Frequently Asked Questions

Frequently Asked Questions About General Illumination LEDs

What is an LED?
Where have LEDs been used in the lighting industry?
Have LEDs always been used in general illumination lighting?
Why have past attempts to create general illumination LEDs failed?
Why don’t LEDs function as efficiently in a traditional fixture housing?
What are the advantages to using LED lights?
What’s the difference between efficiency and efficacy?
How do you evaluate LED products?
How are LEDs able to outperform HID?
If an LED fixture has lower initial lumen output than a traditional HID light, how can an LED fixture claim to deliver lumens more efficiently than an HID?
How does ambient temperature affect LED efficiency?
What is junction temperature?
Do I have to replace LED diodes?
Why is the life span of an LED measured as lumen depreciation?
How long is 50,000 hours?

What is an LED?
LED is the common abbreviation for a light-emitting diode. Each individual LED consists of a semiconductor diode that emits light when a voltage is applied to it. The electronics industry has used LED technology for several decades as indicator lights for various electronic devices. In more recent years, LED technology has progressed to the point where it is viable for general lighting applications.

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Where have LEDs been used in the lighting industry?
LEDs are commonly used in aesthetic, effect, or specialty lighting applications, including architectural highlighting. Many traffic lights and exit signs, for example, now use LEDs.

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Have LEDs always been used in general illumination lighting?
No. Early attempts to apply LEDs in general illumination lighting failed because they didn't meet the lumen-per-watt output or color requirements. Technology has advanced to the point where using LEDs for general illumination is now viable. Lighting industry experts are gaining a better understanding of how to capitalize on that technology.

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Why have past attempts to create general illumination LEDs failed?
Conventional approaches to developing general illumination LEDs often involved retrofitting existing fixtures to house the new LED technology. Instead of investigating the benefits and challenges of LEDs, many early attempts simply used traditional lighting standards and housings. The problem was that LED technology breaks all traditional rules, and it quickly became apparent that old thinking couldn’t be applied to this new technology.

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Why don’t LEDs function as efficiently in a traditional fixture housing?
An LED module may physically fit into an existing housing, but that housing doesn’t leverage the inherent qualities of the LEDs. Standard housings can’t handle the challenges of LED thermal management, which is vastly different than thermal management for traditional incandescent or fluorescent lighting. Also, the optical design used in most traditional fixtures doesn’t maximize the LED’s efficiency.

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What are the advantages to using LED lights?
LEDs bring several advantages to the lighting industry, including high efficiency and durability, and, with superior life over other lamp sources, their required maintenance is greatly reduced. This translates into energy savings, maintenance savings and an overall reduction in cost of ownership over the product’s lifetime.

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What’s the difference between efficiency and efficacy?
Efficacy is a term normally used in cases where the input and output units differ. In lighting, we are concerned with the amount of light (in lumens) produced by a certain amount of electricity (in watts).  

On the other hand, efficiency is a term that is typically dimensionless. For example, lighting fixture efficiency is the ratio of the total lumens exiting the fixture to the total lumens initially produced by the light source.

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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 US Department of Energy concluded in its Solid-State Lighting (SSL) 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.”

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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. This means these LEDs can be used to replace traditional HID luminaires. LEDs also have a vastly longer life span than traditional lamp sources. The fixture design also must be designed to leverage these inherent advantages of LEDs. A Total Systems Approach is needed for an LED product to bring all these features together.

LED fixtures also have an environmental advantage in that they contain no mercury, last longer and produce less waste, and they are made from fully recyclable materials. Furthermore, the extruded aluminum heat sink is manufactured using 77% post-industrial recycled material.

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If an LED fixture has lower initial lumen output than a traditional HID light, how can an LED fixture claim to deliver lumens more efficiently than an HID?

When you average delivered lumens over the course of 60,000 hours, you’ll see that an LED fixture 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, an LED fixture 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. LED outperforms MH over the course of the life of the fixture. Result: the LED’s average delivered lumens is 46% higher than HID over 60,000 hours.

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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.

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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: drive current, thermal path, and 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.”

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Do I have to replace LED diodes?
An LED does not burn out like a standard lamp, so individual diodes do not need to be replaced. Instead, the diodes gradually produce lower output levels over a very long period of time. If one LED fails, it does not produce a complete fixture outage.

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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. As LEDs never really burn out, product life span is therefore measured by lumen depreciation.

The Illuminating Engineering Society (IES) currently recommends calculating the life of an LED as the point at which the LED reaches 30 percent lumen depreciation.

Remember, a 50,000-hour rating is not equivalent to lamp life rating. LED life is rated where it has reached 30 percent lumen depreciation. At 50,000 hours an LED would still be operating, but at a decreased lumen output.

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How long is 50,000 hours?
Based on how long a fixture is illuminated per day, here’s what 50,000 works out to:

Hours of Operation: 50,000 hours is:
24 hours a day 5.7 years
18 hours per day 7.6 years
12 hours per day 11.4 years
8 hours per day 17 years