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Absolute and Relative photometry

December, 2017

Lighting designers use photometric data on a daily basis when preparing a lighting design. All luminaires have photometric files that are produced by the luminaire manufacturers, which can be imported into lighting design packages such as Lighting Reality, DIALux, Relux etc.  But do we really understand how this data is presented and does it affect the end result?


There are two commonly used formats of Photometric data: Absolute and Relative photometry, but does one method produce a better, or more accurate result than the other?


For many years, HID luminaires used Relative Photometry. Relative Photometry measures the lumen output of the light source in free air on its own and compares it to the measured lumens from the light source when it is placed in the luminaire.  The difference in the two values is the Light Output ratio (LOR). The difference in the values (the LOR will be <1) occurs due to the luminaire losses from various factors - heat, internal reflectors and transmission through diffuser material etc. The advantage of relative photometry is that you can measure the light source without luminaire interference and because you have known losses, you can apply an LOR to many different variants of the same luminaire.


Absolute Photometry precisely measures the lumens emitted by a specific luminaire. The light source is measured inside the luminaire, so the flux produced is from the complete system, therefore already includes any thermal or optical loses giving an LOR of 1.0. The limitation of absolute photometry is that it is only applicable for the exact luminaire/light source combination tested.


The LOR is automatically integrated within the photometric file and is therefore automatically accounted for when using the file to perform a lighting calculation.  So whether you are using Absolute or Relative Photometry the results are absolutely the same! 

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