What Size Beam Spread for Low-Voltage Landscape Lighting

FWHM

• Once light leaves its point of origin, it begins to spread out to each side. Even a narrow spotlight lights up objects to some degree several feet in all directions. The technical term for the measurement of the spread of a beam of light is called the full width half maximum (FWHM) divergence. Looking at a light beam similar to a bell curve the FWHM would be approximately the center 50 percent of the bell. It is expressed as an angle, not a width measurement.

Bulb Shape

• The shape of the light bulb is critical to producing the beam spread, according to Phillips, an international producer of light bulbs. Spherical lights such as found in home interior lighting fixtures produce beams of 360 degrees, while spot lights are much narrower. Some manufacturers produce low-voltage light bulbs with spreads varying from an extremely narrow 8 degrees to what is considered a standard spotlight of 35 degrees.

Same Power

• The spread of a beam is not influenced by its voltage, as both narrow and wide beams may be the same watts and use the same energy. A narrow beam may look brighter if stared at directly since the beam is more focused in one direction with less spillover and diffusion. The object where the center of the beam hits will also be brighter.

Different Needs

• Both wide and narrow beams have their place in landscaping. Fewer wide beams are required along a path, as they direct more light side to side. They are also less likely to be an annoyance if shining from the direction of where you are looking. The narrow beam allows the homeowner to keep the light from spilling over into areas where it is not wanted, such as into a window or in the neighbor's yard.