What is LM-79 goniophotometry photometric measurement?

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Intensity meters like the LM-79 may be used to determine how many candelas of light are emitted from different angles of an LED light source. Together, these candela measurements can be used to calculate total or zonal lumen production, or for illuminance forecast within an application.

The test cannot begin until the device being tested (DUT) has been properly connected to a steady power source. The ideal temperature for the room in which the fixture will be installed is 25 degrees Celsius. Expect between 30 minutes and 2 hours for this.

The LM-79 document provides specific guidance on the installation, calibration, and operation of the designated apparatus. Both integrating spheres and goniophotometers are mentioned briefly in LM-79 as two of the most important pieces of equipment that may be employed.

To prevent inconsistencies and promote uniformity, goniophotometers of type C are the only kind allowed for use with LM-79. The DUT is stationary throughout these tests, and the light intensity is measured while sensors spin around it along various axes. The DUT will move relative to a stationary sensor or sensors in goniophotometry types A and B. Inaccuracies may result because the cooling effect caused by airflow around an LED light fitting reduces the efficiency of the LEDs. This impact is reduced to a minimum with type C goniophotometers due to the immobility of the DUT relative to the revolving sensors.


Absolute v Relative Photometric Measurement

The LM-79 situation calls for absolute photometric measurements. The evaluation must thus specify the precise lumen output of the device under scrutiny (DUT).

They are novel compared to photometric evaluations done before the introduction of LED technology. In the past, when incandescent bulbs were the norm, publishing standardized data for a given lamp and then describing the light output from a fixture in terms of its light output ratio was commonplace (LOR). For instance, if the LOR of a fluorescent louver fitting was 0.7, then 70% of the light from the bulb escaped the luminaire and the other 30% was lost inside. This procedure allowed us to rapidly obtain photometric data for a wide range of light sources (for example, data for a 4 x 14W and 4 x 21W fixture could be generated from just one photometric test). Analogous to relative photometry.

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