Calculating obtrusive light: whose job is it?

29-03-2023

Obtrusive light. What is it and why are we increasingly being asked to calculate it? More to the point, why is it something that more manufacturers are being asked for. I mean . . . . what’s going on? Surely, that’s the job of the lighting designer. Isn’t it?

thanks to the lia for use of the image

Whether as lighting designer, engineer, architect, or even client, you may well have been involved in exterior lighting projects. You may have noticed that, apart from the aesthetic elements of what exterior lighting looks like, the last few years has seen increasing concerns around the impact of artificial light on our night-time environment. Put those concerns alongside the embarrassing reports about professional ability coming out of the Grenfell enquiry and we shouldn’t be surprised that competency is fully in the frame.

So, to the crux of the matter . . .

Obtrusive light (sometimes referred to as light pollution) is generally based around sky glow, glare, light spill and light intrusion. And it’s something that can be legislated for if it is deemed to be a ‘statuary nuisance’. It doesn’t matter how good your design looks; it only takes one upheld complaint for the design to be challenged, revised and, in the worst-case scenario, turned off altogether.

Maybe not so bad if it’s a neighbour who’s taken that flat-glass luminaire and decided that it works better when aimed up at 80 degrees; a polite conversation will often suffice, assuming that they are reasonable people, which they surely will be (?!?!) Though, thinking about it, don’t you just love looking out of your window into the face of someone else’s floodlight?

But ramp it up another level and it’s a different kettle of fish. Let’s consider a large commercial/industrial development, where serious money has been spent. Then we can look forward to one super-angry client and a cartoon cloud of dust as those involved make a run for the exit, because the next question will be who is going to pick up the the bill for sorting out the changes that are needed?

If you are working for a manufacturer it is more than likely that, should this situation occur, you’ll be asked (expected?) to pick up the tab. This situation risks becoming most pernicious, given that the manufacturer has usually worked from minimal information and has simply interpreted someone else’s layout. We all know how often site conditions differ from the purity of a nice CAD clean drawing, and it’s a massive assumption that whoever did the layout in the first place knew the whole story.

So what’s to be done ?

There seems to be a trend for manufacturers being asked to mop up problems, post-installation. Perhaps the client didn’t want to pay for the obtrusive light calculations, maybe because ‘the manufacturer can do it’ or ‘the contractor deemed it unnecessary’. But regardless of reason, it was a bad decision

So, who’s responsibility is it to get the job done properly? The client? The designer? The installer? It seems grossly unfair to expect the manufacturer to carry responsibility for something that they may not have had the full competencies to carry out, by which I mean that they are unlikely to have received all of the information required to provide the obtrusive lighting assessment. That’s the kind of information that can only be gleaned by a full site survey . . . that no one wants to pay for.

And that’s the core of the problem. Unless we make our clients aware of this situation we can expect more of project failures – and, ultimately, less support from our manufacturing colleagues..

‘Competency can be determined through membership of a professional lighting body supported by the appropriate qualifications and experience in the application of lighting required.’

Obtrusive lighting calculations need to be carried out by those competent professionals (engineers and designers) who have all of the data required to properly assess the likely impact of an exterior scheme. They will be the people who are in a position to make a full survey of site conditions, the likely neighbouring conflict areas and a full understanding of how the lighting installation is intended to be used.

Lighting design is not well-served by design studios gaming the situation by promising a specification provided that the manufacturer provides an overview of the likely obtrusive light situation. That situation gets even more poor when the designer isn’t able to provide a full brief for the calculations. That’s where the incompetencies come from. And its simply not good enough.

This is getting a bit personal, but this is an example of what I’ve been faced with recently:

With our finger on the stuttering pulse of the jobbing lighting designer, the part of obtrusive light that has caused most conversation and angst amongst our community is the part that deals with limits of luminous intensity. Or, in plain English – the glare from the face of the projector. Its a calculation that can only be made when the designer knows where the observer is placed in the overall scene.

Essentially, it’s about the projected area of the product, which is affected by the difference caused by the viewer’s perspective, as the apparent projected area is different for each product seen from a single observer position. Perversely, it’s the one with the smallest projected area (or Ap) will give the greatest cause for concern.

The trusty workhorses of Dialux and Relux have yet to catch up with this aspect, as I found to my surprise (and displeasure) when recently working on an exterior facade. The only program I know of that can calculate it is AGI, which I found quite tiresome and expensive, although I did appreciate it telling me to ‘chill out’ on occasion.

I was fortunate to have a good lighting friend (we all need those!) to talk the problem through with, because a solution exists for the competent designer in GN01/21 from the ILP, with some worked examples. You can unknot your brow and head to pages 23,24 and 25 where the answer lies. Phew.

Thanks to Chris Fordham for this article, originally published at The LIght Review