Volume 9 Issue 3
July 2006    
application - The use to which a lighting system will be put; for example, a lamp may be intended for indoor residential applications. ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural. compact fluorescent lamp (CFL) - A family of single-ended fluorescent-discharge light sources with small-diameter [16-millimeter (5/8-inch) or less] tubes. high-intensity discharge (HID) - An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps. grid - The combination of electric power plants and transmission lines operated by an electric utility. lamp - A radiant light source. lumen (lm) - A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT. efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. halogen lamp - An incandescent lamp that uses a halogen fill gas. Halogen lamps have higher rated efficacies and longer lives than standard incandescent A-lamps. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. luminance - The photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (square feet or square meter). glare - The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. footcandle (fc) - A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. illumination - The process of using light to see objects at a particular location. PN junction - For light emitting diodes, the portion of the device where positive and negative charges combine to produce light. fluorescent lamp - A low-pressure mercury electric-discharge lamp in which a phosphor coating on the inside of the glass tubing transforms most of the ultraviolet energy created inside the lamp into visible light. inverter - Also known as “power inverter.” A device used to convert direct current (dc) electricity into alternating (ac) current. irradiance - The density of radiant flux incident on a surface. light-emitting diode (LED) - A solid-state electronic device formed by a junction of P- and N-type semiconductor material that emits light when electric current passes through it. LED commonly refers to either the semiconductor by itself, i.e. the chip, or the entire lamp package including the chip, electrical leads, optics and encasement. photon - A small bundle or quantum of electromagnetic energy, including light. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels.
What factors should be considered when selecting a luminaire for PV lighting?

Luminaires for photovoltaic (PV) lighting systems are similar to those for other lighting systems; however, more components generally need to be included with PV-powered luminaires than with traditional grid-powered luminaires. The main functions of a luminaire for a PV lighting system are to:

  • House some or all of the system components, i.e., PV panels, batteries, electronics, and light source
  • Control the light output from the light source so it is directed where it is needed to light a particular area effectively

Light output control is generally a function of the luminaire's optical components (i.e., reflector and lens). These components typically are the same for PV-powered luminaires as for grid-powered luminaires. The optical efficiency of a luminaire determines what portion of the light output from the light source can be delivered out of the luminaire. The higher the optical efficiency, the better; however, issues such as glare, light distribution, and illuminance uniformity must also be considered.

A major difference between PV-powered and grid-powered luminaires is the need to house additional components within or adjacent to PV-powered luminaires. Some of these components, such as PV panels and batteries, can be relatively large. In small PV-powered luminaires such as garden path lights, these components are generally incorporated directly into the luminaire itself. In larger luminaires such as parking lot and roadway luminaires, various components are generally attached to the pole or other structure that supports the luminaire.

The illustrations in Figure 6 represent the approximate size of PV lighting systems required to provide two different light levels on the ground beneath a pole-mounted luminaire: 10 lux or 100 moonlights and 0.5 lux (5 moonlights). This assumes a luminaire that has a single light source and an optical efficiency of 50%, which uniformly distributes all light output on a circular area with a radius equal to the pole height.

A system able to provide sufficient luminance to act as an indicator light, such as garden pathway luminaire, is also illustrated. Figure 6 provides a means to compare the relative size of the PV panel, mounting structure, and battery necessary for each of the three systems.

Figure 6. Illustration of three PV lighting systems

As seen in Figure 6, the PV panels required to power larger scale luminaires designed to provide 10 lux (100 moonlights) or more on the ground tend to be relatively large in comparison to the luminaire itself. This may present some aesthetic concerns, especially in applications where decorative luminaires are used or in historic areas, where the public expects luminaires with a traditional style. The mounting pole for larger scale PV lighting systems typically will need to be stronger to support the PV panel and handle the wind load on the system. Finally, a weather-tight compartment of some type will be needed to house the battery required by the system. All of these components add to the cost of the lighting system.

In comparison, the smaller-scale post-top luminaire designed to provide 0.5 lux (5 moonlights) on the ground is much more compact, and both the PV panel and battery are small enough to be incorporated directly into the design of the luminaire itself. Because of the small size of this luminaire, the mounting pole can also be smaller.

When low light levels such as 0.5 lux (5 moonlights) are considered, only light-emitting diodes (LED) and incandescent lamps are available in lumen packages small enough to provide these levels effectively. It is interesting to note that while respondents on the survey conducted by NLPIP felt that LEDs were suitable light sources for PV lighting systems, they did not consider incandescent lamps to be suitable (see "What are some common beliefs about PV lighting systems?"). This holds true in applications requiring high light-levels and/or using high-mounted luminaires because a large PV panel would be needed to meet the power requirements of the light sources, which would be expensive and cumbersome. However, in situations using lower mounting heights (8 ft [2.4 m] or less), where less than five moonlights are required, incandescent lamps become a viable option because they can provide the suitable lumen package to meet the required light levels at low mounting heights. However, LEDs are able to provide moonlight levels most efficiently over the widest range of mounting heights. This is because the light output provided by each individual LED light source can be very small, and can be easily adjusted by adding additional LEDs to a luminaire design to increase the light output when needed.

A final consideration when selecting a luminaire for a PV lighting system is electrical safety. This is similar to a grid-powered luminaire, and care should be taken that the luminaire and other PV lighting system components meet all applicable safety codes and standards.


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