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Comparison of kinds of lighting

The ballast controls the starting of the lamp and maintains the voltage going to it. Of the two types, the electronic ballast is preferred over the magnetic ballast (source: City of Austin Sustainable Building Sourcebook).

Color Rendering Index (CRI) is used to measure a lamps ability to effectively render color. Based on a scale of 0 to 100, natural light is represented by 100. Ratings between 60 and 70 are considered acceptable; between 70 and 80 are good; between 80 and 90 are better; and, between 90 and 100 are the best (source: Light Energy Source).

Lumen output is a standard measure of the light emitted by a lamp. Although it is a standardized measurement, the lumen of the lamp actually changes depending on the visibility conditions. There are three light conditions-photopic, scotopic, and mesopic. When light levels are very high, photopic conditions exists. Scotopic refers to conditions of low light. Mesopic identifies twilight conditions, when the levels of light are neither high nor low. For example, in scotopic conditions the eye's response to blue lights heightens while its response to yellow and red lights is reduced. In contrast, in photopic conditions the eye has greater sensitivity for yellow light and reacts less to blue light. In photopic conditions, lamps producing a high percentage of yellow light will appear brighter than lamps with lower levels. Likewise, high producers of blue light appear brighter in scotopic conditions. Lumen output represents the overall production of light, but does not reflect the actual color of light emitted. This makes the use of lumen output to determine the brightness of a lamp difficult (source: Better Roads Magazine: Should vision influence roadway lighting design?)

The efficacy measures the amount of power going into the lamp that is being converted into light. A higher efficacy rating indicates that the lamp is more efficient (source: City of Austin Sustainable Building Sourcebook).

Lamp Families

Incandescent lamps, the most prevalent residential lamp, are inefficient light sources. Of the incandescents, the A-lamp is the most common. The tungsten-halogen lamp is the most efficient. Compared to other incandescents, they produce more light, have longer lives, and use less energy. In fact, halogens require 30 percent less energy to run than A-lamps. Although halogens are much more efficient, they produce large amounts of heat and have been know to start fires.

Fluorescent lamps, commonly used by the commercial sector, have relatively high efficacy. Their long operating lives also make them attractive. Improved phosphor coatings have also enhanced fluorescent lamps' color rendering. The use of electronic ballasts eliminates the hum and flicker often associated with fluorescent lamps.

In order for a fluorescent light to operate efficiently, the temperature of the glass tube must be at a particular temperature. The lamps' heat output and its environment determine its optimal temperature, usually close to 105 degrees Fahrenheit. Although the lamp generally starts at a lower temperature when initially turned on, it will heat up to its optimal temperature. Well-ventilated indoor areas provide the best locations for fluorescent use. Lamps will not function efficiently if they are not at the optimal operating temperature.

The T8 has become the industry standard. New technology has produced the T5, but this model has yet to become the norm. Fluorescents are also available with mercury. (sources: Green Building Concepts, Oikos Green Building Source, City of Austin Sustainable Building Sourcebook)

Compact Fluorescent Lamps (CFL) are good substitutes for incandescent lamps because they have long operating lives and are energy efficient. In fact, they require a third less energy and last 10 times as long. CFL's ballast and lamp can also be purchased separately. Because the ballast's life is approximately two times longer than the lamp, the lamp can be replaced without wasting the working ballast (See "Replacing Incandescents with CFLs").

High Intensity Discharge (HID) lamps were originally developed for use outdoors but now include indoor lamps. This family of lamps has a re-strike time of 5 to 15 minute. This long start up time makes HID lamps best suited for environments where they are infrequently switch on and off. Despite this drawback, HID lamps are the most energy efficient light sources available commercially.

Of the HID lamps, Mercury Vapor Lamps have the lowest efficacy. Their lumen output reduces rapidly over the lamp's life cycle and they have low values on the Color Rendering Index. Mercury vapor lamps are not considered to be good green options.

Metal Halide (MH) Lamps, similar to mercury vapor lamps, use metal halide additives in addition to mercury and argon in the arc tube. MH lamps have good CPIs. Metal halides can be used both indoors and outdoors, but are best suited for lighting sports arenas, stadiums, convention halls, and large auditoriums.

High Pressure Sodium (HPS) Lamps are used for both industrial and outdoor applications. They have high rates of efficacy, as much as 140 lm/W. Technology is also working to improve HPS's CPI with optical coatings. Unfortunately, these coatings reduce their energy efficiency.

Low Pressure Sodium (LPS) Lamps are the most efficient lamps of the HID family but they also provide the poorest quality of light. Because LPS lamps are monochromatic, making all colors appear in a grayscale, they work best for outdoor use (sources: Green Building Concepts, City of Austin Sustainable Building Sourcebook)

Replacing Incandescents with CFLs

When selecting CFLs to replace incandescent lamps, you should keep several factors in mind. The CFL's wattage, lumen output, and shape are important to keep in mind when making your selection. When selecting the wattage of the new CFL, using a ratio of 3:1 is helpful. For example, a CFL of 20 watts can replace an incandescent of 60 watts. Lumen output can be helpful in determining how bright the CFL will be.

Because incandescents produce high levels of yellow light, a CFL with the same lumen output may not appear to be as bright. If your new CFL appears duller, consider choosing a CFL with a higher lumen output. Remember to account for the 20 percent decrease in output at the end of the CFLs' life cycle.

Also take into consideration the lamp fixture in which the CFL will be used. Because the CFL emits the majority of light perpendicular to its tube, installing a lamp in a recessed downlight would be inefficient. The fixture itself would absorb most of the light. Integral reflectors or diffusers can be used to ensure that light is distributed better.

Be cautious purchasing cheap CFLs. They often have efficacies of less than half that of the better units. You will be happier investing in a quality lamp. Although the first CFLs purchased may not be exactly want you wanted, through experimentation you will find the CFLs that best fit your needs (sources: Portland's Green Building Resource, Oikos Green Building Source, City of Austin Sustainable Building Sourcebook).

Lighting Controls are an effective means of reducing the operating hours of lamps, thereby maximizing energy savings.

Motion Control and Occupancy Sensors can be used to turn lights off and on automatically. These sensors use passive infrared and ultrasonic technology to turn the lights on when movement is registered and turn them off when movement has not been registered in a given period of time. Motion control can shorten the lives of the lighting units and do not hold up as well in humid climates.

Scheduling Controls turn lights on and off on an established schedule. While many systems are simple, they can also be highly sophisticated. These schedules can be set to specific times and dates as well as changes in the amount of daylight available.

Panel-Level Dimming systems can be used with HID lamps and fluorescents. Installed at the electrical panel, panel-level dimming devices uniformly control all of the lamps on a particular circuit.

Although not a device itself, "Light Trespass" refers to light that falls into unintended areas, for example, a neighbor's yard. Light trespass is a waste of energy and can also have negative affects on the environment. For instance, light affects bird migrations and insect pollinations. Light trespass can be reduced with the use of shields, hoods, and the redirection of light fixtures (source: City of Austin Sustainable Building Sourcebook)

Comparison of kinds of lighting






Standard Incandescent

        Warm, full spectrum color

        Can be sharply focused

        Easily dimmed

        Low initial cost

        Wide variety of bulbs

        Inefficient (less than 25 lumens per watt)

        Short life (typically 1, 000 hours)

        High heat output

        Accent, atmospheric lighting

        Decorative sparkle

Halogen Incandescent

        Same as standard incandescent, plus

        More efficient

        Longer life (up to 2,000 hours)

        Very high heat output

        More limited bulb and fixture selection

        Accent lighting

        Tasks requiring very high visibility


        More efficient (up to 90 lumens per watt)

        Very long life (10,000 20,000 hours)

        Lower heat output

        Wide variety of fixtures and bulbs

        Less precise focus control

        Limited dimming options

        Higher initial cost

        Older models may flicker, hum and be slow to illuminate

        General, diffuse lighting

        Indirect lighting

        Task lighting

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