LETTERS
Photometry – some common misconceptions cleared up

Dear Sir,

I read Alan Tulla’s feature in the July issue ‘Why all LED downlights are 100% efficient’ which referred to the fact that LED luminaires have an LOR of 100.

This raises some other points. There seems to be a common misconception that LED photometry must be presented in absolute format and that IES files are the only valid format for transferring LED luminaire data.

Relux and Dialux implemented a change to EuLumdat about one year ago to make LDT files available in absolute (Cd intensity) format as well as the usual relative (Cd/kLm) format. However, as we at 42 Partners keep pointing out, it is completely unnecessary.

Even without the ‘absolute flag’ in the file, as long as you correctly measure the lumen output and scale the file correctly in Cd/KLm all is well. We do however maintain that the @Lux_magazine presence of the flag encourages people to believe the file is measured in absolute units – the photometer measures candelas.

This is a very dangerous way to measure and very error prone – I don’t know of a lab that would attempt it. Most photometric laboratories still produce the data in a relative measurement mode then just process the data so that it is in absolute units – exactly as we do and the standards still ask for.

For the past 15 years we have been making one set of measurements and from it producing IES and Eulumdat and TM 14 and Phillum data files. Properly scaled data can be arranged to produce any file type.

The difference between a relative and absolute file is just down to which flavour the interpreting software prefers – US software has always used absolute, European has always used relative.

As it turns out relative still has advantages for HID, fluorescent and most lamp types because the same physical size lamp with the same power loading can have different lumen outputs, this is easy to handle in relative systems.

Absolute is advantageous with lamp types where the lumen output is fixed. As it happens this would be favoured for LEDs, but both systems are equally valid for all types of luminaires.

Can we please just stop the uninformed thinking that only IES files can be used for LEDs? An article ridiculing this notion would be great for the industry.

Richard Hayes
Director
42 Partners
Wolverhampton

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