Warm or Cool? Lights influence on thermal comfort

The purpose of the project was to show whether or not using the CCT-adjustable LED light in a building can extend the temperature range that users of the building perceive as comfortable.

The project is based on previous research results on colored light and could lead to a reduction in the heat consumption of buildings in winter as well as reduced cooling in the summer.

Previous studies show the potential for using colored light to influence the experience of thermal comfort, where a warm color temperature of the light causes an experience of warmer surroundings than with a cold color temperature of the light. In the present project, the connection between experienced temperature and the color properties of light was tested without daylight in laboratory environments at DTU - Build. The experiment's hypothesis was that at particularly cool or hot air temperatures there was the potential for the color properties of the light to influence the thermal temperature experience. This hypothesis was rejected, but on the other hand, results close to the comfort temperature 22 ° C tended to have the ambient temperature experi-enced cooler at higher color temperature than at lower color temperature. The experiment took place at a high light level, 1000 lx on work tables.

The next step was a parallel course with laboratory tests and field trials. The laboratory experiments focused on further investigating the effect found at the 22 ° C and the field trials were aimed at investigating the correlation between experienced temperature and the color temperature of the light in a commonly used office environment at DTU - Photonics, Risø campus.

The new laboratory experiments focused solely on exploring the effect of color temperature on experienced temperature at 22 ° C and 24.5 ° C. The trials were conducted with 500 lx on work tables. Here it turned out that after changing the color temperature of the light, there are signs that this has an influence on the temperature experienced, but after half an hour in the room there was no effect of the color temperature on the experienced room temperature. Moreover, the effect could not be detected when subjects used computer screens nor be detected in subjects over 30 years of age.

The purpose of the field studies was to test whether the effect found in the first laboratory experiments also existed in an ordinary office environment and with changeable surroundings. The field surveys took place at Risø DTU Fotonik in offices with daylight. The light fixtures in the offices were with programmable color temperatures and were set to 3 levels respectively. 3200K, 3600K and 4600K, covering their maximum range. The illuminance of the work area from the electric lighting was programmed to be the same for the 3 color temperatures. Measured illumination intensity varied between 373 - 573 lx during the experiment. The field studies did not show a connection between the color temperature of the light and the experienced temperature. Based on our studies, there appears to be an effect of the color temperature of the light on the temperature experienced in rooms without daylight and at 22 ° C. The effect seems to disappear after 30 minutes of acclimatization to the surroundings.

The results indicate that white light under normal conditions is not sufficient to significantly affect the temperature experienced and therefore light that is more saturated or not affected by daylight is required. The effect of color temperature seems to be negligible after half an hour of acclimatization.