Study finds light spectrum changes how people feel heat and cold

Two lamps that look identical to the eye may not produce the same experience of temperature, according to a study published in Energy and Buildings.

Researchers tested two white-light sources with the same brightness and appearance. One contained a higher proportion of blue wavelengths, while the other contained more red. None of the participants noticed a visual difference in the room.

The experiment involved 10 adults, five men and five women aged 18 to 35. Each participant took part twice, once under each type of light, at the same time of day. After 30 minutes of adjustment at about 24°C, the room temperature gradually rose or fell while participants worked, read or solved puzzles. Every five minutes, they rated their thermal sensation and comfort, and they could press a button when they felt discomfort.

When the temperature rose, participants under blue-enriched light tolerated an average of 0.7°C more before asking for relief from the heat. The difference remained significant after researchers accounted for body weight and body temperature, although participants did not report feeling cooler. When the temperature fell, red-enriched light affected how warmly participants said they felt.

The team, led by Julian Wang, a professor of architectural engineering at Penn State, said the results offer early direct evidence that the spectral composition of light, rather than only its visible colour or brightness, can influence thermal sensation. The researchers are examining whether lighting could expand the range of temperatures people find comfortable, potentially reducing energy use in building heating and cooling.