Today lighting designers, specifiers and facilities teams are learning a new language of light-emitting diodes (LEDs) as well as an important distinction between lumen output and delivered lumens. The latter two lighting metrics are related yet different, and this is causing confusion in the field.
Read on to learn how to design with delivered lumens from LED lighting fixtures.
Decoding lumen outputs provided in spec sheets for conventional light fixtures
Spec sheets for conventional light fixtures list the lumen output of the light source as a defining characteristic of fixture performance, but the perceived importance of this metric is misleading. The lumen output (measured in lumens) describes the total quantity of lumens emitted by the light source; however, in most cases, light does not pass directly from the light source into the interior space. Instead, the light source emits light into a fixture housing, which reflects and focuses the light to deliver it to the target area or areas in a space. This light path is important, because light loses energy and intensity with every reflection (also known as the light loss factor). As a result, the amount of light leaving a fixture is often significantly lower than the light first emitted by the light source.
Accurately measuring fixture efficiency and light output
Fixture efficiency, a metric also included on fixture spec sheets, quantifies the percentage of the light emitted from the light source that is distributed by the fixture. Many in the field consider the illumination of conventional light sources in terms of lumen output rather than the light that actually reaches the target surface.
In contrast, LED fixtures describe light output in terms of delivered lumens, which refers to the amount of light delivered from the LED fixture to the target area. This delivered lumens metric discounts the lighting energy that is wasted when it bounces around inside the fixture, is absorbed by the fixture lens, or is emitted and scatters off into areas outside of the target surface.
Understanding the difference between lumen output and delivered lumens
While lumen output and delivered lumens are often mistaken for being interchangeable, they do, in fact, represent different aspects of a fixture’s performance. This common misperception is creating confusion and chaos in the lighting world. Lighting designers and facility managers are transitioning from conventional lighting sources such as fluorescent to LEDs; however, many select an LED fixture with delivered lumens that is equivalent to the lumen output of the conventional fixture it is replacing. The result is an LED-illuminated environment that is far too bright.
Preventing the unintended over-illumination of the LED environment
“When replacing a conventional fixture with an LED fixture, it is important to consider both the lumen output of the conventional light source and the efficiency of the conventional fixture,” said Anne Kustner Haser, president of Anne Kustner Lighting Design in Evanston, Illinois. “These two metrics enable a designer to determine the amount of light distributed by the conventional fixture into the surrounding space. It is that value that designers should attempt to match in the delivered lumens performance of the LED fixture they select.”