Non-pharmacologic interventions, such as high-intensity white light with a high output in the short-wavelength part of the spectrum can play an important role in the care for older people with dementia. In order to assess the effects of prolonged exposure to low intensity light, i.e., E <500 lx, from a light source with a high correlated colour temperature (17,000 K versus 2700 K) on behaviour and circadian rhythmicity of institutionalised older adults with dementia, a cross-over design field study was carried out in a psychogeriatric day care ward in May and June 2008. Effects of the lighting intervention were assessed by the Dutch Behaviour Observation Scale for Intramural Psychogeriatrics (GIP), and tympanic temperature measurements. The two lighting solutions installed, particularly the 17,000 K lighting, led to much lower colour temperatures at eye level in practice. No significant improvements in behaviour and in the range of tympanic temperature were found for the lighting interventions tested. This might indicate that higher illuminance levels are the important factor in establishing successful light therapy, and that higher colour temperature may add up to the effectiveness. At the same time, the 17,000 K light tubes did not result in ultrahigh colour temperatures at the eye level of the subjects, and may even have an adverse effect on some persons with dementia.