How do you evaluate LED products?
Lumen output is only part of the story and can be misleading. To fully evaluate an LED product one needs to review the overall system efficiency, optical control, thermal management of the LEDs, and know at what point in time the fixture will reach 30 percent lumen depreciation. Products with good optical efficiency and thermal management will be able to deliver more lumens, on average, than traditional HID products.
As the Department of Energy concluded in its Solid-State Lighting Commercial Product Testing Program:
"Until the field of SSL technologies and supporting knowledge matures, any claims regarding performance of SSL luminaires should be based on overall luminaire efficacy (i.e., from testing of the entire luminaire, including LEDs, drivers, heat sinks, optical lenses and housing), to avoid misleading buyers and causing long-term damage to the SSL market."
How are LEDs able to outperform HID?
Super-bright white LEDs have the advantage of minimal lumen depreciation, better optical efficiency and high lumens per watt. LEDs also have a vastly longer life span than traditional lamp sources. The luminaire must be designed to leverage these inherent advantages of LEDs.
If an LED fixture has lower initial lumen output than a traditional HID light, how can LED claim to deliver lumens more efficiently than HID?
When you average delivered lumens over the course of 60,000 hours, you'll see that LED outperforms a 400-watt MH lamp operated in a horizontal position. (60,000 hours is used for this comparison to show three full life cycles of the HID.) The MH's lumen depreciation, as well as optical and ballast losses, quickly reduce output of the HID system. Note that there are three relamps over 60,000 hours. Conversely, LED has significantly better lumen maintenance and a more efficient driver. Also note that the LED fixture typically doesn't need relamping from zero to 60,000 hours. Combine this with Beta's exclusive NanoOptic and LED outperforms MH over the course of the life of the fixture. Result: the LED's average delivered lumens is 74% higher than HID over 60,000 hours.
How does ambient temperature affect LED efficiency?
LED fixtures must be designed with junction temperature thermal management as a key component and use the correct LEDs. These products will then be robust enough to operate in most ambient temperature applications. Unlike fluorescent sources, cold temperatures do not impact the performance of LEDs.
What is junction temperature?
Junction temperature is the temperature at the point where an individual diode connects to its base. Maintaining a low junction temperature increases output and slows LED lumen depreciation. Junction temperature is a key metric for evaluating an LED product's quality and ability to deliver long life. The three things affecting junction temperature are: 1) drive current, 2) thermal path, and 3) ambient temperature. In general, the higher the drive current, the greater the heat generated at the die. Heat must be moved away from the die in order to maintain expected light output, life, and color. The amount of heat that can be removed depends upon the ambient temperature and the design of the thermal path from the die to the surroundings. [Source: DOE]
The Department of Energy advises: "Heat management and an awareness of the operating environment are critical considerations to the design and application of LED luminaires for general illumination. Successful products will use superior heat sink designs to dissipate heat, and minimize junction temperature. Keeping the junction temperature as low as possible and within manufacturer specifications is necessary in order to maximize the performance potential of LEDs."
Why is the life span of an LED measured as lumen depreciation?
The life span of an LED is vastly longer than that of incandescent, fluorescent or HID lamp sources, generally lasting 50,000 hours or longer. Although the LED never really burns out, product life span is measured by lumen depreciation. The Illuminating Engineering Society's (IES) current standard for calculating the life of an LED as the point at which the LED reaches 30 percent lumen depreciation. Remember, a 100,000-hour rating is not equivalent to lamp life rating. LED life is rated where it has reached 30 percent lumen depreciation. At 100,000 hours an LED would still be operating, but at a decreased lumen output.
How long is 100,000 hours?
Based on how long a fixture is illuminated per day, here's what 100,000 works out to:
- 34.2 years @ 8 hours per day
- 22.8 years @ 12 hours per day
- 14.8 years @ 18 hours per day
- 11.4 years @ 24 hours a day