Colour is in the Eye of the Beholder
Pumpkin seed oil and water. Credit: iStockphoto/Glenn Bristol
The unique makeup of the cells in our retina, as well as the specific physical properties of substances themselves, explain why we occasionally see things change colour before our very eyes! Samo and Marko Kreft from the University of Ljubljana in Slovenia investigated this phenomenon using pumpkin seed oil as an example. They have just published their research online in Springer’s journal Naturwissenschaften.
In some regions of Central Europe, salad dressing is made preferably with pumpkin seed oil, which has a strong characteristic nutty flavor and striking colour properties. Indeed, in a bottle it appears red, but it looks green in a salad dressing or mixed with yoghurt.
Samo and Marko Kreft’s paper examines the remarkable two-tone (or dichromatic) colour of pumpkin seed oil, by the use of a combination of imaging and CIE (International Commission on Illumination) chromaticity coordinates. The paper also explains why human vision perceives substances like pumpkin seed oil as dichromatic or polychromatic (exhibiting a variety of colours).
Two phenomena explain the perceived shift in colour of pumpkin seed oil from red to green:
Firstly, the distinctive change in colour shade of the oil is due to a change in oil layer thickness. As the oil layer thickens, the oil changes its appearance from bright green to bright red. The observed colour is neither dependent on the angle of observation nor on the direction or type of light.
Secondly, the shift in colour is due to the unique characteristics of the cells in the human retina. Our eyes have two types of photoreceptor cells: rods and cones. Rod photoreceptor cells are very sensitive and operate in dim illumination conditions. Cone photoreceptor cells function well in bright light conditions. They are also the basis of colour perception in our visual image. It is the presence of multiple classes of cone cells, each with a different spectral sensitivity, that gives us the ability to discriminate colours.
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