Your questions answered: LED codes and standards
In lighting, one size/type does not fit all. LEDs, or light-emitting diodes, have gained prominence in lighting design for their energy and operational savings. More information is provided by the presenters of the July 23 webcast.
During the July 23 webcast about LED codes and standards, more than 100 questions were left unanswered. Responses have been supplied by presenters Michael Chow, PE, CxA, LEED AP BD+C, Principal, Metro CD Engineering, Columbus, Ohio; and Robert J. Garra Jr., PE, CDT, Vice President, Electrical Engineer, CannonDesign, Grand Island, N.Y.
Question: You are not discussing the difference in lamp life definitions. Can you please explain?
Michael Chow: For most lamp sources, life is measured in the number of hours it takes 50% of a given sample of lamps to fail or burn out. For example, suppose you have a manufacturer claiming 20,000 hours for a T8 lamp. This means 50% of the lamps in their testing failed at 20,000 hours. LEDs require a different type of lamp life measurement. L70 as this is the accepted standard for the “life” of an LED light source. LEDs use lumen maintenance life (different from rated life, the rated lumen-maintenance life is defined as “the elapsed operating time over which an LED light source will maintain the percentage of itsinitial light output”). Recall from the webcast that LEDs turn to fade over time instead of a catastrophic failure. L70 is the amount of hours where an LED has 70% of its initial lumen output.
Question: Is there an organization that is working on standardized specs for LEDs?
Michael Chow: The DesignLights Consortium has compiled a list of high-quality, high-efficiency LED products. MasterSpec and other standardized specifications have included standardized LED specifications. SpecsIntact also is a good resource for standardized LED specifications.
Question: Any info about tunable (color) LEDs?
Rob Garra: Additional information can be found by reading these articles:
- Tunable LED lighting and controls are starts of show
- Photonic frontiers: RGB LEDs for illumination: Color-tunable RGB LED lighting goes far beyond replacement bulbs
Question: Have any long-term studies verified the accuracy of TM-21? It appears to assume a linear relationship in lumen output versus age for the life of the lamp.
Michael Chow: I am not aware of any long-term studies to verify the accuracy of TM-21.
Question: Do you usually specify the fixture to have a certain L70? Some manufacturers we use have an L90 and L80 instead of L70. Is L70 the standard?
Michael Chow: I recommend specifying to L70 as this is the accepted standard for the “life” of an LED light source. LEDs use lumen maintenance life (different from rated life, the rated lumen-maintenance life is defined as “the elapsed operating time over which an LED light source will maintain the percentage of its initial light output”). Recall from the webcast that LEDs tend to fade over time instead of a catastrophic failure.
Question: What are the average min/max working temps for LED temps?
Rob Garra: See links below:
- Lighting Research Center
- Temperature Derating in High-Power LED Applications
Question: You discussed LED versus T5/T8 fluorescent lamps. How would you compare compact fluorescent lamps, such as in downlights, versus LED; and high-intensity discharge (HID), such as roadways and area lighting?
Michael Chow: Compact fluorescent lamps (CFLs), HID (roadway and area lighting) can be compared to LEDs by looking at the rated life compared to the LED L70 lumen maintenance life: efficacy (lumens/Watt) and color rendering index (CRI). Remember to make sure you are getting the IES LM-79, LM-80, and TM-21 data from the manufacturers of the lamps you are comparing. LEDs are directional light sources. When evaluating lighting fixtures with CFLs, HIDs, etc. with reflectors to LEDs, find the delivered lumens or foot candles to the target (e.g., work plane, grade if outdoor parking lot, etc.). A particular LED fixture may have less efficacy at the LEDs, but the delivered lumens may be higher than with HID because most LED fixtures do not use reflectors. Typically a lot of lumen output (40%) can be lost through reflectors.
Question: Are LEDs qualified for hazardous areas?
Michael Chow: LEDs are not always qualified for hazardous areas. You will need to make sure a particular LED fixture is listed for hazardous areas.
Question: Can you purchase LED lamps with more lumens than desired and add dimmers to them to dim to the desired light level and when the lifespan of the LED fixture is reached, increase the lumen of the LEDs by increasing the dimmer?
Michael Chow: This is a great idea and many lighting designers incorporate what you have asked.
Question: Do line voltage fluctuation (typical summertime brownout variations of 5% to 10%) impair/damage the drivers?
Michael Chow: An LED’s driver should protect the LEDs from line-voltage fluctuations. The IES does not address this with their LM-79, LM-80, or TM-21 documents. Engineers should examine and LED’s driver
specifications to see if it is UL listed for the range of voltage listed.
Question: What depreciation factor (light loss factor, or LLF) is typical for LED fixtures?
Michael Chow: LLF takes into account many factors such as lamp lumen depreciation, luminaire dirt depreciation, ambient temperature effects, etc. Information that addresses this can be found in the presentation Solid-State Street Lighting Calculating Light Loss Factors.
Question: Are there special considerations for the power distribution system that will extend the life of LED lighting? For example, installing panels dedicated only for lighting and protected with asurge protection device (SPD).
Michael Chow: It is a great idea to protect LED lighting from surges. Your example of installing panels dedicated for lighting with a SPD is an excellent idea. Additional protection can be provided by installing a SPD at each LED lighting fixture. For example, installing a SPD at each LED lighting pole is a great idea.
Question: Can you please address what causes LED flicker, even with integral drivers (non-screw-based LED lamps) and how can you prevent it?
Michael Chow: Flicker is undesirable with LEDs and is usually the result of an LED and driver that are not matched well together. Recommended practice includes using reputable manufacturers.
Question: Any problem with using LED lamps with a basic photo cell control? Many screw-in CFLs are marked with warnings not to use with photocell controls.
Michael Chow: Photocells are not standard. Due to their differences (e.g., some required a minimum load to work) it is recommended that the for a specific photocell application, the photocell specification be examined to determine if an LED lamp can be used.