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 the most suitable applications for photovoltaic lighting?

In remote locations such as mountain areas, nature preserves, national and state parks, or rural villages and towns, where the electric grid is far away, providing lighting at night is usually difficult. Under very dark visual conditions found in remote areas, moonlight, or even star light, often provides enough lighting for people's basic needs such as walking or finding a house or car. In many pathway lighting applications, for example, the lighting system needs only to provide enough light to strike the surface of the path at very low levels. Similar light levels are appropriate for a garden or residential landscape. The same can be true for a parking lot in a remote area. Because car headlights will generally provide enough light for a driver to navigate the parking lot safely, fixed luminaires need only provide sufficient light levels to reduce the risk of tripping for pedestrians as they locate and walk to and from their cars. In these types of applications, moonlight illuminance can be used as a reference value for setting expectation of suitable light levels in remote locations.

Based on the average luminance (2500 candelas/square meter) and diameter of the moon (347,900 meters), and the distance between the moon and the earth (384,385,000 meters), the illuminance on the ground on a full moon night with clear sky condition is approximately 0.1 lux. Because this prediction is based on an average moon luminance, actual illuminance from moonlight will vary based on lunar phase, atmospheric conditions, time of year, and other variables. Empirically, the earth's surface illumination from moonlight can vary from 0.005 to 0.5 lux (Courter 2003; Krisciunas and Schaefer 1991). For convenience, the illuminance level of 0.1 lux can be defined as one unit of "moonlight." Although this light level may seem low, an average illuminance of between 0.1 and 0.5 lux is sufficient to read a newspaper. While higher nighttime light levels may be desirable in heavily trafficked or hazardous areas, a wide range of lighting applications can be lighted safely to less than 0.5 lux, including residential areas, gardens, parks, and landscapes where people may walk at night, provided, people are able to see the path and any hazards that might be on the ground.

In campgrounds, as well as state and national parks that people use at night, it is often desirable to maintain low levels to allow people to see the stars while providing lighting where needed for navigation, direction, and safe passage. Dock and waterfront areas along oceans, lakes, and rivers can also be lighted effectively at low levels, if these areas are used only intermittently at night and are not heavily trafficked.

Lighting applications requiring "moonlight" levels are generally well suited for photovoltaic (PV) lighting systems. In these situations, only a small quantity of dc power needs to be stored in a battery to provide a low level of light over a limited period of the night. This allows PV panels to be smaller in size, less expensive, and more easily integrated into the design of a luminaire. This also allows batteries to be smaller and less expensive.


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