differential vs. absolute photometry
What is relative photometry?
Total Lumen output of a bare lamp is measured in relative photometry for more clarity; the total luminous flux of the lamp is measured separately without the Fixture. In IES LM-80- 08, IES LM 82-12, the relative photometry is used. The purpose is to find light output, Efficacy, color over temperature for light engines.
Relative photometry can measures:
- Total luminous flux of the bare lamp
- Luminous intensity distribution
- Scale the luminous intensity values by the ratio of the measured lamp lumens to the rated lamp lumens.
Some important features regarding relative photometry:
- Relative photometry is performed for luminaries with conventional sources.
- Luminaries test is referenced to the luminous Flux measured for bare lamp.
- The result produced is in the units of candela per rated lumens( cd/rated lm)
- It can also measure the efficiency of luminary.
- Normalize ballast factor, Lamp age.
- Specific lamp performance doesn’t matter
Relative photometry is not used in SSL with integrated lamps due to many reasons:
- The LED‘s are difficult to remove from the Luminaries.
- A large number of LED devices will not work properly without heat sinks provided with luminaries.
- The thermal environment that the LED devices undergo within the luminary is often radically different from the thermal environment it will practice in its “bare lamp” configuration.
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