Kruithof Curve

The Kruithof Curve is a graphical representation of the relationship between illuminance (brightness, measured in lux) and color temperature (measured in Kelvin) to define what is considered "pleasing" or "comfortable" lighting for humans.

It was introduced in 1941 by Dutch physicist A. A. Kruithof while he was working at Philips. His research demonstrated that human perception of light changes depending on the intensity of the light source, proving that we inherently prefer warmer hue colors as lights dim and become less bright.

Our preference for the Kruithof Curve is no coincidence, but rather the result of the inner construction of our vision. Our eyes possess two types of photoreceptor cells: rods and cones.

Rods are highly sensitive to blue hues, while cones are more attuned to red hues. When we're in a brighter space, our eyes use red-sensitive cones, which is why we prefer cooler (bluer) lights. As we enter darker spaces, our vision transitions to rods (blue-sensitive), causing us to conversely prefer warmer (redder) lights.

Historically, halogen bulbs naturally had a "warm dim" effect. The more power went through the bulb, the whiter the light became. As lights were dimmed, less current ran through them, naturally warming the color of the filament (typically from 3000K down to 1850K).

Modern LED lights, however, maintain a consistent color regardless of brightness. To simulate a true warm dim, high-quality LED fixtures use two or more LEDs of different colors acting together. This technology helps ensure your lighting feels comfortable at any brightness, whereas a standard LED fixture would feel cold and clinical as it dims.

While the Kruithof curve is an excellent general rule of thumb and widely taught in lighting design, modern research suggests it may not be a strict law for all environments.

• Original Study Size: Kruithof's original 1941 study was based on a very small sample size and used the incandescent and fluorescent lamps of that era.
• Cultural & Geographic Differences: More recent studies suggest that cultural background and geographic location play a huge role in preference. People closer to the equator often prefer cooler lighting at home, while those in northern latitudes prefer warmer lighting.
• Task Dependency: A person's task greatly influences their preference. High illuminance cool lighting might fall in the "pleasant" zone, but if you are trying to relax before bed, it will feel glaring and disruptive regardless of the curve.

Architects and lighting designers rely heavily on the principles of the Kruithof Curve when specifying fixtures for different spaces. The intended function of a room dictates where its lighting should fall on the curve.

The Kruithof Curve aligns beautifully with our modern understanding of circadian rhythms. Our bodies evolved under the sun, which perfectly traces the pleasing zone of the Kruithof curve throughout the day.

• Midday (High Brightness, High CCT)At noon, the sun provides up to 100,000 lux at a cool 5500K - 6500K. This intense, blue-rich light maximizes melanopic lux (the light that suppresses melatonin production), keeping us awake and alert.
• Sunset (Low Brightness, Low CCT)As evening approaches, the sun drops low, scattering blue light and leaving only warm, reddish hues (2000K - 2500K) at a much lower intensity. This reduction in melanopic lux signals our brain to release melatonin, preparing us for sleep.

Modern Human-Centric Lighting (HCL) systems use tunable LED technology to automatically adjust both brightness and color temperature throughout the day, artificially recreating this natural cycle indoors while safely staying within the comfortable zone of the Kruithof Curve.