LEDs were once touted for their energy savings—and little more. Now, after years of rapid advancement, the technology dominates the commercial lighting market. Here is a quick primer to help architects and designers speak the lighting language when specifying this solid-state source. As an example, we use Durham, N.C.–based Cree’s LN Series indirect/direct LED luminaire to illustrate the concepts.
Correlated Color Temperature (CCT): Measured in degrees Kelvin, the lower the CCT value, the warmer the tint of the source. Candlelight is rated at 1850K while daylight is 5000K and greater. LEDs were once notorious for their blue tint, but no more. In fact, several manufacturers offer products with tunable white-light technology, which allows users to vary the output of an LED source from warm to cool temperatures. Cree's LN Series, however, comes in two specific CCTs: a warm 3500K and 4000K. The slightly cooler temperature is suitable for spaces that also rely on natural light.
Color Rendering Index (CRI): CRI values run from zero to 100 and indicate how well a light source shows the colors of the objects it illuminates as compared to the performance of a reference source. For lighting sources with a CCT below 5000K, the reference source is a blackbody radiator—an opaque, non-reflective body object that emits a spectrum of light based solely on temperature. For sources with a CCT exceeding 5000K, the reference is daylight. The 200 white LEDs along the spine of the LN Series have a CRI of 90-plus, while a typical fluorescent lamp has a CRI in the 80s, says Gary Trott, vice president of product strategy for Cree.
Optics: The concentrated light output of an LED isn’t useful without lenses and reflectors to control and direct its distribution. To minimize light loss, total internal reflection (TIR) optics incorporate a reflector and a refractive lens to control direct light from the source and to repurpose reflected light that may otherwise go astray. In the LN Series, TIR microlenses extract up to 90 percent of the light fired from the LEDs into the lightguide, or the luminaire's acrylic wings. The facets, spaced at approximately 1/8-inch on center, help distribute light uniformly.
Efficacy: Not to be confused with efficiency, luminous efficacy measures how much visible light is output per unit of electricity. (Efficiency, in regards to lighting, indicates how much light reaches the intended area to be illuminated.) The LN Series delivers 110 lumens per watt, about 30 percent more than a typical fluorescent source, and 500 percent more than an incandescent.
Dimming: Many luminaires now integrate occupancy and daylight sensors to determine when to power down. The LN Series can dim through zero-to-10V analog dimmers (similar to legacy sources) or Cree’s digital “SmartCast” technology. LEDs may flicker when dimmed depending on their compatibility—or lack thereof—with their dimming control device. In the past, some LEDs also experienced extreme color shifting. Now lighting manufacturers have introduced LEDs that emulate the progressively warm light of halogen and incandescent sources when dimmed.