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.
Can dynamic outdoor lighting save money?

The Institution of Lighting Engineers (2006) estimates that the amount of money saved by dynamic outdoor lighting because of reduced energy use is significant enough to create a net savings, despite higher initial costs. This finding is supported by an economic analysis conducted by NLPIP (Table 1) of a hypothetical parking lot lighted by ten luminaires, each containing a 250 watt (W) lamp, or 300 W after accounting for the ballast. In one scenario, the parking lot lighting system would operate each night at full output, and in the other scenario, motion-sensor control is assumed to dim the luminaires to 50% output during unoccupied periods (as illustrated in Figure 4c) with a resulting energy reduction of 30% overall. Cost data are based on published estimates (Leslie and Rodgers 1996; Leslie 1998; Southern California Edison 2007). The control system in this example is a bi-level switching controller with an integrated infrared motion sensor that is installed on each luminaire. The estimated costs can be applied to installations with fewer or more luminaires by scaling all costs relative to the ten luminaires in the example.

The increased initial cost of the motion-based control system is offset by operating cost savings with a resulting simple payback of nearly 4.5 years. In principle, reducing the target light output below 50% would result in further economic savings. Although this is not practical with many HID dimming systems, this strategy could also be cost-effective for other sources, such as LEDs.



Table 1. Cost comparisons between a conventional parking lot system and one using motion sensor controls to reduce light output by 50% during unoccupied periods (as illustrated in Figure 4c), resulting in 30% less energy use overall

Initial Costs
Baseline* Occupancy Control*

Number of luminaires
10
10
Cost per luminaire ($)
300
300
Total luminaire cost ($)
3000
3000
Lamps per luminaire
1
1
Total number of lamps
10
10
Lamp cost ($)
54
54
Total lamp cost ($)
540
540
Number of poles
10
10
Pole cost ($)
940
940
Total pole cost ($)
9400
9400
Control system cost ($)
200
1085
Total equipment cost ($)
13,140
14,025
Labor ($)
13,140
14,025
Total installation cost ($)
26,280
28,050

Annual Costs
   

Average daily use (hours)
12
12
Annual operating time (hours)
4380
3066**
Rated lamp life (hours)
24,000
24,000
Annual lamps used
1.83
1.83
Relamping labor cost ($)
23
23
Lamp replacement cost ($)
77
77
Annual maintenance cost ($)
141
141
Input power (W)
300
300***
Annual energy use (kWh)
13,140
9198
Electricity cost ($/kWh)
0.10
0.10
Annual energy cost ($)
1314
920
Annual operating cost ($)
1455
1,060

* All monetary units are in U.S. dollars.
** Motion-control system assumed to be equivalent to 30% reduction in energy use, equivalent to a 30% reduction in operating time.
*** Input power at full output.


Dimming HID lamps such as HPS or MH can reduce luminous efficacy, whereby a reduction in light level is proportionally greater than the reduction in input power (Ji and Wolsey 1994; NEMA 2002; Guo 2008). However, this potential effect is not included in the above analysis because of differences among lamp types. There is conflicting evidence on whether or not dimming HID lamps reduces their lifetimes. To understand the potential economic impact of the reduced operating life, NLPIP calculated the simple payback period for the scenario in Table 1, but with a reduced lamp life in the occupancy-controlled system of 16,000 hours (an arbitrary, but substantial reduction in lamp life of 33%). This reduction in lamp life would increase the simple payback period from 4.5 years to 5.5 years.

 



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