An experimental study to evaluate visual flicker caused by an opaque and transparent ceiling fan

Ceiling fans are used to help cool people when they are either inside buildings or outside (Figure 1), while at the same time – field studies in Singapore and California have both shown – that they can promote significant energy savings and increase satisfaction with the thermal environment. Despite these benefits, some design considerations need to be carefully assessed when they are installed and operated. One specific aspect is the visual phenomenon known as flicker, which is caused when the light from ceiling fixtures is intermittently blocked by the rotation of the ceiling fan blades. To avoid visual flicker, we think ceiling fans should be mounted at the same height as the light fixtures. However, in some applications, this is not the case (Figure 1). Instead, the ceiling fan is positioned away from any nearby light fixture to avoid visual flicker, but this does not necessarily prevent it from occurring. Since visual flicker is also dependent on where an occupant is sitting or standing in the building, from some viewing angles, it is still visible (Figure 1). However, there is limited published research that has evaluated the influence of visual flicker caused by ceiling fans.

Figure 1. Application of ceiling fans found across many different building types in Singapore.

We designed an experiment to test how visual flicker is perceived from two different ceiling fans. One ceiling fan had opaque (non-transparent) blades and the other design had transparent blades (Figure 2 and video). These ceiling fans were mounted inside two identical rooms and rotated at the same operational speed and generated comparable air flows. In other words, the only difference across the two test rooms were the ceiling fan blades. Since the light from ceiling fixtures can pass through the transparent blades, we hypothesized that exposure to visual flicker produced by this ceiling fan would cause fewer and/or smaller adverse symptoms. Forty-six people participated in our experiment. To measure some of the adverse effects associated with visual flicker, they performed three different mental performance tests and provided several self-assessments. These measurements were used to evaluate any potential reduction in work performance and how uncomfortable the conditions were under the two different ceiling fans.

Analyses of the data revealed there was a small (effect size, r= 0.24), yet a statistically relevant increase of approximately 3 % in the number of errors made when test observers performed one of the mental performance tests under the opaque ceiling fan. For the other two tests, no relevant differences could be found across the two ceiling fan conditions. In other words, the mental performance was slightly reduced when observers were exposed to the ceiling fan that caused more visual flicker. Since the tests were used to evaluate different aspects of mental performance (e.g. memory), this may have been why flicker influenced one of the tasks but not all three. We also found that 24 % of the people who had worked under the opaque ceiling fan experienced discomfort caused by visual flicker (Figure 2). When compared to the design with opaque blades, these results in general suggest that the visual flicker produced by the transparent ceiling fan created fewer adverse effects.

Figure 2. View from the test observer inside both test rooms containing the opaque (left) and transparent (right) ceiling fans. Below show the results when observers provided self-assessments based on the degree of discomfort due to visual flicker. Note: “D” is the percentage of people that did not find visual flicker to be a problem (“Strongly disagree” or “Disagree”) and “A” is the percentage of people that found visual flicker to be a problem (“Agree” or “Strongly agree”).

While the mental performance tests showed that the effects in this study were small in one test and negligible in two other tests, observers were only exposed to a condition that caused higher visual flicker for short periods of time (i.e., less than one hour). It is not yet clear how they would have reacted if they were exposed to the conditions for a typical working day (i.e. eight hours). The findings indicate that if ceiling fans are not carefully coordinated with the lighting fixtures, the adverse effects produced by visual flicker can be felt by the occupants. This is practice could translate to reductions in work productivity and economic losses. Although transparent blades can reduce visual flicker, while retaining the energy savings and increased thermal satisfaction produced by ceiling fans, they may raise issues of safety in spaces with low floor-to-ceiling heights (i.e. when rotating, they are less visible than opaque blades).

Acknowledgments

The Republic of Singapore’s National Research Foundation funded this research through a grant to the Berkeley Education Alliance for Research in Singapore (BEARS) for the Singapore-Berkeley Building Efficiency and Sustainability in the Tropics (SinBerBEST) Program. The University of California, Berkeley established BEARS as a centre for intellectual excellence in research and education in Singapore. This research is carried out within the state of the art SinBerBEST Testbed (link) located in Singapore.

References

[1] Lipczynska A, Schiavon S, Graham LT, Thermal comfort and self-reported productivity in an office with ceiling fans in the tropics, Build. Environ. 135 (2018) 202–212 (free version).

[2] Raftery P, Miller D, Zhang H, Peffer T, Brager G, Graham LT, Present E, Arens E, Douglas-Jaimes D, Paliaga G, Brooks A, Cohn S, Greene M. 2020. Integrating Smart Ceiling Fans and Communicating Thermostats to Provide Energy-Efficient Comfort. Final report to CEC. April. (free version).

[3] Kent MG, Cheung T, Li J, Schiavon S, An experimental study to evaluate visual flicker caused by ceiling fans. Build. Environ. 10.1016/j.buildenv.2020.107060 (free version and paid version).