Entry lighting indicates where the entry is, safely lights the way in and out of the building, and lights signs or building numbers.
Less is more & many are better than one.
Light levels and fixture placement. These seemingly contradictory principles applied to entryway lighting result in, inviting, uniform light levels. Use multiple low light level fixtures instead of one very bright fixture. If you use just one fixture, the area nearest it is too bright and the areas further away are too dim. Using multiple fixtures lets you put the light in the key areas of interest: near signs, by doorways and stairs, thus more evenly distributing the light.
Light levels at the entry.
Glare is caused either by direct view of an unshielded light source or by too much contrast if the rest of the exterior is relatively dark. A contrast ratio of less than 5 to 1 is desirable to softly accent an area without creating glare. When someone leaves an excessively lit building, it takes the eye longer to adapt from a bright entry to the darker walkways outside, making it difficult to see.
Where does extra light go?
In addition to causing visual difficulties, excessive light adds to other problems as well. All this extra light reflects off the surrounding walls and walkways, bouncing into the sky, creating "light pollution" (even concrete has a reflectance of up to 40%!). Light trespass invades adjacent buildings and light pollution reduces our ability to view the moon and stars, compromising our quality of life.
As our cities and neighborhoods grow, the overall night time lighting levels become brighter. This is due in part, ironicaly, to the availability of very efficient new light sources. Unfortunately, energy efficient light sources tempt property owners into using higher light levels instead of lower power consumption (see table below). As you can see, the 100 watt HPS is far brighter than the base case 100 watt incandescent. The result is light pollution, sky glow and light trespass. Additionally, extra light does not always mean we can see any better. The eye can adapt to very low light levels (moonlight is less than 1 footcandle) and very high light levels (a sunny beach can be over 20,000 footcandles), but it only adapts to one light level at a time. A really bright entry makes the walkway approaching it appear dark. Low level lighting on building A, next to bright glary lighting on building B, makes building A appear dark in comparison. This may result in an upward spiral of increased light levels. From the table, a much better choice is the lower wattage metal halide or compact fluorescent (CFL), providing similar light levels to the base case but with reduced wattage.
- • 100watt incandescent = 1700 lumens.
Same wattage, much higher light level.
- • 100watt High Pressure Sodium = 8500 lumens
Lower wattage, similar light level.
- • 32watt metal halide = 1900 lumens.
- • 32watt triple biax Compact Fluorescent (CFL) = 1870 lumens.
Aesthetics, functionality, and maintenance are major points to consider as you design a lighting system.
- • Will it work? Is the light delivered to the right places?
- • Are there appropriate light levels for safety? Not just minimum light levels, is there too much light?
- • How does it look? Do the fixtures and lighting conform to an appropriate design aesthetic?
- • Is there even light distribution and low glare? Uniformity and low contrast can aid in visibility.
- • Will there be high long term maintenance costs?
For example, a 300 watt halogen flood light may cost $10 to buy while a 22 watt CFL costs $30, making the halogen appear the better buy. But if the fixtures are on 10hrs/day, they will have an annual operating cost of; $6.92 for the CFL and $63.70 for the halogen! (at $.05/kWh). Plus, the CFL lasts 10,000 hours while the halogen lasts only 2,500 hours. Good exterior entry lighting is based on many decisions, not just on first cost.
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