Lighting Research Center Lighting Research Center
    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 are some important considerations in choosing PV lighting?
Amount of solar irradiance
Solar irradiance for photovoltaic (PV) power is affected by location, weather, time of year, and surrounding structures. The solar radiation received by the earth's atmosphere is 1367 watts per square meter (Messenger and Ventre 2004), but this amount is reduced when the solar radiation passes through the air mass. Solar radiation is the radiant energy emitted by the sun. The term "solar irradiance" refers to the amount of radiant flux incident on any surface, including buildings. The solar irradiance is lower at sea level, for example, than it is on a mountain top. It is also generally true that the farther away a location is from the equator, the lower the solar irradiance will be available at ground level. In summer, solar irradiance is available longer than in winter. Weather also affects the amount of solar irradiance. Cloud cover, for example, will reduce solar irradiance. Finally, solar irradiance may be blocked by buildings, trees, or snow and dirt on the PV panels (see "How does solar radiation vary by location?").

Aesthetic and structural concerns
Some of the components in PV lighting systems, such as the PV panels, tend to be large and awkward. Structural support and wind load should be considered to ensure that the PV panel does not pose a safety hazard to people. A smaller PV panel may reduce both aesthetic and structural concerns but will limit the power capacity of the PV panel.

Cost vs. performance
Cost is an important factor in any investment. From the end-user's point of view, PV lighting systems include more components and therefore are more expensive to purchase than traditional, grid-powered lighting systems. However, in areas where electricity from the grid is not accessible, extending the power lines is often prohibitively expensive. For example, it would be economically ineffective to extend a power line to a remote mountain area only for powering luminaires at a campground's parking lot. In these types of remote locations PV power may be a good alternative for providing lighting at night.

The performance of a PV lighting system is related to the quality of its components. Generally, the higher the quality, the more expensive the system. Technologies associated with PV lighting are not fully mature (at time of publication), and some components may not be reliable even if they are expensive. These costs and risks have often pushed PV lighting out of consideration for use in a wide variety of lighting applications. However, the increasing desire for energy independence and the rising cost of energy may change this situation in the future. By carefully matching PV lighting to appropriate applications, PV lighting may find its way to more lighting markets and thereby reduce both the costs and risks associated with these systems (see "How does the cost of PV lighting systems compare to grid-powered lighting systems?").


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