When purchasing lighting elements for your outdoor space, forget about wattage, which only describes the amount of power the light uses and tells you little about actual illumination. Instead, look for information on the packaging about lumen output. Typical bulbs give off around 1,000 lumen, which cast about 1,000 lux onto objects inside the immediately surrounding square meter; remember, though, that lux diminishes as you move objects away from the light source.
Illumination levels describe how much light falls on a specific object in a given area. The International System of Units (SI) measures light output levels by lumens; the light source's brightness as it illuminates an object within a square meter is called lux. Total interior room luminance, which is (roughly) the amount of lumens in the room divided by the amount of square meters in the room, usually averages at or below 100 lux. Larger rooms or spaces without walls, such as outdoor areas, require a greater number of lumens to achieve the desired total lux.
Ambient lighting elements give off the general light used in a space. For outdoor spaces, keep the ambient lighting illumination level low. Aim for between 20 and 30 lux. Lower ambient light creates a relaxed and romantic atmosphere that won’t disturb your neighbors or compete with other, more specifically lighted outdoor design elements or task areas. Overly bright outdoor lighting also diminishes your ability to see details in the night sky and can cause dark, possibly dangerous shadow pockets in areas the light doesn’t reach. If you don't trust your eyes, use a light meter to determine the overall lux of your outdoor space; anything brighter than the average living room, which is usually at between 50 to 80 lux, is probably too bright for everyday use and may disturb your neighbors.
Decorative and Ambient Lighting
Decorative lighting exists purely for aesthetics, while ambient lighting allows for overall illumination in a space. Whenever possible, combine decorative lighting with ambient lighting to minimize costs and to avoid excessively high outdoor illumination levels. For example, an elaborate grouping of paper lanterns or artfully strung fairy lights serve as both ambient light and decorative lighting. Since most outdoor lighting schemes involve low ambient lighting levels to avoid waste and light trespass, additional task lighting might be needed for comfort. Light up the center of eating areas with candle groupings or a low-hanging lantern. A group of 10 candles, at 1 lux apiece, nestled inside a candelabrum or cluster of lanterns, should cast enough light within the surrounding meter to comfortably illuminate place settings and faces.
Stake landscaping lights along footpaths to avoid accidents; use LED or solar landscaping lights, which often have lower lumen outputs, to cast a just-bright-enough to be safe glow onto pathways. Stairs require extra lighting to head off possible stumbles; use one or more directed, shielded outdoor sconces or pole lamps to cast at least 200 lux onto the steps. Use one or two mini-pendant lights over outdoor kitchen work surfaces, which need at least 200 lux for precision-requiring tasks such as chopping or measuring. Select opaque, downward-pointing shades that completely cover the sides of the bulb, so the light doesn’t travel too far and overly brighten the area.
Accent lighting illuminates specific decorative objects or architectural details, which allows you to enjoy your backyard’s best decorative elements after nightfall. Strategically place your accent lighting elements so light doesn’t escape onto other surfaces and overly brighten your outdoor lighting scheme. Place landscape lights behind large plants to create dramatic a silhouette effect without adding much to the area’s overall lux. Angle upward-pointing and specifically directed, low-lumen spotlights to highlight showcase plants and your home’s best architectural details.
- “Landscaping Principles and Practices”; Jack E. Ingels; 2009
- “Sustainable Landscape Construction: a Guide to Green Building Outdoors”; J. William Thompson, et al.; 2007
- Energy Books; Measuring Light Intensity; D.R. Wulfinghoff; 1999
- Scope Calc; Low-Light Performance Calculator; Mikk Thomann
- “The Earth Friendly Home”; Nancy Hajeski, et al.; 2009