Lighting Research Center Lighting Research Center
    Volume 11 Issue 2
September 2010    
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. bi-level switching - Control of light source intensity at two discrete levels in addition to off. 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. continuous dimming - Control of a light source's intensity to practically any value within a given operating range. 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. restrike time - The time required for a lamp to restrike, or start, and to return to 90% of its stabilized light output after the lamp is extinguished. Normally, HID lamps need to cool before they can be restarted. visual performance - The quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy. high-pressure sodium (HPS) - A high-intensity discharge lamp type that uses sodium under high pressure as the primary light-producing element. HPS lamps produce light with a correlated color temperature (CCT) of approximately 2000 kelvins, although CCTs for lamps having higher CRI values range from 2200 to 2700 kelvins. Standard lamps have a CRI value of 22; others have CRI values from 60 to 80. HPS lamps are among the most efficacious light sources, with efficacies as high as 150 lumens per watt, although those with higher CRI values have efficacies as low as 25 lumens per watt. lamp life - The median life span of a very large number of lamps (also known as the average rated life). Half of the lamps in a sample are likely to fail before the rated lamp life, and half are likely to survive beyond the rated lamp life. For discharge light sources, such as fluorescent and HID lamps, lamp life depends on the number of starts and the duration of the operating cycle each time the lamp is started. 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. 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. metal halide (MH) lamp - A high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements. Metal halide lamps have better color properties than other HID lamp types because the different additives produce more visible wavelengths, resulting in a more complete spectrum. Metal halide lamps are available with CCTs from 2300 to 5400 K and with CRI values from 60 to 93. Efficacies of metal halide lamps typically range from 75 to 125 LPW. phosphors - Materials used in a light source to produce or modify its spectral emission distribution. In fluorescent and high intensity discharge lamps, the phosphors fluoresce (emit visible light) when excited by ultraviolet radiation produced by mercury vapor inside the lamp when energized by an electric arc. In a light emitting diode, phosphors convert short-wavelength light or ultraviolet radiation produced by a semiconductor die into longer-wavelength light, usually with the goal of producing white illumination. 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. sky glow - Brightening of the sky caused by outdoor lighting and natural atmospheric and celestial factors. ambient temperature - The temperature of the surrounding air that comes into contact with the lamp and ballast. Ambient temperature affects the light output and active power of fluorescent lamp/ballast systems. Each fluorescent lamp-ballast system has an optimum ambient temperature at which it produces maximum light output. Higher or lower temperatures reduce light output. For purposes of lamp/ballast tests, ambient temperature is measured at a point no more than 1 meter (3.3 feet) from the lamp and at the same height as the lamp. 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. light trespass - A undesirable condition in which exterior light is cast where it is not wanted. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. illumination - The process of using light to see objects at a particular location. pulse-width modulation - Operating a light source by very rapidly (faster than can be detected visually) switching it on and off to achieve intermediate values of average light output; the frequency and the duty cycle (percentage of time the source is switched on) are important parameters in the modulation. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. 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. 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. dynamic outdoor lighting - Outdoor lighting that varies light level or other characteristics automatically and precisely in response to factors such as vacancy or the type of use of an outdoor location. light pollution - An unwanted consequence of outdoor lighting that includes such effects as sky glow, light trespass, and glare. reflectance - A measure of the ability of an object to reflect or absorb light, expressed as a unitless value between 0 and 1. A perfectly dark object has a reflectance of 0, and a perfectly white object has a reflectance of 1.
How can specifiers select appropriate light levels for dynamic outdoor lighting?

An important consideration for the implementation of dynamic outdoor lighting is the level of reduction relative to full light output. At present, there are few guidelines for identifying the appropriate light level when an area is usually unoccupied but may be occupied at any time. One approach is to rely on precedent. Some jurisdictions require that light levels be reduced by half during periods of less frequent use (for example, Fairfax County 2003). The basis for this level of reduction is not clear and may be related to the amount of dimming that is practical for HID light sources (see "What light sources can be used with dynamic outdoor lighting?").

A second approach to selecting the target light level is to base it on visual performance. For example, Rea and Ouellette (1991) developed the relative visual performance (RVP) method that estimates a person’s ability to detect an object based upon its luminance, contrast, and size. Calculated levels of RVP are based on an arbitrary but high level of visual performance (reading black type on a white page under office lighting conditions). As an example, if there were a tripping hazard on a sidewalk or parking lot having the characteristics listed below, then a person’s expected RVP as a function of illuminance is shown in Figure 5.

  • shape: cube, an arbitrary object shape
  • size: 3 inches (in) (7.5 centimeters [cm]) along each side, defined as a critical tripping hazard size (Zeller et al. 2006)
  • viewing distance: 10 feet (ft) (3 meters [m]), a typical distance ahead on the ground at which a pedestrian's attention might be focused while walking (Sammarco et al. 2010)
  • luminance contrast: 0.7, typical of a shadowed portion of a three-dimensional object
  • ground reflectance: 0.1, typical of asphalt (IES 1981)
  • age of observer: 60 years, the upper age limit of the RVP model (Rea and Ouellette 1991)

As shown in Figure 5, the calculated RVP values exhibit a plateau characteristic: below 0.2 footcandles (fc) (2 lux [lx]), RVP values drop precipitously, but above 0.2 fc (2 lx), further increases would provide little improvement in visual performance. Estimates of visibility such as those in Figure 5 might be useful in establishing a minimum illuminance of 0.2 fc (2 lx) as adequate for providing visibility "coverage" (Rea et al. 2010) in a particular location.

Figure 5. Relative visual performance for a tripping hazard as might be seen by a 60-year-old, as a function of horizontal illuminance (Rea and Ouellette 1991)

Another visual performance-based approach is to compare illuminances from electric lighting with those from moonlight. Lighting Answers: Photovoltaic Lighting (Zhou and Frering 2006) describes a unit of light called a "moonlight" that is equivalent to the typical illuminance produced by light from a full moon (0.01 fc [0.1 lx]). When electric street lighting systems were first developed and installed, for example, the lights might not have been switched on when a full moon was present (Hyde 1910) because electric light was thought to be redundant with the visibility benefits of full moonlight. Lighting of just a few moonlights (up to 0.05 fc [0.5 lx]) might be acceptable in applications where only basic visual orientation is required of occupants (Zhou and Frering 2006).

A third method of determining minimum illuminance levels is based on people's perceptions of safety and security from outdoor lighting, which can be quite different from an individual’s level of visual performance under the same lighting. Figure 6 illustrates the results of people’s perceptions of many different outdoor lighting installations in Albany, NY and New York City, NY (Leslie and Rodgers 1996; Boyce et al. 2000). Participants in those studies visited outdoor areas with various light levels and judged their agreement with the statement "This is an example of good security lighting." Although ratings of agreement tended to saturate for illuminances higher than 5 fc (50 lx), agreement was much lower at levels below 1 fc (10 lx). Lighting specifiers should be clear regarding their objectives in planning dynamic outdoor lighting; providing good visibility may result in different decisions about target light levels than providing perceptions of safety.

Figure 6. Average ratings of agreement with the statement "This is an example of good security lighting" by observers in Albany, NY and New York City, NY for outdoor areas illuminated to different light levels (Leslie and Rodgers 1996)


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