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4 CHAPTER TWO CURRENT PRACTICES RELATED TO LED TRAFFIC SIGNAL MONITORING, MAINTENANCE, AND REPLACEMENT ISSUES This chapter focuses on current practices related to LED traffic deficient material that forms a junction. Power applied to this signal monitoring, maintenance, and replacement. However, junction excites the electrons in the electron-rich material it is useful to understand how we progressed to the current set leading to photon emission and the creation of light. Depend- of issues. ing on the chemical composition of the semiconductor layers, the color of light emission will vary within the electromagnetic EVOLUTION TO LEDs spectrum. Historically traffic signals were illuminated by turning on an The individual diodes are grouped together to form a traf- incandescent (60 W to 150 W) lamp behind either an 8 in. fic signal where, depending on the individual LED size, up to or a 12 in. lens tinted red, yellow, or green. Conventional several hundred "lamps" are packaged into an array to form pedestrian and arrow signal indications are similarly illumi- a traffic signal head. The "Vehicle Traffic Control Signal nated by incandescent lamps. Heads: Light Emitting Diode (LED) Circular Signal Supple- ment," July 27, 2005 (VTCSH LED 2005), the current ITE Incandescent lamps produce light by passing electrical performance specification for circular signal indications, current through a (typically, tungsten) filament. The efficacy specifies a light display more consistent with traditional of light production depends on the temperature of the filament. incandescent lamps and lens (1). A specification for arrows, Higher temperatures yield a greater portion of the radiated "Vehicle Traffic Control Signal Heads: Light Emitting energy in the visible spectrum but may adversely affect fila- Diode (LED) Vehicle Arrow Traffic Signal Supplement," ment life. The electrical resistance to the flow of electric cur- has also been published (2). rent in tungsten is 12 to 16 times greater when hot than at cold temperatures. The lower cold resistance produces an LEDs are much more energy efficient than their incan- inrush current that lasts about a tenth of a second. Light out- descent counterparts for several reasons. LEDs are very put (measured in lumens) depreciates over the life of the energy efficient, producing light output with very little heat lamp; typically less than a 20% to 25% reduction at the end while incandescent lamps use a lot of energy generating heat. of rated life. Incandescent lamps only produce white light, which must be filtered for traffic signal use, and this leads to an additional General purpose incandescent lamps typically have a loss in energy. LEDs, on the other hand, produce colored rated life in the 750-h to 2,500-h range. Incandescent lamps light that quite often does not need to be filtered--all of the sold specifically for traffic signals typically have a rated life energy is concentrated around one color band and none is of approximately 8,000 h. Typical initial lumen output for a "wasted" on undesired colors. 135 W traffic signal incandescent lamp is approximately 1,750 lumens. One significant difference in LEDs is that they rarely experience catastrophic failure, as do all incandescent While incandescent lamps have been used in a variety of lamps, although their light output continuously degrades applications for many years, alternatives have been slowly over their significantly longer life. This has the potential to replacing incandescents in many homes as well as outdoor have a "dim" indication that will not be detected by electri- and industrial locations. In most cases, the change from cal current monitoring methods that determine failure by a incandescent has been driven by more efficient light sources. total lack of output resulting from a failed filament in an However, until the 1990s, the incandescent lamp was the pri- incandescent lamp. Also, because of the electronics power- mary light source in traffic signal control applications despite ing the LEDs, the LED traffic signal design must account changes in other applications. for a number of electrical issues including turn-on time, turn-off time, and failed impedance state in order for the An LED is a semiconductor device that creates light safety monitoring device [conflict monitor (CM) or mainte- using solid-state electronics. A diode is composed of a layer nance malfunction management unit (MMU)] to perform of electron-rich material separated by a layer of electron satisfactorily.