LED Terminology in Plain English
Junction Temperature (Tj)
The critical thermal measurement of a diode's operating temperature. This largely determines the life of the LED, the amount of light the LED produces, and the color stability of the output over time.
Lumen Maintenance (L70)
The amount of time it takes for a fixture's lumen output to decrease by 30% from its initial lumen output. 70,000 hrs represents 16 years if the lights are run 12 hours/night, 7 days/week. If they're run at the national average of 10.5 hours/night and 7 days/week, then 70,000 hours extends just over 18 years. Go to: www1.eere.energy.gov/buildings/ssl/reliability_device.html
Downward Delivered Lumens
The amount of usable light emitted from a fixture that actually reaches the ground. This is separate from the rest of the light being emitted from the fixture that goes outward and/or upward.
SSL - Solid State Lighting
This refers to a type of lighting that uses semiconductor light-emitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination.
CCT - Color Corrected Temperature
A measurement of the visual color produced by a light source. LEDs in the 6000K CCT range produce a more bluish-white color. LEDs in the 4500K CCT range produce a white light virtually identical to that of Metal Halide. LEDs in the 3500K CCT range have a color more similar to incandescent lamps. What's important to note is that higher color temps produce higher efficacy (lumens per watt).
CRI - Color Rendering Index
A measurement of how accurately the color of objects appears to us under different lighting conditions. Natural sunlight has a value of 100, in which all colors appear as they should. HPS lighting generally produces CRI of around 21 which is why everything is color shifted and somewhat "washed out". White light, such as LED, Metal Halide and Induction lamping, generally produces a CRI in the range of 75-80.
Lumens per Watt (Efficacy)
The number of lumens produced by the light source or fixture, versus the amount of wattage required to operate that light source or fixture.
IESNA LM-79-08: Approved Method for the Electrical and Photometric Measurements of Solid-State Lighting Products
The guidelines by which we have shifted from relative photometry to absolute photometry.
IESNA LM-80-08: Approved Method for Measuring Lumen Maintenance of LED Light Sources
The guidelines by which LED manufacturers perform the testing that yields their life rating data.
DOE CALiPer: Commercially Available LED Product Evaluation and Reporting
"Consumer Watchdog" program by the Department of Energy to independently test various LED light fixtures and/or LED systems available to the market, with an initial focus on this that may not live up to their marketing material. For more information: http://www1.eere.energy.gov/buildings/ssl/caliper.html
Remember that LEDs are not just another white light source, but a technology that is completely different from HID light sources. The thermal dependence of LEDs is of the utmost importance, and if properly addressed and controlled, these lights can provide very efficient light output for decades with little to no maintenance while reducing current energy consumption by upwards of 50%.