Start with the test report
The most reliable numbers on a spec sheet come from an IES LM-79 test. LM-79 is the standard method for measuring a complete LED luminaire: its total light output in lumens, the power it draws, its efficacy, how its light is distributed, and its colour characteristics.
Ask for the report itself, not just the sheet. Check that it was made by an accredited laboratory, that it tested the model and configuration you are buying, and that the figures on the sheet match the report.
The IES file
The IES file holds the photometric data from that test, written in the IES LM-63 format. It records the luminaire's light intensity in every direction.
Lighting designers load it into calculation software to model a real site: fixture positions, mounting heights, spacing and surfaces. That is how a design shows whether a luminaire will meet the required light levels and uniformity. A luminaire without an IES file cannot be properly designed with.
Light output and distribution
Lumens measure light output; watts measure power. Efficacy, in lumens per watt (lm/W), tells you how efficiently a luminaire turns power into light. Compare luminaires on delivered lumens and efficacy, not on wattage.
Distribution describes where the light goes. Roadway and area luminaires are classified by IES distribution types I to V: Type I is a narrow pattern along the road, Types II to IV reach progressively wider and further, and Type V is a round, even pattern. Floodlights and indoor fixtures are usually described by beam angle.
Correlated colour temperature (CCT, in kelvin) describes how warm or cool the light looks. The colour rendering index (CRI, or Ra) describes how faithfully it shows colours; higher is better.
Ratings that matter outdoors
TM-21 limits how far a projection may go beyond the hours the LEDs were actually tested, so a very large L70 figure deserves a question.
- BUG rating: backlight, uplight and glare, each scored from 0 to 5, where lower is better
- Lumen maintenance: LM-80 tests the LEDs and TM-21 projects their life; L70 is the number of hours until output falls to 70 % of its initial value
- IP rating: protection against dust (first digit) and water (second digit), under IEC 60529
- IK rating: resistance to impact, from IK00 to IK10, under IEC 62262
- Surge protection, in kilovolts: the voltage spike the luminaire is built to withstand
- Operating temperature: the ambient range the luminaire is rated for
Listings and marks
Safety listings such as cULus or ETL show that a luminaire has been tested to the applicable North American safety standards by a recognized certification body. Some programs also qualify luminaires against performance requirements, and certain utility rebates ask for that qualification.
A mark is only as good as the sheet it is printed on. Check that it appears on the current revision and applies to the exact model, and where possible confirm the listing in the certifier's public database.
Red flags
Treat any of these as a reason to ask more questions before you buy.
- No test report, or one from an unnamed laboratory
- Figures that do not add up: lumens divided by watts should match the stated efficacy
- A headline figure that does not appear in the sheet's own tables
- A rated life with no basis, such as no L value or no test data
- Marks shown as logos with no listing behind them
- No IES file available