TUNNEL VISION?

Our traditional photopic-focused understanding of light is increasingly irrelevant and narrow, argues Stephen Cannon-Brookes. Is it time for a radical rethink?

s we struggle to accommodate the emerging goals generated by rising concern for human wellbeing, siege is about to be laid to many of the core tenets that underpin our current understanding of light. It is easy to underestimate the influence that has been exerted by the definition of visible light using A

the photopic spectral sensitivity function known commonly as Vλ. It has given us a tool to measure and deliver light, but it has also encouraged us to regard the visible spectrum of light independently of its close neighbours, ultraviolet and infrared radiation, encouraging the implied disconnection of vision from the wider sensory system. At the

9 E risk of mixing metaphors, future generations will regard this period as one of tunnel vision. The discovery of the blue-light-stimulated melanopsin generators in the retina has rendered our classical photopic-focused understanding of light an increasingly creaking edifice. Addressing this, however, is a challenging prospect for the multiple generations brought up on an orthodoxy that also unpins our understanding of colour. However, in doing so we may be able to appreciate what has made it so difficult to understand a range of phenomena and cultural interpretations or beliefs that seem alien or inexplicable. Examples of the latter can be seen in relating colours to different energy centres or chakras in the body, and our still rudimentary engagement with a new, or perhaps rather old, awareness of the natural environment known as biophilia.

10 Our photopic world has also been conditioned by an evolutionary and technological addition, namely fire or, to the lighting world, incandescence. This has literally coloured our perspective of lighting and how we address artificial light sources in far-reaching ways, for example, the initial standard being a candle and hence our use of candela. While this is not surprising given that nearly all artificial light sources were incandescent until the arrival of discharge tubes, it has in practical terms bestowed a benchmark for lighting technology and raised a number of challenges – a good example being the apparent paradox that a red-rich light source may have the same quality of colour rendition as one with a continuous and evenly distributed spectrum. It takes almost an exercise of faith to believe that there can be a smooth transition in correlated colour temperature (CCT) of a light source on the black body locus from low to high values (in kelvin). Much effort has been devoted to papering over the dislocation. If we step back for a second, there is something innately nonsensical in trying to define the CCT of (cool white) sources against the temperature of a notional radiator that doesn’t exist. In the practical world we inhabit the answer has been to use a function based on the spectral distribution of sunlight or overcast skylight and thus render CCT measurements as purely mathematically derived values, helpfully overlooking the reality that no one has ever made a light source with this spectrum to act as a comparison. At the risk of making a plea, language really does count since the way metrics and units are named and/or defined has conditioned how we approach the subject.

A range of proposals has been offered to try to reconcile the increasing complexity of how we address light and lighting. These mostly focus on the assimilation of the sensitivity function derived for melanopsin production and range from use of direct measurement, a ‘new’ lumen, through to comparative measurements against Vλ, and a number equivalence measurements, some permitting the direct use of photopic illuminance. All are aimed at users answering the question, is there enough? This bottom upwards approach has been a feature of artificial light/modern photometry, based on the dialogue between task performance and the economics of delivering light. Both are brakes on extending our perception of lighting, but they are starting to loose their stranglehold. Lighting designers are beginning to address the wider visual field, escaping the flat lands of horizontal task illuminance targets so beloved of professional bodies and governments, while the price of a lumen (definition at the choice of the reader) has dropped to a point where energy saving is no longer a primary issue, especially when maintenance is properly costed.

These warm thoughts should not distract from the prospect of the rug being pulled from underneath the whole edifice. We are being propelled towards an increasingly compromised situation where different metrics and units will serve to confuse the wider audience and quite possibly participants in the lighting world. Many are being adopted so swiftly that they have had marginal validation, mostly in clinical contexts, and yet are being adopted in building guidelines that promote targets as aspirational standards.

Is it time to redefine what we know? Clearly, we should encompass our understanding of our various responses to light (though this now sounds a little human-centric). The implications are now being addressed, but once again we are seeing an incremental process, largely in response to discoveries. The best-known example is Uλ, proposed by Mark Rea, which encompasses all the currently known spectral sensitivity curves and as a consequence the majority of the visible spectrum (400-700nm). This looks tidy, but begs a bigger question, namely is it not time to make the leap and once again acknowledge that our (and most animals and plants) ‘We should acknowledge that in evolutionary terms light from the sun is our benchmark source’

evolutionary light source is daylight, either direct or diffused sunlight?

Time for a modest proposal, hopefully less monstrous than Swift’s?

First, we need to acknowledge how little we actually know. The discovery of iPRGCs in the retina is humbling because it suggests that we should expect to discover more spectral sensitive curves and these are unlikely to be directly related to vision. Beyond this, we should expect that the way the body interrogates stimulus from these spectral sensitive responses is not compartmented. This would help to explain why sunlight exposure is (largely) beneficial: it stimulates vitamin D production and helps to reduce symptoms of depression, while healing a range of diseases. Let's remind ourselves that much of our understanding of the medical aspects of sunlight were shelved following the introduction of antibiotics.

Secondly, we should acknowledge that in evolutionary terms light from the sun is our benchmark source and, as Nick Baker sought to remind us, we are 'outdoor animals'. It is reasonable to expect that we make use of the whole spectrum of sunlight (radiation), even if our understanding of the processes involved is still fragmentary.

It is thus a simple step to propose that we reconsider our definition of light, effectively back to one before the adoption of the lumen. Does the word sunlight come as a surprise?

Stephen Cannon-Brookes is a past president of the SLL, and works both as a private lighting consultant and an associate professor leading UCL's module on lighting and wellbeing