Relative Photometry - measures the photometric qualities of a luminaire relative to the rated lumen output of the lamp(s). This type of testing is used for traditional (fluorescent, incandescent, metal halide) luminaires.

How does it work? A stabilized reference lamp is measured and normalized with the lamp factor (Lamp Factor = rated lamp lumens/measured lamp lumens) to the rated lamp lumens. The stabilized lamp is then installed in the luminaire and the complete unit is photometrically measured. The final candlepower output is corrected with the lamp factor from the reference lamp and gets recorded into the test report.

Absolute Photometry - is required for LED fixtures and measures the complete lighting system including lamp, ballast/driver, thermal management and all fixture components. This ensures all combined characteristics (stabilized temperature, color temperature, thermal management, etc.) of the system are represented in the test results.

How does it work? Measured candlepower and wattage values are recorded, with no modifications or corrections. IES document LM-79 provides the requirements for testing LED luminaires with Absolute Photometry. Fixture efficacy should be reported based on the tested results of the complete unit, not the LED array/module alone.

One type of test is not superior to the other. Both present useful data and both can suffer from shortcomings if used incorrectly. Be careful comparing Relative to Absolute tests; insure that the all factors affecting both tests are included for fair and meaningful comparisons.

Focal Point provides independent Absolute photometry tests for LED products and Relative photometry tests for traditional products for your use.

References: IES G-2-10 – Guideline for the Application of general Illumination (“White”) Light-Emitting Diode (LED) TechnologiesIES LM-46-04 – Approved Method for Photometric testing of Indoor Luminaires Using high Intensity Discharge or Incandescent Filament Lamps

Additional Information: Efficiency, Efficacy and Photometric Distribution, from Illuminate Magazine Jan-Feb 2011, written by Kevin Willmorth

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