they are rarely viewed directly. Light from a lighting source is viewed in reflection off of the illuminated object. Second, the unit of measure (flux) is the kilo lumen (klm) or higher, not the milli lumen (mlm), lm or worse yet the candela (cd) often used for indicator LED lamps. Finally lighting sources are predominantly white, producing good to excellent color rendering. Today there really is no such thing as a commercial “solid state lamp” for use in illumination. However, a branch in the evolutionary tree is forming and differences are beginning to appear in the technologies used for low power LED indicators and the high power LED light sources that will evolve into lighting sources. In this paper we look into the various solid state light sources. Also to analyze the performance of an LED light source against the conventional lighting sources, take a look into its advantages and disadvantages, explore the areas of application and also see how high power LED’s may be the path for solid state lighting.

Basic Terminologies

Luminous Flux

Energy radiated by luminous source per second weighted against the spectral sensitivity of the human eye. It is denoted by

Lumen (lm) is the standard unit for luminous flux.

Luminous Intensity

It is the amount of luminous flux radiated in a particular direction per unit solid angle. It is denoted by I. The unit of luminous intensity is candela (cd).

Illuminance

Ratio of luminous flux incident on a surface to the area of that surface. Also referred to as lighting level. It is denoted by the symbol E and its unit is Lux.

E= {Lumens × (design factors)} / Area

Luminance

Ratio of luminous intensity from a surface in a given direction to the apparent area of the surface. Its  denoted by L and its unit is candela/m2

Types of Solid-State Light Sources

1) Light-emitting diodes: The most common solid­ state lamps are light-emitting diodes (LEDs). They operate at tens of milliamps and a few volts, and have response times of roughly hundreds of nanoseconds. This makes LEDs compatible with integrated circuits and ideal for use as status indicators. In illumination type applications, however, common LEDs display some drawbacks.  First, while the light is proportional to the current at low values, at higher values the intensity saturates, defining a threshold above which the process is inefficient. Second, an LED radiates brightly over only a narrow solid angle, a few tens of degrees. Third, single LEDs emit light over only a narrow spectral range. Fourth, LEDs based on technology from the 1970s are still widely available; their low-rated performance has led to poor perception of solid-state lamps i n general. However, recent brightness and efficiency gains have led to devices comparing favorably with the performance of some conventional lamp systems. New manufacturing methods, materials and ultra-large scale processing signal a possibility for lower costs, contributing to a resurgence of interest in LEDs. An example of this is the replacement of conventional bulb-reflector systems in some automobile brake lamps with LEDs or laser diffuser systems.

2) Diode lasers: Under optical resonance conditions, solid-state light devices may emit coherent light, in which the output is extremely monochromatic, collimated and in-phase. These devices are called diode lasers, and are used in digital signal storage and retrieval, such as for optical discs, and in telecommunications. While the output of diode lasers are typically higher than in LEDs, the emitted light is coherent, and highly directional. In addition, the bulk of commercial laser diodes radiate in the infrared.

3) Super luminescent diodes: Recently, a light source with properties intermediate between laser diodes and LEDs has been demonstrated. These devices are called super luminescent diodes (SLDs). Fabrication methods arc similar to those for diode lasers; an added step "spoils" the process leading to laser action, yielding  high output high efficiency devices with incoherent, narrowband emission. SLDs were developed for sensor and printing applications, but their high power output and efficiency, and low coherence make them good candidates for use in lighting applications. SLDs, however, are not yet widely available commercially.

4) Electroluminescent devices: Electroluminescent (EL) lamps consist of a thin-film or powder phosphor sandwiched between two electrodes. Under an applied electric field, the phosphor fluorescence, and light is emitted through the transparent front electrode. Most phosphor systems involve a ZnS host and an orange activator, Mn. Alternatives include Ce and TbF3 for green, and Pr for white light. EL devices are found in computer display screens; recently, panels have been used as backlighting for exit signs. EL sources are relatively inexpensive, but are relatively inefficient and dim. Their luminance may be increased by raising the operating voltage, but the lifetime then drops drastically.

Applications

Solid-state lamps cannot compete against high­ intensity lamps such as sodium and high-intensity discharge. However, solid-state lamps are com­ pact, robust, use a tenth the power of incandescent, produce no heat and are long-lived. As industry produces less expensive, brighter and more efficient models, we may expect solid-state lamps to make inroads in some niche applications now dominated by incandescent, and perhaps even fluorescent, lamps.

Illuminated Signs

Illuminated signs may be floodlit, backlit (Internally illuminated), or element-lit. Floodlit signs depend on area and brightness for attention, and thus require uniform luminance over a large area. While the inhomogeneous output of LEDs over wide solid angles makes it unattractive for floodlit applications, LEDs are ideal in backlit or element lit signs.

1) Internally-illuminated signs:  In this type of sign a printed translucent panel is backlit by an internal light source. Examples include sign identifying stores, cinema billboards, and some posters in malls and subways.  Light sources include fluorescent lamps, diffused incandescent and mercury lamps and luminous (neon-type) tube lamps. Typical luminance required are 70 to 350 cd /m2 for ordinary signs, 250 to 500 cd/m2 for brighter signs located in malls, 700 to 1000 cd/m2 for commercial signs, 1000 to 1400 cd/m2 for areas of high sign  competition,  and  1400  to  1700 cd/m2   for emergency  traffic  control  conditions .

2) Element-lit signs: These include applications where lamps form alphanumeric or graphic elements. Examples are neon-type advertising and message signs. Unlike backlit signs, where uniform monochromatic illumination is ideal, clement-lit signs make use of color and apparent motion (flashing lights). For low brightness areas, incandescent up to 11W are utilized; for medium-brightness areas, 25W; and for bright areas, up to 40V. Proportional wattages balance the effect for colored bulbs: clear 10W, yellow 10W, orange 15W, red 25W, green 25W and blue 40W. Motion effects running borders and flashing signs-usually use 20W plasma lamps, which require 0.05 to 3 W, and have efficacies up to 0.3 lm/W and lifetimes of 20,000 hours. Solid-state lamps are good candidates in these applications. They exhibit lower power requirements and are longer- lived than incandescent; LED power requirements are similar to glow lamps, are 10-100 times brighter.  The drawback is unit cost. The larger the sign is, the more LEDs are needed, and the higher the system cost. It is thus to be expected that the first impact will be small scale signs, such as exit-signs. Other applications already utilizing LEDs include element lit house numbers, scoreboards in sports venues and tunnels

 Traffic Control Lights

Lamps used in traffic applications required, green and amber output. Thus, the recent development of commercial, green GaP and orange AlGaInP LEDs is crucial to the success of solid-state lamps in this application.

1)Traffic signal lamps: These consist of rugged, long-life (2000-8000 hours) 60-165 W incandescent lamps housed in a fixture containing a reflector and a circular  aperture and lens plate in an appropriate color (green, amber and red). Initial average lumens range, from 675 to 2250 lumens.  From geometric considerations, solid-state signal lamps would require a cluster of some 200 LEDs each of red, amber and green for a single three-light system, or 200 multicolor LEDs. The total power dissipated would be roughly 10 watts, assuming the luminance of green LEDs can be increased to that of red LEDs, at similar power requirements. Even if green LEDs consumed ten times more power than red LEDs, the total power dissipation would be about 40 watts. A three-light traffic signal based on 60W incandescent would consume 60 watts. If the 40W LED system described above were available, it would represent a difference of 47W per system. This estimate would, of course, be revised by actual performance ratings for green LEDs. One can also envision a hybrid system, in which an incandescent is used for the green signal, and solid-state lamps for red and amber signals.  Such a hybrid system would consume approximately 29 watts at any given time, a 50% electrical advantage.

2) Directional arrow signals: Arrow signals consist of long-life 60W to 165W incandescent filament lamps behind green and amber filters and a die-cut arrow mask. A solid state system requires development of green LEDs with ratings higher than that available now. However, the potential unit savings would be greater than for traffic signal lamps, because conventional arrow signals are internally lit, and use 60 W to 165W incandescent. LED arrow signals would be element lit, and require fewer LEDs than for a traffic signal lamp. From size considerations and the same assumptions above, one unit would require 40 LEDs, with a power usage of 1W.

3) Pedestrian signals: These are internally illuminated signs with green (or white) "WALK" and red (or white) "DON'T WALK" or "walking person" pictogram masks. All use incandescent lamps, from 60 to 100 W each. Since the pictogram versions are more universal, these are the versions which have been developed first for solid-state systems. Around 90 to 100 green or red LEDs outline the figure, and the whole fixture is rated at approximately 1 to 5 W, with an electrical benefit similar to that of directional arrow signals.

Because pedestrian lights must be visible over distances shorter than for traffic signal lamps, luminance requirements are less stringent. In addition, the 30-70° fields of view of most LED lamps are adequate for this application. This has made LED­ based systems attractive, and LED versions are beginning to be tested in various areas.

4) Flashing warning beacons: This includes red and amber railway lights and overhead pedestrian crosswalk lights. A particular advantage is that the colors required are those for which commercially-available LEDs perform satisfactorily. Train warning lamps located before a railway crossing, use rugged 75W incandescent lamps, rated at 500 to 1000 hours. LED replacements indicate potential savings in power consumption, maintenance and lamp replacement costs.

Decorative Purposes

This category includes such applications as seasonal holiday lighting, store displays and amusement parks. Solid-state lamps are ideal for these purposes, even relatively low-performance types, because al¬though the lamps must be bright enough to be seen against their backgrounds, the primary concern of these applications is not illumination.  

Other incandescent in use for outdoor decorative purposes are 5W and 10W lamps. Here the energy savings could increase by almost 40% over the exam¬ple above. A typical indoor Christmas tree system consists of a string of 100 miniatures incandescent with a total power consumption of 50 W. A string of 100 LED clusters would have dissipate 2-10W, rep¬resenting a savings of up to 48 W per string.

The promise of solid state lighting

With the convergence in the mid-1990s of major advances  in AlInGaN and AlInGaP material technologies by the turn of the millennium LEDs were rapidly surpassing the efficiency of color filtered fluorescent light bulbs and white incandescent and halogen light bulbs. LEDs inherit other important advantages including lifetimes measured in tens of thousands of hours, ruggedness, environmental friendliness (no mercury), compact size, low operating voltages, and cool operation. Their small size allows design flexibility in the control and steering of the emitted light by utilizing sophisticated secondary optics. However, today’s lighting applications which require a light source to illuminate a desk, a screen, or  a room demand not only high efficiency and long life, but also high flux, all at a low unit cost. A single 60-W incandescent bulb emits 1 klm of white light with a color rendering index near 100; that is 300 times the amount the light emitted by a typical phosphor converted indicator white LED (pc-LED) at a small fraction of the upfront cost. The challenge is designing LED devices and packages that sustain two to three orders of magnitude higher input drive power than traditional  60 mW) indicator LEDs whilst retaining the same high efficiency and re- liability.

The pioneering work on high-power LEDs began at Lumileds Lighting in 1998 with the introduction of the first commercial high power LED. At 1-W input power, Luxeon devices operate at power levels 20 times that of traditional 5-mm indicator LEDs with efficiencies that can be as much as 50% greater. Lifetimes extrapolate into the tens of thousands of hours. Commercialization of high-power LEDs in 1998 has impacted the decades old Haitz’s Law (Fig 1) manifesting as a knee in the lm/LED versus time plot, defining the point in LED evolution when power LEDs diverged from indicator LEDs. Key among Lumileds’ achievements is a dramatic reduction in package thermal resistance from the 300 K/W level of indicator LEDs to less than 15 K/W for the Luxeon line of LEDs. This 20× reduction in thermal resistance enables    devices to be pumped to 20× the input power whilst emitting 55-lm red, 30-lm green, 10 lm blue, about 25-lm (pc-LEDs) white light, for 1W of input power. At 0.025 klm, the white devices are still 40 times below the 1 klm per unit flux threshold for entry into general illumination as single device sources. Philips Lighting utilizing a cluster of 12 Luxeon white sources generating 0.3 klm of white light. Also a next-generation Luxeon LED, which is the world’s brightest white LED at 0.15 klm at 5-W drive, next to a 15W incandescent bulb. This LED generates nearly 40% more light, occupies 1% of the package volume, and requires only 33% of the power of the incandescent lightbulb. Twelve of these high powered 0.15 klm devices are sufficient to make a blue tinted high-intensity discharge (HID) equivalent 1.8 klm automotive headlamp. Single color green versions of the 5-W Luxeon devices offer luminous fluxes in excess of a 0.13 klm per package. Two of these Luxeon sources can replace the 150-W light bulb in a typical or US traffic signal resulting in 90% energy savings. RGB combinations of Luxeon sources have efficiencies that even rival those of cold cathode fluorescent lamps. For example, applications such as backlighting for LCD televisions and monitors take advantage of the compact source size and narrow color bands, while providing extended color range, ruggedness, and eliminating high-voltage power supplies.

The future of solid state lighting

Very efficient white LED bulbs are replacing inefficient incandescent bulbs and while there are many technical and market challenges to sort out, it is becoming clear that new capabilities and functionality beyond simple bulb/socket lighting systems will become the future of solid state lighting. These LED replacement bulbs will only be the first wave of solid-state lighting, and digitally controlled full gamut light sources, high performance light sensors and new control and communications methods will create what could be called the second wave of solid state lighting, or Smart Lighting. Smart Lighting systems are comprised of illumination technologies that fully utilize the spectral and temporal capabilities of LED technology to provide light, automatically adapt color and intensity to changing illumination requirements, and provide enhanced services beyond simple illumination. Smart Lighting functionality will position illumination as a vital, interactive part of building, energy, information and even healthcare and entertainment systems. The key will be the development of lighting systems that can think, providing exactly the right light where and when it is needed.

Fig 1: Haitz’s Law for LED flux—LED flux per package has doubled every 18–24 months for 30+ years. The Luxeon series of high power LEDs has dramatically improved the performance of LEDs causing the graph to manifest a knee in its plot.

Fig 2: Evolution of the LED lighting technology.

First Wave Status

Solid State Lighting solutions are now capable of replacing most conventional lighting sources, and technical development has largely turned from increasing the efficacy of LED sources (which is rapidly approaching the point of diminishing returns) to reducing the cost of LED light sources to drive more rapid market adoption. The focus on cost is driving continued rapid technology development across the entire solid state lighting supply chain. Everything from substrates used for epitaxial growth of LED structures to new integrated chip designs (high voltage and AC LEDs) and integrated control systems are under development. New thermal and optical materials are also being developed to address both performance and cost issues associated the design of LED packages and operation of lighting modules. Even the rules of illumination design specifications are being rewritten to accommodate the idiosyncrasies and new capabilities of solid state lighting. It is quickly becoming apparent that solid state lighting will outgrow the constraints of conventional bulb/socket lighting system design.

Smart Lighting - the Second Wave

Spectral tunability and high speed LED switching capability, combined with networks of advanced sensors and adaptive lighting communications and control architectures will lead to the development of sensory lighting systems that adapt illumination to the needs of those occupying the illuminated space. New features will enable improved lighting system energy efficiency as well as improved human health and productivity. Future illumination systems will feature both intra-system (light-to-light) optical communications and data communications that will augment RF based communications systems for scalable, secure wireless communications. In addition, digitally modulated (and structured) lighting offers novel approaches to occupancy sensing (of both animate and inanimate objects). By improving the sensory functionality of illumination systems, adaptive lighting control methods superior to the current lighting management methods in use today will be possible.

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