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 507 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 gradually at dusk³. 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 one reason night scenes in films and on stage are often tinted blue⁵.

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 lime shades are easiest to see in dim light⁴.

How common it is

The Purkinje effect happens in everyone with functioning rod and cone cells, so it has never needed a prevalence study². It follows directly from the different colour sensitivities of the two systems, first noticed by Jan Evangelista Purkyně from his own observations in 1819 and since confirmed by measuring the sensitivity curves of rods and cones directly¹². Someone who is completely colour blind for lack of working cones would not notice the shift, since they would see through one system throughout.

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.

Is it a problem?

No. The colour shift at dusk is a normal handover between two working parts of the eye, not a fault¹². Noticing that reds fade and blues stay bright as light dims is expected, and is actually a sign your rods are working.

There is nothing here to mention to a doctor. It is only worth asking about if colours look wrong in normal daylight, or if night vision seems far worse than in the people around you, since those point to a different problem with the rods or cones².

History and culture

Jan Evangelista Purkyně, a Czech physiologist, discovered the effect in 1819¹³. He noticed that his favourite red flowers looked bright on a sunny afternoon but very dark at dawn¹.