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Trousers and Illuminant Metameric Failure


This is about not forgetting the fundamentals or ever imagining that someone else will take care of the fundamentals for you. It's often said that we don't know what we've got til it's gone; that we choose to live in blissful ignorance of all the good stuff that goes on around us. What a piece of work is man!

I want to talk about colour rendering, and for my part, its been so long since I thought about this issue that I've even forgotten the technical term for what I'm writing about but not to worry, because Wikipedia rides to the rescue to remind me that what I'm looking for is Illuminant Metameric Failure (IMF). (There I do the research so that you don't have to).

There used to be a lot of IMF about, chiefly because we had such a variety of light sources to work with; tungsten; tungsten halogen, halophosphate fluorescent, tri-phosphor fluorescent, metal halide, ceramic metal halide. We've got rid of some of those, but now, of course,  you can add an entirely new source spectrum courtesy of our good friends in LED development. These days some think that IMF is a thing of the past - let me tell you something . . . no it ain't.

And what the devil is Illuminant Metameric Failure?  I hear you cry. 
Listen up.
It used to be that we only cared about the amount of light that we could create in a space; chiefly because for many millennia there simply wasn't enough artificial light to go round. But once we got used to the idea of having light a-plenty and I'm going to date that at 1973, the year of the first oil crisis, because it's the first time in history that we deliberately reduced light levels from some of the insane levels that were begin recommended - and I know about that because I worked in energy marketing for the electricity industry in the 1970s and was trying to sell you more of the stuff. Anyway once we had enough light we started paying attention to the quality of that light and, in particular, to the way it handled colours. And so colour rendering became an issue and the colour rendering index (CRI) became a lodestone for lighting specifiers.

And because we're lazy and like things to be pigeon-holed by numbers, we decided that anything with a CRI of 0.8 and above was good enough for us, and anything with a CRI above 0.9 was truly a gift from the gods.

But it's not that simple; we forgot that there are other things that really need care and attention, not the least of which is Illuminant Metameric Failure.

Yes but what is it?  you say again. 
OK - here goes.
Everything that we can see, we only see because of its colour (please don't get me side-tracked with day and night vision just stay with me) and most objects are blessed with METAMERIC pigments. And that is to say, colours that look the same under a variety of light source. The red socks that your granny gave you for your birthday will look the same red regardless of what light you choose to wear them under. And this makes metamerism Very Important, as you can imagine.

But it's not always the case. There are pigments that are susceptible to IMF, and here are a few stories for you, starting with the reason for me writing this piece now.

The Case of the Black Trousers.
Having lived through the 1970s (see above) I still ear the trauma caused by extreme brownness and refuse to wear any clothing of that shade, or any shade near it. Trousers, therefore, shall be BLACK. I had cause to re-visit an M&S store to exchange a new pair for a different size (sadly, it's not because I've got taller). We'd had problems choosing the chinos in the first place. It was tough enough sorting out the blues and blacks in the first place; when we got back and found a replacement pair of the same colour . . . imagine our surprise when the lighting above the services counter showed them as two distinctly different colours not shades of blue; one was definitely black, the other definitely blue.
So, M&S: long-time heroes of mine for selling clothes that real men can buy, as well as being the first high-street store to adopt tri-phosphor fluorescent lighting you're letting yourselves down. That combination of fluorescent and metal halide that you're working with just doesn't cut the mustard. Please try harder.

The Case of the Banana Corridor.
I was the lighting designer on an interior project to deck-out the executive floor of an international bank. The quality of finishes was, not surprisingly, to be of the highest order. The interior designer selected one of those wonderful polished plaster finishes in a subtle creamy-yellow along the main corridor. Very nice. I was using some new-fangled low-energy compact fluorescent lighting. Tri-phosphor technology, of course.
Unfortunately, the interior designer (despite my warning, I may add) put the finishes board together under a tungsten anglepoise lamp. The impact of IMF was immediate as the soft creamy-yellow went straight into banana-overdrive. Hmmmmm.

The Case of the Mis-matched Office Furniture.
This was my very first experience of IMF and such was the dramatic effect that it's still in my head after 30-odd years.
I was working in London's famous West End, surrounded by architects, interior designers and greek tavernas. It was a time to be young. Two designers walked into the showroom, each carrying a blue office chair. There was a secretarial chair (a wheely-one) and a meeting room chair (with four legs); both with pads covered in blue fabric, looking for all the world like the same colour. Herein lay the problem.
The designers had presented the fabrics to the client in daylight and under (as ever) tungsten anglepoise lamps as well as whatever passed for the ID zeitgeist in the early - 80s. And, having gained approval, they hadn't looked at the chairs again until they had all been delivered to site which, to no one's surprise, was using fluorescent lighting. And it was at this point that one of the chairs did a kind of electro-pop shimmy across the floor as the discreet blue fabric lit up like a disco. 
This was definitely Not Nice.

How can such things be resolved?
1. change the lighting. This is expensive.
2. change the finishes / fabrics. This is either expensive or impractical.
pretend its not happening. This is the cheapest, though fundamentally unsatisfactory solution.

But how do we stop it happening?
Ah yes: we start looking a bit closer at WHAT we're lighting, not just at light levels and tick-boxing colour rendering indices. And I'll now leave that to M&S.


RIBA CPD in 2015

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