August 1996 Volume 2 Issue 15
Lighting Efficiency: A Comparison by Lighting Type
There are many types of electric lighting available today. The lamps in these systems are energy conversion devices that take in electrical energy and put out light energy. None does a perfect job of this, but some types do a much better job than others. This article discusses the most common types of lighting used in industry and commerce, and highlights those types that can reduce the electricity bill.
Much light out and few watts in is the combination we’re looking for. It’s very much like efficiency, but in lighting it is called efficacy. The visible light produced by a lamp is measured in lumens. The input power needed to produce that light is measured in watts. Efficacy is lumens out divided by watts in: lumens per watt.
Artificial lighting of some sort is needed in every indoor working environment. In the present survey, lighting sources have been put into two groups. In the first group we have the types most suited to general office lighting and the lighting of specific tasks. In the second we have the types of lighting suited to high-ceiling environments like factories and warehouses, and to outdoor situations.
OFFICE AND TASK LIGHTING
The first practical electric lighting was incandescent lighting. It dates back to 1879 and the carbon filament lamps of Edison in the U.S. and Swan in England, and to 1906 and the introduction of the tungsten filament bulb. Incandescent lighting’s positive features are the low cost of the lamps and the excellent rendering of colours. Incandescent lamps have a near-perfect CRI (Colour Rendering Index) rating of 97.
Incandescent lighting’s highly negative feature is the large amount of electricity needed to produce the light. As the chart indicates, incandescent lighting is by far the worst choice from the standpoint of energy consumption and operating cost.
Relative Energy Cost for Equal Light*
(Standard Fluorescent = 100%)
|----------Office / Task------|------Plant / Outdoors-------|
*Ballast losses included
During the 1950s, ‘60s, and ‘70s, T-12 “Cool White” and “Warm White” fluorescent lamps and associated iron-core ballasts provided the lighting in almost all offices. A great many of these systems are still in use, so we have selected as the lighting-cost reference on our lamp comparison chart the
classic office fixture: Two 4-foot 40-watt tubes and
standard ballast. Efficacy: 68 lumens/watt (ballast loss included). Relative energy cost: 100%. Colour Rendering Index: 62 for Cool White; 52 for Warm White.
Compact Fluorescent Lamp is the term given to various fluorescent screw-in replacements for incandescent lamps. For the same light output, these devices require as little as 1/4 of the input power required by an incandescent lamp. With CRI ratings as high as 82, their rendering of colour is better than standard fluorescents. These lamps cost much more than incandescent bulbs, but they last 10 times longer. Furthermore, they use so much less energy over their useful life that purchase price is not a major factor.
Although not nearly as efficient as compact fluorescents, two types of improved incandescent lamp are also available. Krypton-filled Incandescent Lamps cost somewhat more than standard incandescents, but last longer and produce the same light output with 10% less power in. Tungsten Halogen Lamps are an even more efficient variety of incandescent lamp. Reflector-type halogen PAR lamps, for example, require 40% less power than standard PAR lamps for the same light output.
T-8 fluorescent tubes used with electronic ballasts currently provide the most energy-efficient lighting for offices and low-ceiling commercial spaces. This is the lighting of choice for new installations, and T-8 retrofits have proven cost effective in many situations. The CRI for T-8 lighting is high: typically 82.
LIGHTING FOR HIGH-CEILING AREAS AND OUTDOORS
High Intensity Discharge (HID) lighting involves high-wattage, very bright lamps, and is thus best suited to situations where people do not look directly at the lamps — or do so from a distance. HID lighting is commonly used indoors to light warehouses, factories and arenas, and outdoors to light parking lots, roadways and walkways. Differences in efficiency and colour rendition characterize the different types of HID lighting. All HID lamps have long operating life: typically 24,000 hours for mercury vapour and HPS, 10,000-20,000 for metal halide, and 18,000 for LPS.
Mercury Vapour lighting was an efficiency breakthrough when it was invented in 1933, but it is obsolete today. For many years mercury vapour lamps were used for street lighting and similar purposes, but this type of lighting has poor colour rendition (CRI = 22) and is now being replaced by other, more efficient kinds of HID lighting.
Metal Halide lamps have the best colour rendition of all HID lighting options — a CRI of 65 for clear lamps, and 70 for the phosphor-coated kind. This, along with its relatively high efficiency, makes metal halide lighting attractive for malls and other high-ceiling commercial areas, factories where colour discrimination is important, and sports arenas.
Where colour rendition is less important, still lower operating cost can be achieved by going to High Pressure Sodium lighting. The CRI for HPS lamps ranges from 21 for standard Clear lamps to as high as 65 for Colour-Improved Clear and 70 for Colour-Improved Diffuse-Coated varieties. HPS lamps are commonly used in warehouses, loading-dock areas, and parking lots.
The most energy-efficient light source currently available is Low Pressure Sodium. Unfortunately, this light is monochromatic — one color only, in the yellow region of the spectrum. This makes LPS light useless for colour discrimination. It is fine, however, for many safety-related purposes, and is increasingly used to illuminate parking lots, walkways and roadways. On P.E.I., low pressure sodium light illuminates rural road intersections, and it illuminates parking lots and walkways on the UPEI campus.
For more detailed information on lighting options and issues, call Mike Proud or Ron Estabrooks at 368-5010 (toll free).