LIGHTING | CALCULATION METHODS The problem is, and has always been, that we are trying to classify light objectively, when human responses to it are subjective and can vary greatly Jonathan Rush Jonathan Rush, partner, Hoare Lea That lux or illuminance is an irrelevant metric in need of a replacement is not in doubt. Anyone with a basic understanding of human responses to light and space knows that measuring the amount of light on a horizontal plane without considering the reflectance of materials, illuminance of the surrounding surfaces, or light on the human face is a poor way to describe quality. Given what we now know about the physiological effects of light on humans, it is also an appalling way of defining compliance. Lux gives a simple answer to a complex question, and the lighting industrys continued championing of the term has given it ubiquity, and reinforced the idea that lighting is easy. Change is necessary, but to address the complexity of the question properly, we cannot replace lux with another simple metric. Ultimately, good lighting design is not going to be summed up by a metric value and trying to do so is perhaps not going to provide the change we want to see. Mark Ridler, head of lighting, BDP Four eminent lighting experts, who command my immense respect, have come together to try to achieve an admirable beneficial change in lighting practice internationally. I broadly support the aims as set out. There are trends in workplace lighting that the Covid-19 pandemic is accelerating; among these are an emphasis on biophilia (read circadian) and a move away from some tasks to an emphasis on face-to-face communication. All of this supports a shift from horizontal task to better facial recognition in other words, ambient illumination. However, we need to be clear about who we design codes for and whose design practice this is going to change. Lighting designers? Unlikely. They tend not to design to code, are guided by experience, and are judged on their portfolio. Engineers? Quite possibly. They will understand the maths and have access to the tools to design to the code. Contractors, manufacturers? Probably not if the mathematics or concepts are too complex to grasp easily. There are risks in this approach, too. Ambient illuminance is defined as the average flux density of the indirect flux field within the volume of a space is much harder for a client to understand than how much light is here. If it is not intelligible or verifiable, it will founder. Alan Tulla, independent lighting consultant and former SLL president I remember my first boss, at the GEC Hirst Research Centre, saying to me: Alan, never forget the vertical illuminance; its so people can see people. Even 40 years ago, illuminating engineers, as we were called then, were emphasising the importance of lighting the space, rather than just the desktop. One reason horizontal illuminance has been around so long is that it has the great advantage of being easily understandable and verifiable. In his 2005 address, Mind the gap, then SLL president Geoff Cook spoke of how little people made of research in their daily professional practice. This is my concern about ambient illuminance. It is defined as the average flux density of the indirect flux field within the volume of a space. I reckon this may be a step too far from where we are now. Might I suggest that, with the amount of Zooming and Teaming we are all doing nowadays, maximum vertical illuminance (Ev) at face height should be the interim transitional standard? It would also be a useful stepping stone to the manifestos goals. CJ T o read the full responses to the Ambient Lighting Manifesto, visit cibsejournal.com 38 March 2021 www.cibsejournal.com CIBSE March 21 pp36-38 Lighting manifesto response.indd 38 19/02/2021 17:46