This article builds upon a previous article in LD+A that presented strategies for delivering circadian-effective light in an office space while minimizing energy use. While various methods for defining circadian-effective light have been proposed, we chose the circadian stimulus (CS) metric because our work is based on the scientific literature showing that rods, cones and intrinsically photosensitive retinal ganglion cells (ipRGCs) participate in how the retina converts light signals into electrical signals for the master biological clock. The CS metric is derived from circadian light (CLA 2.0), which characterizes the spectral sensitivity of the circadian phototransduction circuits in the human retina.1, 2 Reflecting the circuits’ operating characteristics, from threshold to saturation, CS is therefore postulated as a measure of the effectiveness of optical radiation incident on the retina for stimulating the biological clock. LHRC laboratory and field studies have shown that exposure to a CS of 0.3 or greater for at least two hours per day, especially in the morning, is effective for improving sleep quality, mood, and alertness and reducing stress in office workers,3 as well as for reducing depression in people with Alzheimer’s disease and related dementias living in long-term care facilities.4, 5
As articulated in the UL 24480 Design Guideline for Promoting Circadian Entrainment with Light for Day-Active People, options other than CS are available for use. The key takeaway is not which metric to use, but the imperative to develop lighting schemes that deliver bright days and dark nights in an energy-efficient and visually comfortable manner. The present installment offers insights and guidance to designers and application engineers who wish to provide circadian-effective lighting solutions in K-12 classrooms while avoiding excessive energy use compared to typical classroom lighting applications.
1. BackgroundThe naturally occurring delay of the sleep-wake cycle in adolescents is often exacerbated by a lack of exposure to a daily robust light-dark pattern, typically consisting of an underexposure to circadian-effective morning light and overexposure to this kind of light during the evening hours.6,7 The two principal factors that contribute to this underexposure are inadequate electric light levels in school classrooms and a lack of exposure to daylight, especially for children living in northern latitudes where school start times can occur before sunrise. When combined with adolescents’ increased sensitivity to evening circadian stimulation and high levels of exposure to self-luminous devices, the resulting light recipe is radically at odds with what is needed for entraining students and improving their sleep quality and overall well-being. As a remedy, the LHRC recommends providing a daytime circadian stimulus (CS) at the eye of 0.3 or greater and limiting evening light exposures (CS <0.1 at the eye). However, a potential drawback of providing high circadian stimulation during the daytime in a classroom setting is that the light levels required to do so are higher than those needed for visual performance alone, and lighting power demand can increase as a result. The goal of our study was to determine the specific lighting strategies and luminaire characteristics that most effectively deliver the target CS to the eye.
2. The StudyThe LHRC used the same general methodology that was employed in a previous study evaluating similar strategies in an office environment, which was reviewed in the December 2019 issue of LD+A and published online in April 2020. The present study employed photometric simulations of lighting solutions in a typical K-12 classroom that delivered horizontal illuminance target values of 300 lx and 500 lx to calculation points modeled at the desktops. CS values were calculated at the simulated occupants’ eye level (Figure 1).
The simulation modeled five ceiling-mounted luminaire configurations at six correlated color temperatures (CCTs) ranging from 2700K to 6500K, and at two horizontal illuminance (EH) targets of 300 lx and 500 lx, for a total of 60 unique lighting conditions. Two troffer layouts (2-ft by 2-ft and 2-ft by 4-ft), two direct-indirect pendant configurations (in two rows perpendicular to the rows of desks, and in three rows parallel to the desks), as well as a 4-ft recessed linear luminaire layout were analyzed. Additionally, two arrangements were explored that utilized specialized SPDs to further maximize the CS to lighting power density (LPD) ratio—the performance metric utilized to quantify the efficacy of the lighting solutions for delivering CS for the least amount of energy. (The higher the CS:LPD ratio, the more energy efficient the design.)
The first additional arrangement utilized the recessed linear luminaire but with a 3500K spectral power distribution (SPD) “optimized” to stimulate the ipRGC photopigment melanopsin by having an additional short-wavelength peak at 487nm, and thus, maximize the melanopic-to-photopic (M/P) illuminance ratio. (It should be noted, however, that an increase in M/P ratio does not necessarily mean an increase in circadian system response, given the previously demonstrated subadditive response by the circadian system.8) The second additional arrangement explored the effects of utilizing narrowband short-wavelength “blue” light (λmax = 452 nm) from an added layer of overhead recessed linear luminaires (Figure 2) in combination with the typical 2-ft by 2-ft troffer (EH of 300 lx and a CCT of 3000K). Calculations were performed utilizing the CS calculator and the blue light layer was mathematically “dimmed” to deliver 8 lx at the eye (in combination with the 2-ft by 2-ft troffer at 300 lx horizontal and 3000K) to reach an average CS value of 0.3, and 16 lx for a combined CS of 0.4 at the furniture locations in the space.
Figure 3 compares the CS:LPD performance of the five overhead luminaire types and layouts evaluated for the study, as well as the “spectrally optimized” source and the configuration with the supplemental layer of blue light in conjunction with the 2-ft by 2-ft troffer delivering 300 lx at 3000K. Data points in the light-gray shaded area of the chart met the CS target value of at least 0.3 while staying under the ASHRAE 90.1 2019 LPD guideline of 0.71 watts per sq ft, and points in the dark gray shaded area did the same but for a target CS of at least 0.4.
Only the configuration with the supplemental blue light layer was able to reach the design CS target of 0.4, and that configuration had the fourth-highest CS:LPD ratio of any configuration that could reach a CS of at least 0.3. (As noted earlier, higher CS:LPD ratios indicate greater energy efficiency.) The troffers, with wide diffuse intensity distributions, and pendants with some direct and a “batwing” indirect lighting component, were the most likely luminaire types to provide the CS target with the lowest energy use. At 300 lx horizontal, the only configurations that were able to reach a CS of 0.3 did so with a CCT of 6500K (excluding the conditions with the supplemental blue light layer), whereas at 500 lx, over four times as many configurations reached the 0.3 target and did so at warmer CCTs (3500K and above). Like the findings of the previous study investigating lighting strategies in an office space, increasing the light level had a greater relative impact on CS than increasing CCT, and luminaire types with a vertical-to-horizontal illuminance ratio of 0.6 or higher were more likely to reach the CS target of 0.3.
To assist designers and application engineers in the process of translating the research into practice, the following performance specifications and design guidelines offer a jumping off point to better understand the luminaire-level characteristics that are most important, as well as application-based considerations to keep in mind when designing lighting for the health and well-being of K-12 students.
3. SpecificationsFor delivering circadian-effective light to promote entrainment and alertness among students in K-12 classrooms, the LHRC recommends providing CS ≥ 0.3 throughout the entire school day (roughly 8 a.m. to 3 p.m.) to ensure students receive at least two hours of continuous exposure. To avoid undoing the benefits of this exposure as the day progresses, exposures should be reduced to CS ≤ 0.1–0.2 in the late afternoon and CS ≤ 0.1 in the evening (Table 1). Receiving a high dose of CS in the morning is the most-effective time of day for promoting circadian entrainment, but in reality, students in the higher grades (i.e., middle and high school) typically change classrooms, reducing both the time they spend in a given space and the likelihood they will be continuously exposed to two hours of high CS levels at any time of day. Providing a CS of at least 0.3 over the entire school day will therefore increase the chance of hitting the two-hour mark plus it will ensure that alertness is also being promoted. But if students remain in a single classroom for most of the school-day (as is commonly the case in elementary school), to reduce energy consumption while still promoting entrainment, it will be possible to lower the CS level to 0.2 from the late morning through the early afternoon. To account for the shading of light by facial features such as the nose and brow, as well as shading and absorption from furniture or materials hung on classroom walls, the LHRC recommends a daytime design target CS value of 0.4 to accommodate these factors and ensure that most occupants of the space will receive the desired CS of at least 3.0.
4. Design Process
- Model your space. Build a 3D computer model of the classroom in a photometric simulation program such as AGi32 and arrange vertical and horizontal illuminance calculation points. Calculate: (1) horizontal illuminance in a 6-in. by 6-in. grid on the work plane and (2) vertical illuminance along a line at the occupants’ eye level (3 ft-6 in. above the finished floor; see Figure 1). Using the SPD of the specified light source, calculate CS using the newly published CS Calculator 2.0 developed by the LHRC.
- Determine the spectrum and light level. Increasing the horizontal light level to 500 lx will increase the likelihood of reaching a CS of 0.3, regardless of spectrum if only white light sources are being used in the space. While more products are becoming available that tout special “optimized” polychromatic spectra for circadian stimulation, this strategy may come at a steep energy cost, as shown in Figure 3. The “spectrally optimized” source provided slightly higher CS than the standard 3500K recessed linear luminaire but required 70% more energy to achieve a CS of 0.3. The luminous efficacy of such sources is likely to improve over time, but care should be taken when considering them for your design. Because the circadian system is so sensitive to short-wavelength light (peak close to 460 nm), supplemental narrowband blue light can be particularly useful when strict energy efficiency requirements, horizontal illuminance, CCT and/or aesthetic constraints make CS delivery difficult from traditional overhead luminaires alone. To achieve a CS of 0.3, 8 lx of blue light at the eye (in addition to the 2-ft by 2-ft troffer delivering 300 lx horizontal at 3000K) was needed from the supplemental layer, for an additional 0.09 watts per sq ft. For a CS of 0.4, 16 lx of blue light was needed for an additional 0.17 watts per sq ft. Using supplemental blue light was the most effective means of providing a CS of 0.3 for as little energy as possible without exceeding a CCT of 5000K.
- If using blue light is not an option, design the overhead lighting to provide adequate CS and maximize CS:LPD. Light must reach the retina to affect the circadian system. Therefore, sources with relatively wider intensity distributions that deliver more light in the vertical plane will provide more circadian-effective light per watt compared to narrow beam sources. It is important to evaluate the intensity distribution of the luminaire you are considering. Look for those with a vertical-tohorizontal illuminance ratio of at least 0.6:1, such as troffers with wide, diffuse distributions, or direct/indirect pendants with a “batwing” indirect component.
The goal of this study was to expand the scope of the design guidelines previously developed for office spaces to schools with the hope of bringing the benefits of circadian-effective lighting to a large population that often experiences sleep disturbances and classroom lighting that is inadequate for the circadian system. Employing a thoughtful design approach can deliver benefits to the health and well-being of the K-12 population while also protecting the health of the environment.
Creative solutions like supplemental narrowband blue light can deliver circadian stimulus without the need for expensive control systems or increased light levels and energy consumption. But beyond the lighting design, it is also important to educate students, as well as their teachers and parents, on the potential benefits of such lighting and their role in the process. It should be made clear that avoiding disruptive evening and nighttime light exposures is equally important for promoting the health of the circadian system. And while designers and engineers can illuminate classrooms in a manner conducive to improved sleep quality and health, only through engagement with the end-users and stressing the importance of “circadian hygiene” can the maximum benefits of such solutions be achieved.
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