The easiest lamps to specify are for path, area, and deck lighting. Most of the fixtures used for these areas are designed to be used with non-directional bare bulbs. The fixture controls where the light is directed since these lamps illuminate in 360°. The only lamp choice that you may have is the amount of light if the lamp used by the fixture is available in more than one wattage. Since the fixture controls the spread of the light, increasing wattage increases the intensity of the light and not the area that is lit. Selection of these fixtures is covered in the next section.

For other lighting tasks, downlighting, uplighting, spotlighting etc., the problem becomes a little more complicated. When designing to light a particular area or object, we need to specify the lamp that will place the right amount of light with a beam spread that only covers the target area and then find a fixture that uses that lamp. To do this we need photometric data for the available lamps. There are several sources for photometric diagrams for various lamps. Many of the manufacturers for landscape fixtures include this data in their fixture catalogs.

Using the photometric charts is relatively easy.

Source:  Kim Lighting

The charts above are both for 35 watt MR16 lamps. The chart on the left is for a 20° spot (SP) and the one on the right is for a 40° flood (FL). On the chart the horizontal axis is the spread of the light beam in feet. The vertical scale on the left is the number of feet from the lamp or fixture. The scale on the right is the footcandles of luminance at that distance. In these examples, both lamps produce a 4’ radius of light around the center of the beam. However the 35MR16/SP (spot) produces that beam spread at 20 feet while the 35MR16/FL (flood) has that same spread at 10 feet. In terms of light intensity, the 35MR16/SP produces 44 footcandles at 8 feet and the 35MR16/FL produces 22 footcandles. Once we have determined the area we need to light and the amount of light needed, we can search through the charts of the various lamps until we find the one that best meets our needs. 

Another easy to use source for this data is General Electric’s Light*Beams Software. Although this program was obviously developed by GE to promote the use of their lamps, there are charts with equivalent lamps from other manufactures available from other sources. According to the GE product description you can “Use Light*Beams to display the candlepower data for any (GE) lamp you choose - Light*Beams will perform the simple point calculations to tell you how much light will fall onto the surface as either a candlepower curve or cone diagram. Select a lamp, define its distance from a surface and its aiming angle with the surface, and Light*Beams will do the rest. If you aren't sure which lamp to choose, let Light*Beams help you. Light*Beams contains Lamp*Scout which will identify lamps meeting your performance criteria.”

The next problem is deciding how much light is needed on the object or area. We normally work in the range from 1/10 footcandle to 5 footcandles. Why 1/10 footcandle? Well that is the approximate light produced by a full moon. While you may not be able to read the paper by the full moon, most people can move about safely with the light of the full moon. Low levels of light do not produce glare or deep dark shadows which can make it more difficult to see. Some other guidelines to consider when determining the amount of light to project:

·     Age of the people using the area is a factor. As we get older we loose some light sensitivity.

·     Typically 5 footcandles is adequate to light an object or specific plant; however signs and house numbers may require more light.

·     Task lighting may have to be in the 20-50 footcandle range. Tasks like reading, playing games, and others that do not include fine detail will require 20-30 footcandles. Tasks like cleaning and fine detail work may require 50 footcandles or more.

·     By national building code, stairways must have a minimum of 1 footcandle as measured at each tread.