Scientific Seen

News, Commentary, and Tutorials from a Scientific Perspective

The Commission internationale de l’éclairage, CIE, is releasing new documents on Human-Centric Lighting. The group, known in their English translation as The International Commission on Illumination, notes the types of issues discussed in this forum a couple months ago.

Specifically, their press release states:

Scientists, the lighting industry, lighting designers and other stakeholders in the lighting community have continued to identify options and to design products and solutions that make use of non-visual lighting effects in a beneficial way, despite the fact that the established knowledge in this field is still premature. Among the few points of general agreement is that the non-visual effects of light exposure depend on the spectrum, intensity, duration, timing and temporal pattern (light history) of the light exposure.

The two new documents will address how to quantify the human non-visual response to illumination (presumably in the same sort of statistically average way as the CIE chromaticity diagrams) and how to identify the human factors that are influenced by non-visual illumination. Just to clarify, “non-visual” in this context doesn’t mean invisible or outside the spectral response of the eye. It refers to light sensed by neurons that are not involved in image formation — that are not rods and cones. The primary receptors involved in the non-visual response are called intrinsically photosensitive retinal ganglion cells (ipRGCs). They have a spectral response that overlaps quite significantly with the action spectrum of green cones.

Intrinsically photosensitive retinal ganglion cells detect light and modify human metabolic activity -- the goal of human-centric lighting.

An intrinsically photosensitive retinal ganglion cell, which is the primary receptor targeted by human-centric lighting efforts. Image from Ning Tian, M.D., Ph.D., photographed by Bryan William Jones, Ph.D, via

As with any field on the edge of knowledge, what we don’t know far outweighs what we do know, but the CIE is looking to turn that around. It would be nice to be able to design human-centric lighting based on sound understanding of its physiological effects.


By now even the most isolated consumer has at least heard of LED lighting. LED replacement bulbs are available in any hardware store larger than a closet. The case for the economics and quality of solid-state lighting has been won. Now the industry can concentrate on bringing true value to LED lighting. That value will not come in the form of lower energy use; it will come in the generation of light that enhances the human experience.

That means influencing some aspect of human comfort, health, or behavior by producing and modifying light’s intensity, spatial and angular distribution, spectral content, and possibly other characteristics. Before LEDs we had only limited control over the characteristics of light. Now we have nearly infinite degrees of control. With this control the question becomes: What is ideal lighting, and how can we produce it?

Schoolchildren in LED-lit classroom.

Philips LED lighting offers teachers the option of four pre-set lighting scenarios, ‘Normal’, ‘Energy’, ‘Focus’ or ‘Calm’. There’s a positive response, but what does it mean? Courtesy of Philips Lighting.

Are we ready to answer that? The just-completed Strategies in Light Conference in Las Vegas featured a workshop led by a truly distinguished panel of experts claiming that human centric lighting is ready now. The tenor and content of the discussion might be summarized by a slightly different title. Perhaps something like, “Technology is Ready to Start Investigating the Desired Qualities of Human-Centric Lighting.”

The argument put forward by the presenters is that they are already proceeding with human-centric lighting projects designed to optimize or improve some aspect of human behavior. Stan Walerczyk introduced the new solid-state lighting at the Seattle Mariners’ stadium — the first Major League Baseball stadium to make the switch — which includes (in the home team locker room only) features to tweak circadian rhythm for optimum alertness at gametime. Doug Steel talked about the growing body of knowledge linking various health conditions with light exposure — and the corollary that light exposure can potentially improve health outcomes. Professor Robert Karlicek described methods for integrating “non-invasive” sensors into lighting systems to produce light optimized for a given activity — hands-free because the sensor’s system autonomously categorizes the human activity. Mike Lambert described some human-centric lighting pilot projects in schools, including a school for autistic children.

The message is that light is already being used in innovative ways to influence human health, behavior, and comfort.

The Big Questions for Human-Centric Lighting

But some big questions remain. Bob Karlicek summarized the situation with a single statement: “We really don’t know what we don’t know yet about how lighting will influence how people learn and work.”

We don’t even really have a grasp of the most basic question:
In what ways do the visual and non-visual light receptors influence human behavior, mood, and health? It’s likely to be a very complex answer with primitive roots. That is, the intrinsically photosensitive retinal ganglion cells (ipRGCs) are very highly conserved, in biological terms. That means they appear even in “primitive” organisms only distantly related to humans, which points to a)their early origin and b)their importance to survival. That early illumination response had nothing to do with the human brain, as it was around far before there were any humans (or, for that matter, any brains). So there are going to be some very basic biochemical responses to light that are far beyond our conscious (or even unconscious) control. On the other hand, the human brain does a really good job of processing and manipulating sensory response. Basically, we can convince ourselves we feel one way when our body is telling us we truly feel something completely different. And that’s even before we introduce “thinking,” which makes everything more complicated. Thus, the complex answer with primitive roots.

Light and the brain.

Light influences the biochemistry of the brain, endocrine systems, metabolic processes, and — well, who knows where it ends? Courtesy of Nat’l Institute of General Medical Sciences.

That raises the next logical question:
How do we go about finding the answer to the previous question? One answer, proposed by Doug Steel at the SIL workshop, is to create environments where users get to choose their own lighting characteristics and then simply monitor what they do. He believes there is a natural tendency for people to drift away from their initial preferences — what they believe they like — to lighting that maximizes whatever it is they’re trying to optimize. If engineers partner with clinical professionals (many of whom are being dropped by pharmaceutical companies) then he believes these types of studies will identify (or at least illuminate ) the influence of light on human activity. On the other hand…there are so many confounding factors that the mechanisms might be hidden beneath layers of competing biochemical, emotional, and cognitive mechanisms. Professor George Brainard emphasized that point, reminding the panelists that there’s a well-developed successful path for performing this sort of research, progressing from animal through clinical studies. Although I don’t recall anyone explicitly stating this, the panelists’ consensus seemed to be that a)there are already companies out there claiming either specific or nebulous health effects of their particular lighting system, and b)traditional scientific discovery will take too long. So we should do what we can to provide at least some sort of scientific investigation into the effects of lighting, and we should do it right now.

Which brings up the final question, at least for this already long post:
What makes good lighting? In an elementary school classroom is the good lighting that which maximizes student performance or that which minimizes behavioral problems? In the work environment, is it that which maximizes worker productivity or worker comfort? If lighting makes me feel keyed up, as if I’ve been physically active, am I likely to eat more, even though I’ve been sedentary? Is lighting that makes me happy better than lighting that makes me more alert? And, distinct from those types of questions, there is the layer of light and health. You can imagine a scenario where I may feel good and work hard under certain levels of illumination, but that ends up triggering some photosensitive biochemical pathway that begins tumor growth.

My concern is that human studies right now will be almost necessarily one- or two-dimensional: looking at the effect of light on one or two aspects of human comfort, health, behavior, or activity. It’s likely that the complex structure of the human brain is reactive to illumination in many, perhaps contradictory, ways and if we really want answers we’re going to need to take as comprehensive an approach as possible.

The Human Centric Lighting Society is working on these sorts of questions, and you should take a look at their site if you want to get up to speed.


From the day I began working as a science writer (starting as a “hobby” while I was a gainfully employed optical systems research engineer) I’ve covered new breakthroughs — discoveries or inventions that have revealed or used new principles or techniques. The challenge of covering that “beat” is that the mere fact that a discovery or invention has been made says absolutely nothing about the use or application of the advance. So there’s a bit of a disconnect between news of technological or scientific progress and the availability of the fruits of that progress to a wider community. But there is a definite process that these things tend to follow: initial work–>use by a handful of specialized practitioners–>availability of the method to a technically capable general population–>accessibility to all. Of course, not all breakthroughs make it through all these stages, and one of the most fascinating aspects of following science and technology is to predict which ones will hit the big time.
Applying that model to the development of general lighting with LEDs, the first breakthrough was simply crashing through the performance, cost, and reliability (i.e., quality assurance and product validation) barriers to bring solid state lighting to market. Any walk through a big box hardware store will demonstrate that basic LED technology is now in the fourth stage of development.

Controls Adding Value

The subsequent stage is to realize even more value from LED lighting by incorporating it into an overall control system, i.e., Smart Lighting. I’d say that’s at the third stage (perhaps on the cusp of the fourth), where folks who are generally capable around technology can now incorporate advanced controls into their lighting systems. Proving the point are commercially available systems such as the Hue system from Philips bringing those capabilities to the general consumer, and companies like Redwood Systems, that offer integrated controls solutions to commercial and industrial customers.

Image of LED lights controlled by smartphone app.

Philips’ Hue system brings LED lighting control within grasp of the tech savvy. (Courtesy of Philips Communications)

Although the market penetration for LED lighting in general is still proportionally very small compared to the overall industry, that’s now a marketing challenge more than a technical one. The next wave in LED lighting is to start to apply the inherent controllability of solid state lighting to use light to define spaces and optimize suitability of illumination for any circumstance. Specifically, LEDs offer an unprecedented degree of control over the distribution, spectrum, and intensity of light in a space — including the ability to vary those parameters over time.
At the Strategies in Light Conference, Hans Nikol, VP for Strategy and Innovation at Philips Lighting, discussed how a significant percentage of the market for the Hue LED lighting system is driven by teenagers — looking to show off the cool lighting at their homespun raves (if anyone uses that word any more). But that’s not the kind of value that’s going to drive widespread adoption of solid state lighting. What’s needed is a way to identify specific benefits of lighting control — aspects of illumination that improve human health, productivity, or perhaps even some more nebulous sense of well-being.

Quantifying the Promise of LED Lighting

Lighting Research Center's Home Lighting Design Tool

Knowledge of the effects of light can help produce illumination designs that influence behavior and health. (Image courtesy of RPI’s Lighting Research Center)

Also at Strategies in Light, Mark Rea, Director of the Lighting Research Center at Rensselaer Polytechnic Institute, spoke about various ways to quantify the value added by the ability to spectrally and temporally tune illumination. He is pushing the industry to help identify quantifiable metrics that can be applied to commercial, industrial, educational, and domestic environments to present a clear story of the value of controlled illumination in those environments. Of course, identifying a metric is different from establishing the connection of that metric to human health and performance. That work is in its early stages, with the exception of a few tantalizing tidbits. But enough work has been done to understand that human well-being, alertness, and productivity are influenced by lighting choices. Although some may quibble, I’d contend that this field is still at the stage where researchers are making their fundamental discoveries, and significant expertise is necessary to investigate and apply these illumination methods. It’s also my contention that herein lies the true value of solid state lighting. Without the ability to control illumination, it didn’t make much sense (and was difficult to do anyway) to investigate the effects of various illumination levels, colors, and timing. Now those investigations are underway, and the results should drive the value of LED lighting well beyond that of simply upgrading incandescent light bulbs.


Solid-state lighting — using light emitting diodes for general illumination — offers a host of advantages over incumbent technologies. For starters, LED lighting is five times (or more) as efficient as incandescent lighting, it has no components requiring hazardous material waste provisions (as do fluorescent lamps), and LEDs instantly respond to electric current, eliminating the delays inherent in high-intensity-discharge at startup and allowing the illumination level to be controlled. And those really are just for starters, as there are many other advantages.

But LED lighting has one big disadvantage: it’s completely different from other lighting technology. For example, LEDs are generally driven with DC current as opposed to the AC that powers just about every other source. Another big difference is that LED packages themselves (the LED chip, encapsulant, and primary optics) shape and direct the light. Contrast that with the tungsten-alloy filament at the heart of an incandescent bulb. When the filament heats up it puts out light (about 5 to 10 percent of its electrical power usage) in every direction.

Manufacturers can make specialty incandescent bulbs that have internal mirrors to direct the light in a certain way, but in general, one light bulb is just like another. You can replace a Sylvania bulb with a General Electric bulb with a Phillips bulb and you can put it in a desk lamp, an outdoor sconce, or a recessed fixture. Incandescent bulbs are commodities because the physics determines how a filament works, and there’s not much any manufacturer can do do distinguish their offering from another company’s. The good part is the interchangeability; the bad part is that the light output of incandescent lamps is cut down by the fixture efficiency (the amount of light that doesn’t get where you want it to go. Because the light from the bulb itself is uncontrolled — spreading out in every direction — you can’t match a bulb to the fixture in which you’d like to put it, so light (and energy) is wasted.

Matching the Light Source to the Task

What if, though, you could buy a different light bulb for every fixture, one tailored to minimize wasted energy for that one particular application? It would be expensive. And every few months or so it would burn out and you’d need to run down to the hardware store and buy a replacement bulb (selecting from the scores your retailer would need to keep in stock). So that’s not really a viable solution for incandescent bulbs. But it is for LEDs.
One of the advantages alluded to above is that LEDs can last a really long time (there are some issues with exactly how long, but that’s for another day). Long lifetime means that it’s reasonable to select an LED lighting solution specific to each application — because you won’t need to change it out for 5, 10, perhaps even 20 years or more. In practice, it’s a little more complicated than that because there’s not just one LED that will go in every desk lamp or cove light. Every luminaire (light fixture) manufacturer selects and arranges LEDs in a different fashion. So if you really want to optimize LED lighting you have to buy an entire fixture. It’s still a money-saving proposition (it’s not uncommon for industrial customers to get payback times of anywhere from a few years to several months), but it creates a dilemma for LED lighting.

LED lighting can be forced into an Edison-Screw bulb.  Image from U.S. Department of Energy.

LED lighting can be forced into a traditional incandescent bulb shape — but should it be?
Image from U.S. Department of Energy.

Retrofit or Redesign?

LED lighting manufacturers have two choices: they can make fixtures that optimize the distribution of light to fully take advantage of the new capabilities offered by solid-state lighting, or they can design retrofit bulbs that can be put into place as one-for-one replacements of existing incandescent or fluorescent fixtures. At last week’s LED Show the dilemma was (quite literally) on display. The purists argue that forcing LEDs to mimic (crappy) incandescent or fluorescent sources will set the industry back because customers will see energy savings, but not much more of the advantages of LED lighting. The retrofit folks argue that the replacement bulb solution lowers the barriers to entry, giving LEDs an absolutely necessary foothold into the general illumination market. At times, there’s some visible contempt expressed on one side or the other.

The logical path (one that’s already being played out) seems to be that retrofit solutions get LEDs in the door, where they then get to display some of their other advantages. Those bonus advantages then create showcase solutions that become selling points for designs that fully embrace the LED lighting solution.


It’s interesting to watch the evolution of a technology.  I attended my first LED lighting conference in 1998 and I’ve attended at least one in every intervening year.  In those first years, Las Vegas was the primary customer.  Nowhere else was lighting such a significant expense — and such an essential part of marketing.  Higher efficiency and lower maintenance represented a huge savings for casino operators on The Strip.  But the ability of LEDs to manage the distribution of light (direct it to the consumer instead of spraying it into space) and the flexible control of LED lighting (to easily reconfigure displays) added to the energy savings to make LED lighting worth the investment.

But there’s a big difference between convincing casino operators to upgrade their high-value, high-maintenance displays and getting consumers to shell out big bucks to replace their 25-cent incandescent bulbs.  The fundamental problem is that LEDs are significantly different than their incandescent predecessors, but it’s difficult to take advantage of their new capabilities within the current infrastructure.  So people pushing for LED adoption have to limit their argument to two factors: it will save energy and it will reduce maintenance costs.  It’s kind of like arguing for replacement of horsedrawn buggies with gas-powered automobiles, but having your arguments for change limited to discussions of lower hay costs and reduced need for street cleaning.

LED lighting squeezed into a familiar incandescent bulb form factor.

LED lighting can be made to look like an incandescent bulb, but that’s like requiring a buggy whip on a Ferrari.

Even with that limited evaluation, LEDs are now well past the point where they are economically viable (after some intense political, economic, and technological growing pains), so just about any lighting project today needs to at least consider LEDs as an option, and for many developers they are the option of choice.  But now that LED general illumination is in place, system operators are realizing some other benefits.

Networked Capabilities of LED Lighting

At the LED Show (starting yesterday in — fittingly enough — Las Vegas) Kelly Cunningham of the California Lighting Technology Center (CLTC) at the University of California-Davis described a networked implementation of LED lighting control on the university’s campus.  Outdoor LED lighting at the campus is triggered by passive infrared sensors that provide little more than a simple present-or-not signal.  Even with that limited input, the control system anticipates pedestrian and cyclist movement, bringing lighting up to full brightness levels before the traffic reaches the lit area.  The ability to remotely control and instantaneously modify the illumination level of LEDs is central to the operation of this kind of system, and the immediate benefits are impressive.

For example, 100 wall packs (those curious rectangular fixtures affixed to the outside of buildings and washing the walls with light) detected only a 28 percent occupancy rate, leading to an 85 percent reduction in energy costs — over and above the reduction simply due to LED efficiency alone.  The CLTC implemented the same kind of system on a stretch of urban roadway.  Although the final report has not been released, Cunningham said the results are similar.

That’s encouraging news for the industry, because those are the kind of integrated lighting systems that insiders have been claiming would lead to additional levels of savings (and other capabilities, but that’s another story), and this provides another fairly significant example of the promise coming to fruition.  It also demonstrates another general truth: if you don’t have a capability, then you don’t have any idea what you’ll do with it; but when you develop the capability you will apply the capability in clever ways.  That’s true for networked lighting now.  In the near future, the precise control LEDs offer over color, intensity, and distribution of light will be used to modify illumination in our work and home environments to enhance our comfort and productivity in ways we can only glimpse today.