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Through the history of electric lighting, we’ve largely relied on advances in lighting technologies to light spaces more efficiently. The first big leap in lighting efficiency corresponded to the commercialisation of fluorescent lamps in the 1930s, with many other types of gas discharge technologies introduced to the market in the subsequent decades. But, these days, we’re most familiar with the energy benefits of solid-state lighting (SSL), particularly light-emitting diodes (LEDs). An analysis conducted by the United States’ National Research Council has demonstrated that the widespread commercialisation and increasing adoption of LED-based lighting products has indeed reduced the energy consumed by lighting,1 at least in some parts of the world. However, there are physical limits to the efficiency with which light can be generated by electricity, so our current strategy of waiting for advances in lighting technologies to achieve energy conservation gains will not pay off indefinitely.


### Luminous Efficacy

Measuring the energy performance of lighting is a bit tricky. Many other fields and industries simply quantify efficiency: the ratio of the power produced to the power consumed. This unitless quantity is often expressed as a percentage and works pretty well when applied to many appliances, like furnaces and water heaters. We don’t use it in lighting, however, because the human visual system isn’t equally sensitive to all wavelengths of light in the visible spectrum. A source might convert electricity to light very efficiently, but the light produced might have a spectrum that we are relatively insensitive to. So, the most widely used measure related to energy efficiency in the field of lighting is luminous efficacy. 

With a unit of lumens per watt (lm/W), this quantity characterises the amount of light, weighted by the sensitivity of the human visual system, emitted by a light source per Watt of electrical power consumed. While luminous efficacy provides lighting manufacturers and designers with an accurate and useful way of quantifying the energy performance of individual lighting devices, it does not fully address the use of lighting in architectural spaces.

In buildings, the primary function of light is to facilitate the visibility of surfaces: walls, people, food, books, In buildings, the primary function of light is to facilitate the visibility of surfaces: walls, people, food, books, artwork, furniture, etc. Light strikes these surfaces, reflects off of them, and enters the eyes of building occupants, resulting in visual perception. Light that does not enter an eye does not contribute to this function – for illumination purposes, it is wasted. And much is indeed wasted. Though luminous efficacy is the most widely used proxy for efficiency in lighting, it only considers one small part of the illumination lifecycle.


“It is the people in the trenches of the lighting industry – product engineers, lighting designers, architects, sales reps, etc. – that have the power to change the way we light the world.”

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### Application Efficacy

Application efficacy is a relatively new lighting term, meant to describe a more holistic approach lighting efficiency – a concept focused on the relationship between the electrical power consumed by lighting hardware and the amount of light that contributes to the visual perception of building occupants. Though it currently cannot be measured, it can be used to generate new strategies for reducing the energy consumed by lighting. 

Of course, this approach isn’t entirely new. We already use occupancy sensing in some commercial spaces to turn off lights when spaces are unoccupied. We are, hopefully, in the habit of switching off lights when we are the last person to leave a room. But these strategies are fairly rudimentary – we have the potential to radically re-think the way that we bring light into architectural spaces.

Current lighting design practices typically result in white light blanketing rooms, fairly evenly across both space and time so that building occupants can engage in any visual task at any location within the environment at any time. While this affords flexibility to our occupants, it is often very wasteful. After all, every photon of light that is emitted into a room, but not absorbed by a photoreceptor photopigment, a molecule that begins the chain reaction that leads to visual perception, doesn’t serve any useful purpose. 

What if we were able to light spaces in ways such that all, or at least more, photons did actually serve a useful purpose? How much energy could we actually save? To begin to answer those questions, we have to be willing to completely reimagine the ways that we bring light into architectural spaces and entertain ideas that depart from current practices pretty drastically.


### Thinking Radically

The Lighting Lab at the University of Sydney tries to do just that. To illustrate, let’s consider the doctoral research of Alp Durmus,5 an alumnus of the lab and now an Assistant Professor in the Department of Architectural Engineering at Pennsylvania State University. The premise of his work was both simple and profound: a portion of the light that strikes illuminated surfaces is absorbed by them. 

When this happens, the light is converted to heat and cannot facilitate people’s vision. However, we also know that light absorption isn’t random – it is related to the colour of the surface. For instance, blue surfaces predominantly reflect short wavelength (blue) light and absorb more of the longer wavelength (green, yellow, orange, red) light. So, if we were to light blue surfaces with blue light, we should be able to reduce the amount of light that gets absorbed and ultimately be able to generate less light to achieve the same surface luminance.

While formulating the idea for this research, we envisioned futuristic lighting systems that used sensors to detect the shape, size, position, and colour of all surfaces within a space. Utilising both spatial and spectral tuning, each surface could be illuminated with light that has a spectrum that minimises the amount of light that is absorbed. The idea is certainly ‘out there,’ but the base technologies needed to create something like 
this already exists.

As you may have guessed, it’s not quite as simple as illuminating blue surfaces with blue light, as this would yield an over-saturated colour appearance. So, a lot of this research involved simulations and computations. From this, we learned that, if we had total control over the spectral power distribution of light (i.e., could create any spectrum that we wanted), we could potentially cut energy consumption in half, while maintaining the expected colour appearance of surfaces. That’s a lot of energy! And it’s saved simply because light that would be absorbed is never even created. Even though product designers have unprecedented flexibility to specify the spectrum of a light source, the level of spectral control needed for those sorts of gains isn’t yet easily attainable. So, we also tested the idea with real sources and real illuminated objects. Optimising a mixture of just nine different coloured LEDs, we found more modest energy savings of approximately 10% were possible. Furthermore, we were able to validate our simulations of the resulting colour appearance with actual people. Our experimental participants found that familiar objects illuminated by absorption-minimising spectra appeared equally natural and attractive as those lit by conventional white LED light sources. Alp’s research further investigated the acceptable optical tolerance for lighting surfaces in such a spatially specific way and applied the idea to the illumination of artwork, since absorbed light damages art.

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Absorption of light is certainly not the only way that light gets wasted in real architectural spaces. There are countless other ways that we could drastically reduce the amount of light that needs to be generated. We could develop dynamic gaze-dependent lighting systems, that illuminate only the parts of rooms within occupants’ fields of view at any given moment. We could leverage sensor technology to create illuminated environments that can detect the activities of buildings occupants and custom the lighting accordingly, in real time. 

We could track occupants’ light exposure over time and leverage visual adaptation to reduce the amount of light needed in interior spaces, without compromising visual appearance. Of course, there are many, many more possibilities. We are studying some of the ideas in our research lab and have plans to investigate others in the future. This type of research is important because it not only tests the general feasibility of these ideas, but the experimental data collected also provides design guidance when the time comes to develop these types of lighting systems for the real-world. However, this type of research is just the first step toward bringing these kinds of ideas to life.


### A Call to Arms

So far, improvements in lighting efficiency have depended largely on the work of physicists, materials scientists, and cutting-edge electrical engineering researchers – these are the people that, most often, discover new ways of generating light with electricity and refine these new lighting technologies. They’ve done amazing work that has propelled our industry forward. But, the approach to lighting efficiency discussed here will not arise from a laboratory. It is the people in the trenches of the lighting industry – product engineers, lighting designers, architects, sales reps, etc. – that have the power to change the way we light the world. These types of changes don’t depend on fundamental advances in lighting technologies, though they certainly pose engineering challenges. 

Perhaps more importantly, they challenge the culture and norms of the lighting community. Adopting new ways of lighting that improve application efficacy would require us to adjust our expectations and be willing to not rely on ‘what’s always worked.’ In addition to the potentially massive energy benefits of this approach, I believe that a transformation in lighting practices of this type would empower lighting designers and make the value of their work more apparent to clients. After all, the specification of sophisticated, integrated systems customised to specific buildings or spaces requires a level of expertise worthy of investment.

I believe there are a couple of initial steps that we can take to move in this direction. The first is to re-think our relationship with lighting application standards and recommendations. Though they vary around the world, they are often simplistic and prescriptive. The simplicity that makes them easy to apply inadvertently makes them an impediment to innovative design practices. 

After all, if horizontal illuminance was measured to assess the performance of something like an absorption-minimising lighting system illuminating a space, the results wouldn’t be meaningful or useful. Secondly, we can open our minds about how product specifications are communicated from manufacturers to designers. When designers become heavily reliant on a handful of metrics to assess products, we’re also impeding innovation. Again, quantities like luminous flux and even luminous efficacy wouldn’t provide meaningful information about the performance of something like an absorption-minimising lighting system.

I know I’m asking a lot. It would make many of your jobs more challenging. I, personally, enjoy doing things that are interesting more than I enjoy doing things that are easy. How about you? 

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