Purkinje Effect

mainEver wondered why the colors seem to change at night? For example, if you look at an air painting, you can see how the colors of objects look radically different in very low light just before dawn or dusk. Consider a red rose, for instance. We know that the flower’s petals are bright red against the green of the leaves in daylight. But, take a look at dusk and you will see that suddenly the contrast is reversed, with the red flower petals now appearing dark red or dark warm gray, and the leaves appearing relatively bright. Bright red doesn’t remain bright red anymore, and green doesn’t remain green either. They all become a bit monochromatic and it becomes difficult to separate them. Why does this happen?  

Before we start, let’s look at the story behind it

PurkinjeJan Evangelista Purkinje (1787-1869) was a professor of physiology at the University of Prague. He is credited with a number of important scientific discoveries. As a young man, he made a simple observation, and came to some interesting conclusions. He was keen on walking outdoors in the early morning, before it was completely bright. He noticed that his favorite 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 became 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.

Why does this happen?

The human retina has two types of cells called cones and rods. There are about 4.5 million cone cells in the retina, and they are responsible for color vision. There are normally three types of cone cells, 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 colors, 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.

rose different colorsThis results us in becoming nearly color blind under low levels of illumination. As the light dims, the rods take over from the cones, and before color disappears entirely, our color perception shifts toward the blue-green spectrum. The Purkinje effect explains why we can’t see many colors at night other than the blues and greens that our rods can sense. 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! Purkinje would not have known about the system of rods and cones in the retina because they were discovered much later. So he just based these conclusions on what he observed. Pretty insightful!

Digital Cameras vs Purkinje

Digital cameras do not model human vision in low light. As light dims, the eye becomes more sensitive to the bluer wavelengths of light, due to the presence of blue-green sensitive (and red-blind) rod cells. No similar mechanism is found in cameras. Invariably, photos taken under very dim light do not match the color that is actually seen. Using a color calibration target under these conditions will certainly be disappointing if you want to reproduce what you actually see.

This leads us to something called “photographic intent”. Sometimes, I want to record the colors and brightness of the various objects in the scene in a way that faithfully preserves their relative tones, while also subtracting out most of the variation of lighting. This is perhaps the most objective way of recording a subject, and this kind of flat uniform lighting is often found in product shoots, or in the exposure blending techniques that are used when taking architectural interiors.

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