Almost all of today’s common electric light sources may be categorized broadly as incandescent, fluorescent, LED, or high intensity discharge (HID). In order to understand the technology that allows each lamp type to produce light, it is important to review certain basic principles.
Watts, Lumens and Efficacy
Many people think that a higher wattage lamp will always produce more light than a lower wattage one. This confuses light output, which is measured in lumens, with the electric power a lamp uses, which is measured in watts. In fact, a 20W compact fluorescent lamp can produce just as much usable light as a 75W incandescent lamp (and save a great deal of energy). The most common way to express the energy efficiency of a light source—its “efficacy” — is as a ratio of the number of lumens it produces to each watt of power it consumes. In today’s energy conscious world, a lamp’s lumens per watt (LPW) performance is one of its most critical characteristics.
Service Life and Lumen Maintenance
The average rated service life of a lamp is based on the point in time when 50 percent of a large sample of those lamps will fail, or “burn out.” Many of today’s most sophisticated lamps offer an extremely long service life as well as exceptionally high color rendering. Lamps may cost more to purchase, but the additional expense can be offset by reduced energy costs and less frequent replacement. The light output of all lamps will deteriorate gradually over time. The rate of this deterioration —known as lumen maintenance— it varies from lamp type to lamp type. It is particularly important to understand lumen maintenance and service life when selecting a lamp for a hard-to reach fixture where replacement is difficult.
Foot-candles and Candlepower
Two important measurements of light are frequently confused. They are foot-candles and candlepower. Foot-candles measure the light that falls on a surface (illuminance) in lumens per square foot. A foot-candle minimum, for example, can be written into lighting specifications based on the Illuminating Engineering Society of North America’s (IESNA) recommended light level for a particular room or task. Candlepower, however, measures the intensity of a light source in a specific direction. Candlepower measurements are expressed in candelas and are independent of any object or surface that is being lit.
Point Source and Diffuse Source
When selecting a lamp for a given application it is important to determine whether a point source or a diffuse source is more appropriate. A point source is a lamp or fixture that is relatively small compared to the area it is lighting and has the potential to direct a concentrated beam of light on a specific surface or object. Incandescent, halogen and HID lamps are typically used in point source applications. A diffuse source, on the other hand, is relatively large and spreads light over a wide area. Linear fluorescent lamps are the most common diffuse source lighting.
Beam Angle and Field Angle
The pattern of light delivered by reflector lamps—a popular point source—is often described in terms of beam angle and field angle. Beam angle is the smaller figure, and refers to that portion of the lamp’s beam where the candlepower is greater than 50 percent of the candlepower measured at the center of the beam. The field angle describes the larger area of the beam where the candlepower is greater than 10 percent of the center beam candlepower.