| The IES-LM-79 standard definitively states that lumen output and thermal issues in solid-state lighting (SSL) are best managed by requiring SSL performance testing to use absolute photometry on a Type C goniophotometer. The organizations that run programs such as Caliper and Energy Star have therefore charged their performance-testing laboratories to use absolute photometry on a Type C goniophotometer for luminaire output, efficacy, intensity distribution and zonal lumen density measurements.
Due to LED manufacturing processes, individual LEDs vary in lumen output. When placed into a luminaire and/or an array, the sum of the individual LED lumens (as modified by the luminaire components) becomes the cumulative light output of that luminaire. Relative photometry prorates test results against self-certified bare-lamp rated lumens. It uses mathematical calculations to remove the differences in the output of the bare lamp(s). Absolute photometry records the actual intensity levels during the test, such that the manufactured differences among LEDs are tested as they performed on the day of testing, in the sample fixture.
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