The Revolution in Street Lighting

The Revolution in Street Lighting

A Brief History

The lighting of streets began during the Greek and Roman civilizations with the use of oil lamps. The first electric streets lights used carbon arc lamps and were first used in the mid-1800’s. By the late 1800’s incandescent lamps began replacing the carbon arc lamps. Incandescent lamps were cheap to produce but were highly inefficient. (80% of the energy lost to heat) The mercury vapor HID (high-intensity discharge) lamps were first introduced in the 1950’s. General Electric and Westinghouse were the major producers of MV lamps for street lighting. In the 1970’s the high pressure (HPS) and low pressure (LPS) sodium lamps were introduced followed by the metal halide (MH) lamp. The HID lamps provide much longer lamp life and efficiency than does the incandescent lamp. Incandescent lamps have been used in traffic signals since their inception, but recently a new kid on the block has shown up – the light emitting diode (LED). Beginning in the 1990’s replacing incandescent lamps in traffic signal heads has transformed the operation of traffic signals to the point where battery backup systems can operate the signals for extended periods of time during power outages due to the low power consumption of the LED’s.

The Introduction of LED Street Lights

Because of their long life and lower power consumption, LED’s are a natural for use in the lighting of streets and roadways. However, before LED’s could be used in outdoor lighting installations the technology and performance characteristics of these devices had to meet or exceed those of the commonly used HPS and MH luminaires. HPS and MH technologies continue to evolve and improve slowly, but the technology for LED luminaires is now improving rapidly. The characteristics commonly measured for luminaires include energy efficiency, color quality, optical design, thermal management, and cost. LED luminaires are now competitive in most all of these characteristics. Because of these factors the number of lighting manufacturers producing LED street and area lighting fixtures is skyrocketing in just a short time.

Effects on Environment

LED luminaires contain no mercury or lead making them environmentally safe to handle, maintain and dispose of.

Luminaire Efficacy

Energy efficacy is measured in lumens per watt (lm/W). (A lumen is a measure of luminous flux.) A comparison of photometric data for three types of luminaires is shown in the table.

The table below demonstrates two things: light distribution efficiency and energy savings.

*fc = foot-candle = 1 lumen/sq. ft.

A more uniform distribution of illuminance on the roadway (lower max/min ratio) reduces bright spots.

Color Quality

The color of illumination can be described by two independent properties: correlated color temperature (CCT) and color rendering index (CRI). For street and area lighting CCT’s in the range of 3000-5000K are good. A CRI of 50 or more is good for street lighting. Table 3 illustrates the color qualities of different light sources.

Optical Design

LED luminaires can provide better distribution of light than HID lamps because the individual point source LED’s in the luminaire array can be directed to illuminate a specific area on the ground. LED luminaires can be made to meet the light distribution pattern types, I-V, as promulgated by the Illuminating Engineering Society of North America (IESNA).

Life and Lumen Maintenance

LED luminaires can be expected to operate for 60,000 hours (13 years) or more whereas HPS lamps have a life expectance of less than 30,000 hours and MV lamps have a life expectance of less than 10,000 hours. Lamp Lumen Depreciation (LLD) is the percentage of initial lumens of a lamp at the end of its expected useful life. The LLD for MH lamps is about 60% and 80% for ceramic MH and HPS lamps. The LLD for LED’s in an outdoor environment is estimated to be over 90%.

Cost

The initial cost of LED luminaires is higher (25-50%) than for conventional HPS or MH luminaires, but the return on investment for LED luminaires because of their lower maintenance and energy costs can be as short as 3-4 years.

Checklist

New LED lighting products are being introduced almost on a daily basis. It is important when considering the installation of new or replacement lighting to consider the following:

• Ask for photometric test reports based on the IESNA LM-79-08 test procedures.
• Ask about warranty; 3-5 years is reasonable for outdoor luminaires.
• Check ingress protection (IP) ratings, and choose an appropriate rating for the intended application.
• Ask for operating temperature information and how this data relates to luminaire efficacy and lumen depreciation.
• Check color temperature for suitability in the intended application.
• Assess glare, preferably with luminaire at intended mounting height and under typical nighttime viewing conditions, compared to incumbent technology
• Evaluate economic payback, based on applicable energy, equipment, maintenance, and control costs for the site.

RKH is a SSL Quality Advocate. When considering new LED lighting make sure the product carries the Lighting FactsCM label.