By Charles Jarboe
Traditionally, architectural lighting has been engineered to deliver horizontal illuminance to the workplane in order to provide adequate illumination for visual tasks. Gradually, lighting designers and manufacturers began to consider the aesthetic as well as psychological effects of light, such as perceived brightness of a space, and developed techniques to distribute the light not just on the horizontal surfaces of a room, but on the vertical ones as well. More recently, the scope of architectural lighting has broadened further to include the physiological effects of light, specifically how light affects the human circadian system and alertness.
Light is the primary exogenous cue for synchronizing the body’s endogenous functions with the 24 hour light-dark cycle at one’s local position on Earth. There are several characteristics of light that are central to this process: the amount (or level) of light received at the cornea, the spectral properties of that light, and the timing and duration of the light exposure. In order to induce a response by the circadian system, the light stimulus must reach the retina and, therefore, vertical illuminance (EV) is crucial to this process.
In addition to stimulating the circadian system, light exerts an acute alerting effect similar to that of a cup of coffee. Recent research suggests that the characteristics of a light stimulus, such as its amount and spectral properties, affect alertness and circadian entrainment differently. Whereas short wavelength “blue” light can elicit alertness as well as suppress the hormone melatonin (an established biomarker for the circadian system), long wavelength “red” light can either maintain or increase alertness without suppressing melatonin.
Given the increased understanding of the mechanisms involved in translating a light stimulus to a signal influencing the circadian system, advances in lighting technology, and the breadth of research indicating that circadian disruption can lead to many negative health outcomes such as metabolic and cardiovascular disease, some forms of cancer, sleep disruption, and various problems relating to mood and general health, it is becoming part of the lighting design process to consider the non-visual impacts of lighting and to help ensure such negative health outcomes are avoided wherever possible. However, because the circadian system has a higher absolute threshold for activation than the visual system, and because light at the eye is required for circadian phototransduction, circadian or “human-centric” lighting designs often require higher light levels than those needed for visual performance alone, and this has the unwanted potential to increase energy use—representing a step backward in terms of design sustainability and efficiency.
The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute recently conducted a study to evaluate the effectiveness of several LED lighting strategies for delivering circadian stimulus (CS) to occupants of a typical office space while minimizing, or preventing, increased energy use. The study employed photometric simulations of these strategies in a typical open-office space, delivering a criterion CS of 0.3 to calculation points modeled at the simulated occupant’s eye level.
The CS metric utilized in the study is derived from circadian light (CLA) which is irradiance at the cornea weighted to reflect the spectral sensitivity of the human circadian system. CS is defined as the percentage of nocturnal melatonin suppression achieved after a one-hour light exposure from threshold (CS = 0.1) to saturation (CS = 0.7). Although melatonin is produced only at night, it is used as a surrogate metric for how light affects one outcome of the circadian system. In fact, recent studies conducted by the LRC both in laboratory settings and field applications have shown that a CS level of 0.3 or greater for at least two hours per day, especially in the morning, was found to be effective at improving sleep quality, mood and alertness, and reducing stress in office workers, as well as reducing depression in people with Alzheimer’s disease and related dementias living in long-term care facilities.
The photometric simulations conducted for the study investigated the performance of six overhead luminaire types at six correlated color temperatures (CCTs) ranging from 2700K to 6500K, two target horizontal illuminance (EH) levels (300 lx and 500 lx), and two intensity distributions (typical and wide), for a total of 144 combinations. Additionally, the study evaluated the performance of a personal desktop luminaire delivering narrowband short-wavelength “blue” light in combination with typical overhead lighting. Figure 1 shows the CS to lighting power density (LPD) performance of the various lighting configurations covering the extremes of the range of CCTs typically specified for office environments (3000K and 5000K).
As indicated in Figure 1, desktop luminaires in conjunction with typical overhead lighting (2-ft by 2-ft troffers delivering 300 lx horizontal at 3000K) were the most effective solution at providing the recommended CS design target of 0.4 while using as little energy as possible. The overhead lighting solutions that performed best were the troffers with wide “batwing” intensity distributions, as well as pendant luminaires with some direct lighting component. In general, the majority of the overhead luminaires that achieved the CS target of 0.3 had a vertical to horizontal illuminance ratio above 0.65:1. Additionally, increasing the horizontal illuminance target from 300 lx to 500 lx had a greater relative impact on the CS:LPD ratio than increasing CCT from 3000K to 5000K.
For designers and specifiers to translate the findings of past research and the present study into practice, the LRC has developed the following lighting design guidelines intended to aid designers in developing circadian-effective lighting solutions, while also maintaining energy efficiency and meeting local energy-code requirements.
For a typical office environment, the LRC recommends providing a CS ≥ 0.3 during the daytime, followed by CS ≤ 0.2 in the evening and CS ≤ 0.1 in the nighttime (Figure 2). However, unlike a typical or virtual illuminance meter used in AGi32, the human eye does not have a cosine spatial response due to the shading of light by facial features such as the nose and brow, and since there may be unaccounted for objects or materials that can block or absorb light in the real-world environment, the LRC recommends a daytime CS design target value of 0.4 to accommodate for these factors, and to ensure that most occupants of the space will receive the necessary amount of light.
To ensure CS is being delivered while also minimizing energy use to the greatest extent possible, the LRC recommends maximizing the CS:LPD ratio. While numerous lighting products and configurations can be used to meet these performance specifications, one of the goals of the study and resultant guidelines is to determine the luminaire-level qualities that should be considered and optimized to increase the likelihood that the specifications will be met in an energy efficient manner, while also keeping in mind aesthetics and human factors considerations.
1. Decide if desktop luminaires can be used.
Given the overwhelming CS:LPD advantage of the desktop luminaire delivering blue light at the eye (only 24 lx of blue light at the eye was needed in addition to the 2-ft by 2-ft troffer delivering 300 lx and 3000K to achieve a CS of 0.4, for an additional 0.07 watts per sq ft), the first step of the design process should be to consider such an additional layer of light in the vertical plane close to the eyes of the occupants. Desktop luminaires, or other similar personal light solutions, can be especially useful when energy, horizontal illuminance and/or CCT constraints make CS delivery difficult from overhead luminaires alone.
2. If desktop luminaires cannot be used, model your space.
Use a photometric simulation model of the space to calculate horizontal illuminance in a grid on the workplane, and vertical illuminance at the eye level of the occupants in the space (Figure 3). The vertical illuminance points should be positioned along a line 4 ft above the finished floor and aimed in the direction of the eyes of the occupant seated at that location. Using the spectral power distribution (SPD) of the light source(s), calculate CS using the web-based CS calculator developed by the LRC: lrc.rpi.edu/cscalculator/
3. Design overhead lighting to provide adequate CS and maximize CS:LPD.
Evaluate the intensity distribution of lighting products and favor products that deliver a high vertical to horizontal illuminance ratio of at least 0.65:1. Troffers with wide “batwing” distributions or pendant luminaires with some direct lighting component and wide indirect component will be the most likely to have a high EV:EH ratio and provide relatively more CS to the eyes for an equal amount of energy (Figure 4).
When feasible, provide higher light levels (500 lx horizontal) during the daytime hours and/or consider increasing CCT from 3000K to 5000K. Increasing the light level from 300 lx horizontal to 500 lx had a relatively larger impact than increasing CCT from 3000K to 5000K. Still, the average LPD of fixtures providing a CS of at least 0.3 at 5000K was 5% lower than the LPD of fixtures providing the same CS at 3000K. It is also important to know the actual SPD of the light source, not just the CCT when designing for CS. For example, eight different light sources all having a CCT of 3000K had a CS range from 0.22 to 0.26 for equal horizontal illuminance.
It is also possible, using tunable or dimmable static white LED lighting systems, to modulate the CS delivery throughout the day and potentially reduce energy use by decreasing light levels in the afternoon hours. However, while morning light is most important for entrainment, afternoon light levels are important for promoting alertness and should not be reduced too dramatically without the addition of supplemental red light from desktop luminaires or other personal sources.
The present study found that providing a CS value of 0.3 for the entire work day was the most energy-efficient schedule option that provided adequate morning light for entrainment, and afternoon light for alertness.
The overall aim of the study and these guidelines is to aid lighting designers and specifiers in the process of identifying the characteristics of lighting products that should be considered and optimized when designing lighting for non-visual effects such as circadian entrainment and acute alertness. Additionally, we urge the lighting community, designers and manufacturers alike, to think beyond the horizontal plane and consider utilizing an additional layer of light in the vertical plane, more personal in scale, that can deliver light for promoting entrainment and/or alerting effects while minimizing energy use and relieving the burden of the overhead lighting system from doing a task it is often not designed to perform.
It is also important to note that aesthetic and human factors considerations should not be neglected when designing lighting with non-visual impacts as a performance criterion. Glare, brightness perception and psychological effects of light should be carefully addressed, and when considered in combination with visual and non-visual effects, lighting design can become “holistic” in the truest sense of the word.
The study was funded by Natural Resources Canada, the Light and Health Alliance, and the Lighting Energy Alliance. Light and Health Alliance members are Armstrong Ceiling & Wall Solutions; Axis Lighting; Cree; GE Current, a Daintree company; Ketra; LEDVANCE; OSRAM; and USAI Lighting. Lighting Energy Alliance members are Efficiency Vermont, Energize Connecticut, National Grid, Natural Resources Canada, NEEA, and ConEd.