Feeling too warm or too cold indoors may depend on more than the thermostat. The spectrum of indoor lighting plays a role in how people perceive temperature, according to research led by scientists at Penn State.
In a study, available in the August issue of the journal Energy and Buildings, researchers found that controlling red and blue wavelengths - two of the bandwidths of electromagnetic radiation that flow through space and are perceived by humans as light - of white light can impact how people experience indoor temperatures. Although the wavelength differences are invisible to the human eyes, they can influence the gap between perceived and real temperature by as much as 1.3 degrees Fahrenheit (F).
According to Julian Wang, professor of architectural engineering at Penn State and principal investigator of the study, if certain lighting conditions allow people to feel comfortable across a wider range of temperatures, that could help reduce heating and cooling demands and ultimately lead to energy savings.
''We wanted to understand whether lighting can widen people's thermal comfort zone," Wang said. "If it can, even a modest shift in perceived temperature can translate into substantial cumulative energy savings over time.''
Previous studies have shown that colored lighting, such as blue and red light, can influence thermal perception, he said. However, colored lighting is impractical for everyday environments such as offices, classrooms, and homes because it can be visually distracting and create visual noise, making it difficult to perform routine tasks comfortably. Through this project, Wang's team found that the "color" doesn't need to be visible to achieve the same effects - subtle differences hidden within ordinary white light can produce similar effects without changing the appearance of the lighting.
''The novelty of this study is that we used white light that looks identical to the human eye but has a different spectral composition,'' Wang said, explaining that their findings provide some of the first evidence that different white light spectra can influence how people perceive indoor temperatures. ''That allowed us to evaluate whether the response was driven by thespectral composition of the light, rather than by people's perception of color.''
To test this idea, the researchers recruited 10 participants, five women and five men, between the ages of 18 and 35. Participants occupied a simulated office cubicle inside a climate-controlled chamber at Penn State's University Park campus while experiencing gradually increasing and decreasing air temperature cycles starting from an initial room temperature of approximately 76°F. During the separate experimental sessions, participants were exposed to two types of white light that looked identical but differed in their dominant wavelengths: one enriched in blue short-wave light and the other enriched in red long-wave light. Throughout the study, the researchers monitored participants' thermal sensation, comfort and behavioral responses.
The researchers found that when participants were exposed to blue-enriched white light in the controlled room, they felt cool enough to tolerate air temperatures about 1.3 F warmer. In contrast, red-enriched white light had the opposite effect, making participants feel warmer.
While the findings are promising, Wang said, this was an exploratory study. In the future, the team plans to expand the research to see whether the results are consistent when utilizing participants from a wider range of backgrounds and age groups. They said they also hope to investigate how lighting interacts with other environmental factors, such as sound, window views and visual patterns, to shape people's perception of indoor comfort.
''The next step is to do research in a larger and more diverse population to determine whether these effects extend beyond young adults and could be applied globally," Wang said.
Ultimately, he said his group plans to test the new lighting strategies under more extreme heat and cold conditions and evaluate how they could be incorporated into real-world buildings to improve comfort while reducing energy use.
''This is just one of our starting points,'' he said. ''Our long-term goal is to expand people's multi-domain interactions, and if successful, this approach could improve comfort and human performance while reducing energy use in buildings, from homes and offices to space habitats.''
This research was funded by the Institute of Energy and the Environment (IEE) Seed Grant at Penn State.
Study collaborators include Jeffrey Muninger and Chenshun Chen, postdoctoral scholars in architectural engineering from Penn State; Anne-Marie Chang, associate professor of biobehavioral health at Penn State; first author Nan Wang, a postdoctoral scholar in civil and environmental engineering at Northwestern University; and Yanxiao Feng from the New Jersey Institute of Technology.
