Purkinje Effect

Purkinje Effect

Purkinje Effect

Jan Evangelista Purkinje, 1787-1869, was a professor of physiology at the University of Prague. He is credited with a number of scientific discoveries, including the discovery of Purkinje cells, which are large nerve cells found in the cerebral cortex. However as a young man, he made a simple observation - something we have all probably seen - but because he noticed what he saw, and thought about it deeply, he came to some interesting and original conclusions.

He was keen on walking outdoors in the early morning, before it was properly light. He noticed that his favourite red flowers, which seemed so bright in normal daylight, seemed so much darker relative to the surrounding leaves, when viewed in very low light conditions. This has become known as the Purkinje Effect, and he speculated that humans have two separate systems for seeing, one that is used in bright light and one that is used in low light conditions.

We now know that the human retina has two types of cells, cone cells and rod cells. There are about 4.5 million cone cells in the retina, and they are responsible for colour vision. There are normally three types of cone cell, but overall they are most sensitive to yellow light. The rod cells, of which we have about 90 million in each retina, work at very low levels of light, but cannot distinguish between different colours, which is why everything seems to be black and white at low light levels. However the rod cells are most sensitive to the blue/green end of the spectrum.

So in normal daylight, red flowers (which contain a lot of yellow) seem to be very bright, as seen by the cones in the retina. Under very low levels of light, the rods which are most sensitive to blue/green wavelengths, see the surrounding leaves as being brighter than the red flower!

Jan Purkinje would not have known about the system of rods and cones in the retina. However he didn't only look at the world around him, he 'saw' the world, and made some insightful conclusions from what he saw.

The second image is a simulation of what the main image would look like under very low light conditions, seen by the human eye.

Photo by Jonathan Zander.