The Kruithof Curve

Arie Andries Kruithof conducted experiments in 1941 where observers were asked to evaluate the appearance of a room lit by discharge lamps of varying color temperatures and illuminance levels. By plotting the combinations that were rated as ‘pleasing’ or ‘natural’, he defined a bounded region on a graph with illuminance on the y-axis and color temperature on the x-axis. For example, a dim light (e.g., 20 lux) was found pleasing only if it had a very warm color temperature (e.g., 2000 K), similar to candlelight. A bright light (e.g., 500 lux) was pleasing only if it had a cooler temperature (e.g., 4000 K or higher), similar to daylight.

While the original experiments had a small sample size and their strict validity has been debated, the general principle described by the Kruithof curve has been highly influential. It provides a simple, intuitive guideline for lighting design that aligns with human experience: the warm, dim light of a fire is cozy, while the cool, bright light of the sky is energizing. This concept is the foundation for modern human-centric and circadian lighting systems that aim to improve health and comfort by modulating light intensity and color temperature throughout the day.