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Pt.12: LED Lighting For Traffic Lights & Signs Electric By JULIAN EDGAR Lighting New manufacturing techniques are producing high-brightness LEDs in a variety of colours. Their applications include traffic lights, street signs pathway lighting and vehicle tail lights. Light Emitting Diodes (LEDs) have been used as indicators and in displays since the early 1970s. However, it is only recently that LEDs have been produced with sufficient brightness to allow their use in applications where they can directly replace incandescent and fluorescent lamps. LEDs can now be found providing the light source in some torches, traffic lights, vehicle tail 82  Silicon Chip lights and even in gardens. In fact, some prototype high-brightness LEDs now have luminous efficacies exceeding those of incandescent lamps and rivalling mercury and fluorescent lamp technologies. Depending on the application, LEDs can give clear benefits in terms of lamp life, lumen depreciation and efficacy. However, LEDs can have some signif- icant disadvantages as well. Light Emitting Diodes LEDs are basically solid-state devices with a p-n semiconductor junction. When a forward voltage is applied to the p-n junction, the charge carriers inject across the junction into a zone where they recombine and convert their excess energy into light. The materials used at the junction determine the wavelength of the emitted light. Fig.1 shows the internal structure of a LED, while Fig.2 shows the performance details of the latest LEDs, ranging from red to blue in colour. The aluminium indium gallium phosphide (AlInGaP) LED is one of the more recent designs and has been used to develop yellow, amber and red LEDs (incidentally, aficionados of LED design pronounce AlInGaP as “Allen Gap” – something to remember if you want to impress!). The use of this material results in much lower lumen depreciation over the life of the LED. More recently, indium gallium nitride (InGaN) has revolutionised green and blue LEDs – just look at the 200 times improvement in the efficiency of the InGaN blue LED over the previous SiC (“sick?”) design! Although the luminous efficiency of LEDs has greatly increased in recent years, many LEDs must be used together to produce a large amount of light. LEDs emit light which is highly saturated and nearly monochromatic. Fig.3 shows the wavelengths of light developed by a variety of Hewlett Packard Super Flux LEDs. White LEDs are a recent development and can be constructed in a number of ways. The first technique is to add a phosphor to the epoxy of a blue LED. The Nichia Corporation of Japan and Siemens of Germany have developed this process, whereby a layer of phosphor material is used to translate most of the light emitted from a blue LED die into a wide band of essentially white light. The first LEDs to use this technique were quite inefficient, with a net luminous output only 17% that of a blue LED operated at the same current. However, the more recent Siemens designs use gallium nitride (GaN) or indium gallium nitride (InGaN) blue LEDs coated with a luminescent pigment based on Y3Al5O12 doped with caesium ions. This phosphor is actually incorporated into the epoxy resin coating of the LED. These white light LEDs are better than earlier designs, being currently about 20% more efficient than incandescent lamps. Fig.4 shows the spectral output of the Siemens white LED. Mixing the light from blue, green and red LEDs can also generate white light. Similar in nature to RGB colour displays, these white LEDs employ three separate colour dies (red, green, FACING PAGE: these traffic lights show their green lights for 99% of the time, 24 hours a day. Replacing the green incandescent bulbs with LED signal indicators would save a considerable amount of energy. blue) in one device to mix the three primary colours and thus produce white light. In summary, it’s now possible to produce high-brightness LEDs in a range of colours. This makes them particularly attractive as light sources in road signs and traffic lights. BALL BOND & TOP CONTACT GOLD WIRE LED DIE WEDGE BOND REFLECTOR CUP ANODE LEAD CONDUCTIVE EPOXY DIE ATTACHMENT Traffic lights CATHODE LEAD Incandescent lamps have been used in traffic lights for over 70 years. Fig.1: the internal structure of a LED. Other lamps that have (Hewlett Packard). been considered in the past include cold-cathode fluorescent lamps, in the traffic signal may be on for more electro­ l uminescent panels and high-frequency fluorescent lamps. than 99% of the time. Inevitably, this means that some of However, LEDs in traffic lights have now become widespread, especially the lamps within the array need replacing earlier than others. However, in the USA. This is primarily for two for safety reasons, all the lamps inside reasons: (1) longer lamp life; and (2) traffic lights are generally renewed at lower power consumption. the same time, rather than when failLamp requirements ure requires it. Long life (8000 hour) Although seldom considered by Krypton gas-filled incandescent lamps are replaced yearly in some locations. most people, traffic lights place unique demands on lamps. First, the This approach results in high mainlamps of a particular colour within the tenance costs and disrupts the traffic array generally burn for longer hours during lamp replacement. The incandescent lamps used in than the others. For example, in many installations, the lamps behind the red traffic lights are quite high-powered, being typically 67-150W. The wattage lenses are illuminated for the longest periods, while in some pedestrian required varies with the colour – red crossing applications the green lamp signals require the highest wattage, Fig.2: LED Performance Colour Material Dominant Wavelength (nm) Luminous Efficiency (lm/W) R ed TS AlInGaP TS AlGaAs AS AlGaAs GaAs 630 644 637 648 15 10 4 0.1 Reddish/Orange TS AlInGaP AS AlInGaP AS AlInGaP AS AlInGaP GaP GaP 617 605 615 622 626 602 20 10 10 8 1 1 Amber/Yellow TS AlInGaP AS AlInGaP GaP 592 590 585 20 10 1 Green InGaN InGaN GaP GaP 525 505 569 560 15 10 3 0.7 B l ue InGaN Si C 470 481 2 .01 MARCH 1999  83 The use of LEDs in traffic light signals gives a massive decrease in power consumption. Signals using red LEDs have been used in the USA for some time and green LED indicators suitable for use in traffic lights have also recently been released. (Dialight), while green and amber signals require lower wattages. In the US, it is estimated that there are 3-4.5 million traffic signals operating, each of which has an approximate annual energy demand of 990kW/h. Together, they use nearly three billion kW/h per annum. The traffic lights in California alone are estimated to consume 310 million kW/h per year. As a result, low current consumption LEDs have major advantages in traffic light applications, particularly While early traffic light designs used over 300 LEDs, more recent designs based on the latest high-brightness devices have reduced this to just 18. This traffic light has a power consumption of just 14.5W, while incandescent lamps vary from 67-150W. (Dialight). when it comes to longevity and saving energy. The LEDs used in red and amber traffic lights use an aluminium indium gallium phosphide (AlInGaP) construction. Special lens structures are used to direct the light and the epoxy packages of the LEDs contain ultraviolet-A and ultraviolet-B inhibitors, to reduce the effects of long-term exposure to direct sunlight. Intensities of up to 4500mcd <at> 20mA are available in LEDs with 15° viewing angles, dropping to 2800mcd Fig.3: this graph plots the wavelengths of light produced by four Hewlett Packard LEDs. As can be seen, most LEDs produce monochromatic light. This gives LEDs advantages in some forms of lighting and disadvantages in others. (Hewlett Packard). 84  Silicon Chip at 23° viewing angles. The red LEDs have a dominant wavelength of 630nm, while the amber LEDs emit light predominantly at 592nm. Green LEDs use indium gallium nitride (InGaN) construction with a wavelength of 505nm and intensities of up to 2300mcd <at> 20mA with a 23° viewing angle. Energy savings In the US, the Massachusetts Highway Department last year replaced all red incandescent bulbs in that state’s highway traffic lights with red LEDs. The $US1.8 million cost was partially supported by a $US250,000 grant from several energy companies, while annual power savings of $US340,000 also helped reduce the financial pain. The state of Philadelphia also has one of the largest LED traffic light installations in the world, with 14,000 LED lights installed since 1992. When the Philadelphia LED traffic light installation program is completed this year, it is expected to reduce power demand by 1MW and save just under $US1 million per year in electricity costs. It is estimated that changing just the red lights for LEDs at an intersection saves $US50-100 per year in reduced energy consumption. In addition, the low power consumption of the LED units allows effective battery backup during power cuts. In a traffic light application, the life of the LEDs is expected to be about 10 years, which is about 5-10 times the life of incandescent lamps. Depending on energy cost, the cost of the LED unit and possible financial incentives offered by government or energy utilities, the payback period can vary between one and seven years. What’s more, the costs are steadily falling. The cost of a red LED traffic light unit has fallen from $US750 when they were first introduced, to $US350 by 1993 and $US230 in 1995. Since then, the price has fallen even further, with the current price now just $US110. The first traffic lights using LEDs had an array of no less than 324 LEDs behind each lens. However, a joint venture between Philips Lighting and Hewlett-Packard has recently resulted a new LED “light engine” that contains just 18 LEDs. When used in conjunction with a special polycarbonate lens, the nominal power rating of the light source has been reduced from 25W to just 14.5W. The new lamp features automatic temperature compensation and includes correction circuitry for power factor and harmonic distortion. This ensures a power factor of greater than 0.9 and less than 20% THD, the latter being important in minimising noise on system lines (early LED traffic signal units had power factors of less than 0.6). Unlike an incandescent lamp (which greatly varies its light output according to input voltage), the High-intensity coloured LEDs can easily be used in arrays to make arrow signals. (Dialight) intensity of the LED system does not alter by more than 10% from the value at 117VAC, over a range from 80-135VAC. Although only the red incandescent lamps are replaced in many installations, green LED traffic signals have also recently been released and these are now also being used in increasing numbers. Temperature compensation Temperature compensation circuitry in LED traffic lights is required because the luminous output of the LEDs varies with temperature. The rate of variation in luminous output depends on the materials used within the LED and ranges from about 1% per Fig.4: the spectral output of the Siemens white LED. The phosphor layer (the “converter”) considerably broadens the spectrum of the emitted light. (Siemens). °C for some red and orange LEDs to 0.4% per °C for some blue and green devices. For example, at -40°C, AlInGaP LEDs have an output that’s 192% of the value measured at 35°C. Conversely, at 55°C, the luminous output is only 75% of that measured at 25°C. Elevated temperatures frequently occur during LED lamp operation. These elevated temperatures are caused both by the ambient conditions in which the lamps are operating and by the heat generated by the LEDs themselves. The latter source can contribute as much as 25-30°C in traffic light applications. The greatest problems are likely to occur when the temperature within Fig.5: a temperature compensation circuit is used to maintain LED brilliance with ambient temperature changes. In this case, a photodiode is used to monitor the LED output and the circuit responds by increasing the current when the LED dims. (Hewlett Packard). MARCH 1999  85 This US pedestrian crossing sign uses a raised hand (for don’t walk) and a symbol of a human figure (for walk). High-intensity blue LEDs are now being trialled for these applications. The elderly, especially, find blue LEDs very visible. (Dialight). the traffic signal housing reaches 75°C. Since most LED modules are retrofitted into unventilated signal heads, heat can rapidly build-up due to solar radiation and adjacent incandescent lamps – this in addition to the heat that the lamps generate themselves during operation. As a result, LED junction temperatures can reach 93°C or more! If steps are not taken to address this situation, the diminution in light output that results can be as much as 65%. It should be noted that such a decrease in lamp output is most likely to occur when the Sun is at its brightest – just when the traffic lights need to be as bright as possible! The internal heat generated by a LED can be minimised by keeping 86  Silicon Chip the thermal resistance of the LED die/lead assembly as low as possible. Using copper lead frames instead of the more common steel lead frames helps to achieve this. Another approach is to automatically supply additional current to the LEDs as they dim, using an electronic control circuit. However, this approach is only feasible if provision for heat removal from the LED dies has been made, otherwise thermal runaway can occur. This means that heatsinks and ventilated traffic signal housings are required when variable current supply techniques are used. Some recent designs include temperature-compensating drive circuitry to maintain legally-required luminous intensities over a temperature range from -40°C to +74°C. Fig.5 shows a suggested temperature compensation circuit for maintaining a constant LED brilliance. It is essentially a current source with feedback to a photodiode. The op amp’s output drives the base of a PNP transistor (Q1) which supplies current to the LED. As the temperature increases, the intensity of light produced by the LED decreases. This reduces the amount of light falling on the photodiode and thus reduces the photodiode current, thereby increasing the amount of current fed through the feedback resistor (Rf). This causes the op amp to increase the drive to the PNP transistor and thus increases the LED current. So the LED’s luminous output is maintained at a constant value. Long exposures to high temperatures can also cause a permanent reduction in LED light output. Indeed, the normally quoted 100,000 hour life (to half-intensity) of LEDs is probably not applicable to the typical operating environment of traffic signals, the LEDs in fact having a much shorter useful life. One study showed that LED traffic signal intensity was reduced by 27% from its initial value after just two years. This means that LED traffic lights need to be tested for light output on a regular basis, as the LED signal may remain operational well past its useful or “safe” life. Colour blindness One potential problem with LED traffic lights concerns recognition by people who are colour-blind. Approximately 8% of men and 0.5% of women have congenital red-green deficiency. Incandescent lamps produce light over a wide spectrum, so even when the light is colour-filtered, it still has a fairly wide spectral band across many wavelengths. While individuals with colour blindness may perceive such lights as being less intense than colour-normal people, the decrease in brightness is moderated because the individual still sees many wavelengths at normal brightness. Slightly increasing the luminous intensity of incandescent lamps above that required for colour-normal people can thus compensate for the colour deficiency. Conversely, LED traffic signals have near monochromatic characteristics – ie, the light produced covers a very narrow spectral band. If this narrow band lies within the spectral region where the individual’s visual sensitivity is poor, the traffic light may not be seen or recognised quickly. Although the intensity of the illumination could be increased, this could cause the light to be too bright for people who aren’t colour blind. To overcome this problem, AlInGaP and InGaN LEDs that produce peak wavelengths throughout the visible spectrum are being developed. SMART FASTCHARGERS® 2 NEW MODELS WITH OPTIONS TO SUIT YOUR NEEDS & BUDGET Now with 240V AC + 12V DC operation PLUS fully automatic voltage detection Use these REFLEX® chargers for all your Nicads and NIMH batteries: Power tools  Torches  Radio equip.  Mobile phones  Video cameras  Field test instruments  RC models incl. indoor flight  Laptops  Photographic equip.  Toys  Others  Blue LEDs One interesting recent development is the use of high-intensity blue LEDs in pedestrian “Walk” signals. In the US, these signals use emblems depicting a walking person (walk) and a raised hand (don’t walk). The use of gallium nitride (GaN) blue LEDs in these signs gives them high visibility, without the risk of the signs being misinterpreted as signals for drivers. Another attraction is that the blue LEDs provide excellent visibility for the elderly. That’s because as people age, their visual colour sensitivity shifts towards the blue end of the spectrum. The first generation of blue LEDs was based on silicon carbide (SiC) and had very poor luminous efficacy. However, several years ago, Japan’s Nichia Corporation developed a new process to produce highly efficient, brilliant blue LEDs. These devices develop light intensities an order of magnitude greater than their predecessors and other manufacturers have since followed suit. Typically these blue LEDs produce dominant wavelengths in the range of 450-470nm. Initial testing of high-intensity blue LED “Walk” indicators was carried out by the Texas Transportation Institute at Texas A&M University. In the daytime, both normally-sighted viewers and those with a degree of colour blindness preferred the blue LED indicators over the standard incandescent indicators by margins of 80% and 50% respectively. However, at night the picture changed. In this case, 73% of people with colour blindness preferred the blue LED signal but this dropped to only 25% for those with normal vision, the latter seeing the sign as too bright and “blurry”. As a result, a digital night dimming Rugged, compact and very portable. Designed for maximum battery capacity and longest battery life. Fig.6: a blue LED walk sign as seen through a pair of blue sunglasses. Because most LEDs emit light over a very narrow spectrum, the effects of blue-blocker sunglasses and other filters need to be carefully researched. The latest bright blue LEDs have a relatively wide spectrum compared to other LEDs, so the sign is still quite visible. (Hochstein). AVOIDS THE WELL KNOWN MEMORY EFFECT. SAVES MONEY & TIME: Restore most Nicads with memory effect to capacity. Recover batteries with very low remaining voltage. CHARGES VERY FAST plus ELIMINATES THE NEED TO DISCHARGE: charge standard batteries in minimum 3 min., max. 1 to 4 hrs, depending on mA/h rating. Partially empty batteries are just topped up. Batteries always remain cool; this increases the total battery life and also the battery’s reliability. DESIGNED AND MADE IN AUSTRALIA For a FREE, detailed technical description please Ph: (03) 6492 1368 or Fax: (03) 6492 1329 2567 Wilmot Rd., Devonport, TAS 7310 system was added to the design. One lingering area of concern regarding the use of blue LEDs for traffic signal applications is the availability of “blue blocker” sunglasses. It has been suggested that these could reduce the visibility of the monochromatic light produced by blue LEDs. However, unlike other LEDs, blue GaN LEDs emit energy over a relatively wide band. For example, the spectral output of the Nichia NLPB500 blue LED is over 75nm wide, whereas a Hewlett Packard CJ-15 “Portland Orange” LED has a spectral output less than 17nm wide. As a result, it is quite difficult to filter out the light emitted by broad­ band blue LEDs using a narrow band optical filter such as a pair of blue-tinted sunglasses. Fig.6 shows the appearance of a blue LED walk sign with blue sunglasses placed on top. While the reduction in luminous intensity is significant, the LEDs are still clearly visible. Next month, we will look at the use SC of LEDs in vehicle lighting. MARCH 1999  87