Purkinje effect

How it works

The retina has two kinds of light sensor. Cones work in bright light and give colour vision. Rods work in dim light and give no colour at all². The two kinds are tuned to different parts of the spectrum.

Cone vision as a whole is most sensitive to yellow-green light at about 555 nanometres¹. Rod vision is most sensitive to blue-green light at about 505 nanometres¹. In daylight, the cones dominate and a red rose looks bright against green leaves. As the light fades, the cones shut down and the rods take over. Rods barely respond to red at all. So the rose goes almost black while the leaves, and anything blue, still look bright¹.

The change happens over about half an hour of dusk. In the in-between stage, both systems work, and colours look strange. Blues glow, reds die, and the whole scene looks cooler than it did an hour before. This is why night scenes in paintings and films are often tinted blue¹.

The effect has practical uses. Red light barely affects the rods, so a red torch lets you read a map without losing your night vision. Astronomers, sailors, and pilots use red lights for this reason¹. Some fire departments have moved from red vehicles to lime-yellow ones, partly because yellow-green stays visible in dim light and red does not¹.

Try it

1. Find a red object and a blue object of similar brightness. Two books or two pieces of paper work.
2. Place them side by side near a window at sunset. Or put them in a room and dim the lights slowly.
3. Look at them every few minutes as the light fades.
4. The red object goes dark and dull. The blue object stays bright. In deep dusk it may look brighter than the red one, even though it did not in daylight.

History and culture

Jan Evangelista Purkyně, a Czech physiologist, described the effect in 1825¹. He noticed that his favourite red flowers looked bright at midday but dark against their leaves at dawn¹³.