SIC-Colour of Brilliance

Science In The City

The Colour of Brilliance

DEAR SCIENCE WORLD,

I bought a poster of my favourite Van Gogh painting after seeing the original in London. But when I brought it home and put it on my wall, I was so disappointed. The colour seems all wrong. What happened?

Art

Dear Art,
I assume you wrote to us because “what happened” didn’t involve dropping your poster in a puddle in the Underground. Before I try to explain your disappointment, please bear with me for a few thoughts about how we see colour.

Suppose you’re gazing at a sunflower in your garden. You can see the flower because light shines on it and is reflected back towards your eyes. The light from the sun, which we call white light, is actually a mixture of all the colours of the rainbow. When the sunlight shines on the flower, the flower’s petals absorb some of the colours of light (red, orange, green, blue and purple) and reflect just the yellow light back to your eyes. Simple, right?
It’s actually a bit trickier. There isn’t just one yellow—check out the sample chips at a paint store for proof of this. Light waves with wavelengths between about 570 and 600 nanometres look yellow to humans. Your yellow flower reflects only some of those wavelengths, and may well reflect some red or green light waves as well. The pigments Van Gogh used to paint the sunflowers also reflect yellow light waves, but in slightly different combinations of wavelengths than the flowers themselves do, so the two yellows appear different.

To make your poster, the printer likely scanned or photographed the original and then printed copies using magenta (pink), cyan (blue-green), yellow and black inks. These four colours are combined in layers and different combinations produce a huge range of colors. But printing inks, which need to be slightly transparent, use different pigments than thick, opaque oil paints. It might be nearly impossible to create an ink combination that reflects exactly the same wavelengths as Van Gogh’s yellow. Your disappointing poster may simply be a victim of print technique.

But before you curse the printer, consider this: the flower, or the pigment, can’t reflect light that doesn’t shine on it in the first place. If only blue light shines on your sunflower, there will be no yellow light to reflect back to your eyes, and the flower will look grey. I’m sure you don’t have a coloured spotlight on your poster. However, even “white” light bulbs give off varying amounts of each wavelength of light. Fluorescent lights, for example, are notorious for producing more green light than incandescent lights do, which makes orange and yellow tones look sickly. Art galleries and museums know this, and choose their lighting to flatter their displays—you can do the same.

Van Gogh was very interested in colour perception, particularly in how some combinations of colours seem to enhance each other. He probably would have been interested in experimenting with the effects of different pigments and lighting on his artistic efforts. So, Art, until you can get back to London to admire the original painting, take comfort in the fact that your curiosity about the poster has brought you closer to your favourite artist.

Sandy Eix, PhD, Colourful Personality