LAMPS: LUMENS, BEAM SPREAD & WATTS
LAMPS: LUMENS, BEAM SPREAD & WATTS
One of the biggest difficulties people have when designing with accent lights is choosing the right lamp or fixture to use in each situation. Let’s take a look at what differentiates lamps and which ones you’ll need for different applications.
FIXTURES AND LAMPS
Accent lights come in two basic varieties: some fixtures are designed to have an incandescent or LED lamp inserted into them, and some fixtures have an LED lamp built in. For our purposes here, we’ll just talk about the “lamp,” whether it’s built in or sold separately. They’re labeled the same way, so you’ll be able to choose the right lamp, or the right fixture, using the same basic information.
LUMENS, BEAM SPREAD, AND WATTS
There are three important pieces of information about a lamp when it comes to designing your landscape lighting system: lumen output, beam spread and wattage. The lumen output tells you how bright the light is, the beam spread tells you how narrow or wide the beam of light is, and the wattage tells you how much energy the lamp uses. You’ll choose the lamp based on its lumen output and beam spread, and you’ll use the wattage to determine the size of the transformer.
GETTING THE RIGHT BRIGHTNESS
In order to get the same brightness level on each of the elements in the landscape that you’re illuminating, choose a lumen output level and beam spread that create equal illumination on the different objects. Often it will take multiple accent lights on larger objects like trees to achieve the same illumination as a wall wash or a smaller shrub. On tall trees, you’ll want to use lamps with high lumen outputs and narrow beam spreads, while on wall washes you’ll want lower outputs with wider beam spreads.
If you want to calculate the amount of illumination more carefully, you can consult the manufacturer’s photometric charts. The chart will tell you both the amount of illumination (measured in foot candles, or “fc”), and the width (in feet) of the illuminated area at different distances from the light source. When choosing lamps, aim for about 1.5 fc at the farthest point you’re illuminating.
One more thing to consider when deciding on lamps is that some surfaces reflect more light than others. For instance, if you used the same lamp as a wall wash on a white limestone block building, it would appear much brighter than a red brick building. So if you want a brick wall to be as bright as the limestone wall, you’ll need to use a brighter lamp. Different trees reflect light at different levels as well—the white bark of a birch or sycamore will be much brighter than the grayer bark of an oak, and the deep green of a spruce tree will be darker still. This means that when you’re illuminating different kinds of trees in the landscape, you will need to choose brighter lamps for darker, less reflective trees.
To sum up, there are three things to pay attention to when deciding on the right lamp for the situation: the size of the object you’re illuminating, whether it’s short and wide or tall and narrow, and how reflective it is. Use this information to choose the right beam spread and lumen output for the situation.
WATTAGE & TRANSFORMER TYPE
The final piece of information that you’ll need to attend to is the wattage. Each lamp will use a certain number of watts. The big difference with LEDs is that they use fewer watts than incandescent lamps. In order to size your transformer, you’ll need to add up the wattage of each fixture. The total wattage of the system will determine how big the transformer needs to be.
The wattage will also affect what size you’ll need your wire to be. Wire is sized based on amp draw, and you can consult a wiring chart to determine what size wire you’ll need for the system. Count up the total wattage being used on each wiring run. To calculate the amp draw, divide the number of watts by 12. Then look up the amp draw on top of the wire sizing chart, identify the length of the run on the left side, and use the wire size recommended in the chart.
The specs on landscape lighting, while they might seem complicated at first tell you just what you need to know to choose the right lamp for the job: lumens to tell you brightness, beam spread to tell you the shape it illuminates, and watts to tell you how much power it uses. With this in mind, you can select your lamps with confidence.