42 Partners - Relux - Absolute photometry in EU-Lum files

The original presumption with photometric data files was that light sources could be changed. Data was structured so that the file contained a degrading factor which is the ratio of the lumen output of the luminaire to the total lumen output of all the sources, called the LOR (light output ratio). The total lumen output of the sources was specified as a separate item in the file.
It was possible to list a selection of sources in the file with differing outputs.
Luminaire lumens = source lumens * LOR
The file then gives information about how much light goes in what direction.
Of all the light from the luminaire (x lumens), y% goes in this direction.

LED luminaires usually have an integral light source that is not meant to be changed and that cannot be separated from the luminaire. There is no need to have a light output ratio figure because the light source cannot be changed, the measured lumen output of the luminaire is the only figure used and is specified as the source lumen output with no degradation, i.e. LOR=1.
The only source output specified in the file is the output of the luminaire itself.
Luminaire lumens = source lumens * 1
The file still gives the same information about how much light goes in what direction by saying:
Of all the light from the luminaire (x lumens), y% goes in this direction.

LED manufacturers then started publishing nominal lumen output value for their light sources, so it became possible to produce a file which incorporated the measured luminaire output and the published source output with a corresponding degrading factor.
The only source specified in the file is the nominal output of the source.
Luminaire lumens = source lumens * LOR
The file still gives the same information about how much light goes in what direction by saying:
Of all the light from the luminaire (x lumens), y% goes in this direction.

All three of the variants give the same calculation result:
The calculated lumen output of the luminaire is the same.
The files still gives the same information about how much light goes in what direction by saying:
Of all the light from the luminaire (x lumens), y% goes in this direction.

Because the starting point is how much light comes from the luminaire, this type of file is called 'relative photometry'.

Some members of the lighting community decided that they wanted a note in the data file to show that the light source in a luminaire shouldn't be changed (why this was needed is matter of debate, you shouldn't just change the lumen output figure in a data file unless you know what you are doing).

This was achieved by specifying that:
the figure given in the file for the number of lamps was a negative value instead of a positive value
the only source that could be specified was the output of the luminaire
This modification was incorrectly called 'absolute photometry'.

Absolute photometry is a system of measurement where you do not measure the lumen output of the luminaire. It is meant for specialist lighting applications where the only interest is in a few measurement points, e.g. specific angles from a car headlight.
Absolute photometry data files give information about how much light goes in what direction by saying:
We don't know how much light comes from the luminaire, but we do know that y candelas go in this direction.

Only measuring a few output angles is not applicable to luminaires for general lighting because we need to know where all of the light goes. If we measure where all the light goes, we know how much light comes from the luminaire. If we know how much light comes from the luminaire, and describe where it all goes, there is no difference between 'absolute photometry' and 'relative photometry'.