Introduction

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.

The sun is an energy resource like no other, providing the sustaining power for life on earth. Harnessing the sun's power through the use of photovoltaic (PV) collection systems can create a sustainable form of clean energy and reduce demand for utility-generated electric (grid) power. PV collection systems have the potential to power lighting systems. However, the effectiveness of a PV system for a given lighting application depends upon a complex set of environmental, technical, and cost variables. Performance expectations and aesthetics also must be taken into consideration when deciding whether to use a PV system to power lighting.

Although solar cells and PV collection systems were introduced in the 1950s (Messenger and Ventre 2004), to date these systems have been used for lighting in only a very limited number of applications. Some of the reasons for their restricted use include high initial purchase and maintenance costs, unreliable light production, and complexities in designing and installing PV systems. Although several field demonstrations have showcased PV lighting systems in parking lot and roadway applications, PV lighting systems are generally not economically and practically feasible when the lighting for these applications is designed according to existing illumination guidelines. Therefore, this Lighting Answers focuses on outdoor applications such as those in rural or remote areas, designed for use where light levels similar to moonlight may be sufficient for the tasks being performed.

Not covered in this report are PV systems that power indicator lights such as lights on ocean buoys, blinking traffic lights, and construction signage, although these are viable applications. Also, hybrid PV systems that combine small generators or grid power with PV panels are not included in this Lighting Answers.

In the early part of the twentieth century, when electric street lighting was beginning to be installed in many areas of the United States, moonlight levels were commonly used as a standard or reference point for outdoor lighting. In many instances, the visual quality of a street lighting design was measured against moonlight. The closer the illumination provided by electric street lighting came to moonlight, the more highly it was rated, even though electric lighting could provide significantly higher light levels (Blood 1907). Moonlight was also believed to provide better visibility than electric lighting. In some areas of the United States, electric street lighting was actually turned off when the moon was full, because it was believed that the electric lighting detracted from the illumination provided by moonlight (Hyde 1910).

When considering the needs of an outdoor lighting installation, it is still helpful to think in terms of moonlight levels. Under very dim visual conditions (i.e., those found in rural areas), moonlight, with an illuminance of approximately 0.1 to 0.5 lux, or 0.01 to 0.05 footcandles (fc) on the ground, often provides enough lighting for people's basic needs such as walking, or finding a house or car. NLPIP proposes that moonlight illuminance can be used as a reference value for setting expectations of suitable light levels in rural or remote lighting applications. For the purposes of this Lighting Answers, one moonlight is defined as 0.1 lux (0.01 footcandle). This is equivalent to the average illuminance on the earth's surface provided by a full moon.

In order to assist the reader to compare the requirements and cost-effectiveness of PV lighting systems for various applications, this Lighting Answers uses three sizes of lighting systems as examples:

	1. A system capable of providing approximately 10 lux or 1 fc (100 moonlights) on the ground beneath the luminaire
	2. A system capable of providing approximately 0.5 lux or 0.05 fc (5 moonlights) on the ground beneath the luminaire
	3. A system able to provide sufficient luminance to act as an indicator light for direction-finding applications.

These lighting conditions were selected because they represent a range over which PV lighting systems might be considered practical.