INDO Reviews BS 5489-1:2020 – Photometry
This is the second in a series of three articles looking at the main changes to BS5489, published earlier in 2020. The fact that BS5489 now confirms categorically that absolute and relative photometry both deliver the same result was seen as a significant clarification
Why was the clarification on photometry important?
In a technical industry where specifications and data are critical, nailing down the absolute versus relative photometry argument was important. The new document clearly states what’s going on with the two different types of photometry and confirming the fact that they both deliver the same result.
It’s vital the industry is able to understand these sorts of technical nuance. Customers should get very clear guidance so that any myths or misconceptions that start circulating don’t simply assume some kind of accepted wisdom.
As LED continues to be taken up, the data must be rooted in fact. It’s an easy trap to fall into, to assume that absolute must be better because it is ‘absolute’ – of course that sounds better, but the reality is that LED technology is just not that simple, so yes, I think it was good that a general misconception has been put to bed formally by the industry.
For us a company, it was very heartening to see that this concept, which INDO has been championing for a long while, has now been formally validated.
What do the new standards say?
The top of the section on photometry in the standard states clearly;
…Lighting design can be carried out using absolute or relative photometry… both have advantages and disadvantages and there is no preferred option as either method is expected to produce the same result
This directly counters the misinformation that’s been going around the industry
You do see specifications issued where it states the photometry provided must be absolute. However, since there are two ways of measuring photometry (absolute and relative), as long as they’ve been measured properly, it will result in the same outcome – both within the client’s lighting design software package and on the ground, once installed.
Of course, there is no issue in manufacturers being able to prepare absolute photometry – we can and will do that, but it is essential to understand that “the photometric file used to determine the performance of the light source is an accurate representation of the luminaires” Again, it doesn’t matter which measure is used, so long as it’s accurate. And that is the critical point.
For more information on this, read our absolute vs relative blog post (It continues to be one of our most visited web pages!).
In reality, everything to do with LED photometry is more complex anyway.
Before LED, when you had lamp photometry, you had a lumen package for a given lamp that was effectively aggregated over the testing of a large number of lamps.
So for example, when we said a Cosmopolis was 16.5k lumens, that was because they’d taken multiple measurements on that lamp type and aggregated the data to state a single lumen package. Then you could do your photometry… you’d use that lamp data, put it into your photometry and then your light output ratio (LOR) would tell you what the actual lumen package was for that luminaire.
Nowadays, manufacturers take a series of LED boards, test them and then measure the lumen package. Inevitably the best result is selected, the one with the highest lumen package and that is the figure presented. Really, if you’re using absolute photometry, you should be obliged to test at least 10 modules for each variant and take an average of that lumen package.
Most people were comfortable with a lamp because it was really easy to understand how it worked. Whoever you bought your 250 Watt SON lamp from, it didn’t matter; they would all be the same and you knew what you were getting. But now, there are so many variables. Even if everyone used the same LED chips, there are multiple other factors in the equation and customers end up with different results.
It’s understandable that people feel more comfortable with absolute photometry because they believe it to be more accurate, and closer to the traditional way of evaluating light sources.
So, while LED technology has changed everything, most people still like to draw equivalents with old lamps. They’re using the experience of old technology to understand the new and unfortunately, that won’t yield accurate results. Customers need LED experts and manufacturers to advise and help them navigate the new product complexities; to be open, honest and clear about data.
It’s a learning curve and that’s definitely why these standards are so important to help us all understand how to do our jobs well and apply the correct approach to current technology.
If you have any questions about this (or any) area of the new BS5489 revisions, do get in touch by email or phone us on 02380 982022
