September 24, 2015
LED Luminaires - Efficiently Delivering Lumens
LED Lamps and Luminaires are well known for having the highest luminous efficacy, producing the most light with the least amount of energy. This has made them an attractive energy-saving alternative for conventional light sources in numerous applications. However, the efficiency of LED luminaires is not only measurable in terms of luminous efficacy; it can also be assessed in terms of efficient light distribution.
Will an LED fixture be bright enough? This question is often asked when considering LED upgrades. For this reason, one of the first things considered is the lumen output. How many lumens does the LED produce compared to the current lamp? While this information can be beneficial, it fails to paint the whole picture.
To better understand this, let’s look at an example. Say we are considering replacing a 400W metal halide flood light with a HyLite LED flood fixture. The initial lumens for a 400W metal halide lamp is approximately 36,000. This is the amount of light that is produced the first time the lamp is turned on. All lamps have some form of lumen depreciation. A better gauge for the lumen output of a particular lamp is the mean lumens. Mean lumens typically list the lumen output left when a lamp reaches 40% of its rated life. The mean lumens for the 400W metal halide lamp is approximately 24,000.
The recommended replacement for the 400W metal halide flood is the 120W HyLite LED Prizm, producing approximately 13,000 lumens. By only looking at lumens, it appears the new LED Flood Light fixture will be only half as bright. Yet, lighting upgrades much like this have been piloted worldwide with overwhelmingly positive results. How is this possible? The answer revolves around luminaire (fixture) efficiency and delivered lumens. The efficiency of a light fixture is a comparison of the light output emitted by a light fixture versus the light output of the fixture’s lamp.
Traditional light fixtures, especially those offering directional illumination, have an inherent issue: light is needed in one direction while the lamp produces omni-directional illumination. The light extends in all directions inside the fixture. To alter the light distribution and allow more light to emit from the fixture, reflectors and lenses are incorporated. These components reflect and distribute the light from inside the fixture outwards. However, even with the best designs, these components are not 100% efficient. A lot of light becomes trapped inside the fixture hampering the overall efficiency of the lighting system. For floodlights, it is common for fixture efficiencies to range from 50% to 70. This means that 30% to 50% of the lumens from a Metal Halide lamp are trapped and wasted inside the fixture. For our example, let’s take the mean of this statistic and assume the fixture efficiency is 60%. This would result in 13,200 delivered lumens.
Conversely, LED luminaires offer superior fixture efficiency. LEDs are a directional light source. The illumination extending from the LED is directed outside of the fixture. Additionally, the illumination can be more efficiently controlled allowing numerous distribution patterns and beam angles with minimal light loss.
To top it off, the lumens tested and reported for LED Luminaires are the actual illumination extending from the fixture. The LED modules or packages are not measured independently before installation in the fixture, like metal halide lamps. The reported lumen output of an LED fixture is the total lumens emitting from the fixture. The fixture efficiency is, essentially, 100%.
The effect of luminaire efficacy on the two technologies:
Once fixture efficiency is taken into consideration, the difference in lumens between the metal halide and LED flood light is much closer. Other factors including distribution, beam angle, and field efficiency should be considered as well. Emitting more lumens outside a fixture is not beneficial if more light is wasted outside of the intended area. Testing and data can show how efficiently lumens are being distributed and delivered within the intended area. LED Luminaires have the ability to tightly control light distribution.
Laboratory testing on the HyLIte LED Prizm showed that over 99.4% of the delivered lumens were emitted within the classified NEMA Spread. This concept is known as field efficiency. Field Efficiency is that percentage of initial lumens from a light source that are utilized within the field angle, or the degree of light to 10% of maximum intensity on either side. Field Efficiency builds off the concept of luminaire efficiency. Depending on the model, metal halide flood light fixtures offer field efficiency between 35% to 55%.
The Table below shows the delivered lumens from both the Metal Halide and HyLite LED Fixtures. The 400W MH floodlight only offers 7,650 delivered lumens compared to 8,429 for the LED Fixture. Because of the LED fixtures ability to efficiently create and distribute illumination, maximum energy savings are realized.