A typical streetlight fixture consists of several components – a luminaire, a pole, and a control device. A luminaire is a complete lighting unit consisting of one or more lamps (the light source), other parts that hold the lamp in place and protect it, wiring that connects the lamp to a power source, and often a reflector that helps direct and distribute the light. The housing of the luminaire is usually made of steel or aluminum. It has a lens that shields the lamp from exterior elements that also allows light to pass through.
The pole, usually steel, is a unit that supports the luminaire and contains the electrical wiring that provides power to it. At the bottom of the pole is a base plate. The base plate is a horizontal plate welded to the pole where it mounts to the ground. Bolts through the base plate hold the pole in place. A cover is often used to protect the bolts.
A street light fixture may have an arm near the top of the pole to hold the luminaire, depending on the design. The arm is sometimes referred to a bracket. A streetlight fixture with no arms, but a single luminaire at the top of the pole, is referred to as a post top.
A device may be added to a luminaire to control its usage. Conventional controls often include a photocell, installed in a National Electrical Manufacturers Association (NEMA) twist lock at the top of the luminaire or below the fixture in the case of a post top. A photocell monitors exterior light levels and switches on/off an individual luminaire independent of adjacent luminaires. The City of San Francisco plans to implement a wireless control system for new lighting installations. A wireless system includes a control device as a component in each luminaire. In some cases, the control device may be located inside the pole or surface-mounted on the luminaire, replacing the photocell. The control device is a receiver and transmitter that collects control information from a remote hub and sends status reports to that hub. The hub is also a receiver and transmitter that networks several fixtures to a data control center. The complete system is known as a streetlight asset management system because it can be configured to send luminaire status reports regarding power usage, outages, etc. and can provide dimming control of dimmable sources.
There are several lamp types currently in use by the SFPUC, including some outdated types such as incandescent and mercury vapor lamps. HPS, ceramic metal halide, and LED lamps are specified in the streetlight catalogue. The following discussion describes the light sources found in the streetlight catalogue, and their benefits.
High Pressure Sodium (HPS) - Lamps
Historically, the HPS lamp has been the most efficacious light source when compared to the common streetlight sources such as mercury vapor and metal halide lamps. The yellow-spectrum HPS lamp slowly dims over time but does not completely extinguish. The lamp is known for "cycling," whereby it switches on and off when it has reached the end of its life. When cycling, the arc within the lamp extinguishes and the lamp must cool down before the starter circuit initializes a new arc. HPS lamps begin to suffer end-of-life cycling before the amount of useful light becomes visibly diminished.
Metal Halide - Lamps
Metal halide has the best color rendering and most “white” light of the high intensity discharge lamps (metal halide, HPS, low pressure sodium) that are typically used for street lighting and so manufacturers are interested in developing its full potential. Ceramic metal halide lamps have better color rendering and color stability than standard metal halide lamps which have color shift over the life of the lamp. Improvements will be made to ceramic metal halide lamp technology and high wattage lamps will be added to the product selection.
The Philips CosmoPolis metal halide lamp, introduced in 2009, is designed to operate with a CosmoPolis ballast. This lamp type is used by one of the fixtures in the catalogue. This lamp/ballast system provides highly efficient light output, 30,000 hours of life compared to 20,000 hours of other metal halides, and “white” light that is stable over the life of the lamp. Most impressive, is the system ability to step dim the light output to 60% while maintaining good color characteristics. It is available in 60 watt (W), 90W and 140W. Light fixture manufacturers are developing streetlight fixtures that use this system. The CosmoPolis lamp cannot be used to retrofit an existing HPS lamp.
Light Emitting Diode (LED) - Lamps
A LED is unlike conventional light sources/lamps. It is essentially a semi-conductor on a circuit board. When used in a streetlight fixture, an array of multiple LEDs is required to produce enough light output. Each unique array design requires a uniquely compatible system housing to manage the heat generated at the circuit board and to control the photometric output. LEDs and their arrays are not interchangeable from fixture type to fixture type as are conventional lamps.
LEDs are point sources that emit light in directional beams. Additional reflectors are not often required in the fixtures. Fixture manufacturers more often instead use lenses to diffuse and modify the beams either over each individual LED or across a full LED array. Because each LED in an array can be aimed individually, LEDs can be designed to provide more precise beam control in streetlights than conventional light sources.
LED technology is the most rapidly advancing category in street lighting today, even though they have been around for a long time. Only recently, due to technological advancements, have they become suitable for general illumination purposes.
Benefits of LED Technology in Street Lighting
• High efficacy light fixtures: 60-80 lumens per Watt
• Long life: 50,000 -100,000 operating hours before lumen output depreciates to 70% of initial output (based on extrapolated estimates)
• More precise beam control than other light sources
• No mercury content
• No ultra violet light
• Dimmable control
• No warm-up time to full output