Mechanics of Iridescence
There are many subjects in nature which are extraordinarily beautiful, and animals which glint and gleam with iridescence are definitely one of them. However, in order to take on the (not inconsiderable) challenge of illustrating iridescent creatures it’s important to have a basic grasp of the processes that cause these animals to shine and glitter as they do.
The colours seen in areas of iridescence are not caused by pigment, but by physical interaction and interference between light waves and the object they’re reflecting off. These can be constructive light waves, which add to the light waves coming from another proximal surface; and which result in brighter colours. Or they can be destructive light rays, which cancel out light waves coming from another proximal surface; these cause colours to disappear or produce no reflected light and thus an area of darkness.
We see these effects in the rainbows of oil slicks or on the surface of soap bubbles:
(Soap bubble photograph by Irfan Mirza)
We only see these effects if they occur on two surfaces very close to each other so the light waves are able to interfere with one another (the inner and outer layer of a soap bubble, or in the case of insects, the inner and outer layer of a wing).
Constructive and destructive interference can occur at the same time and at the same point (think of the areas of dark on a beetle, caused by destructive interference, bang up next to gleamingly bright patches of shining colour which result from constructive interference).
Another way some birds, butterflies, beetles, fish and reptiles produce iridescence is by layering multiple micro-thin ridged layers ontop of each other, in a spiral or helicoidal pattern. Again, the interference between light waves reflected from each surface results in glorious colours such as the wing of a morpho butterfly.
(Blue morpho butterfly by Fran Henig)
The more reflective layers there are, the brighter the colours produced by constructive interference will be. If all the layers are the same thickness you’ll get one colour reflected; if the layers vary in thickness you’ll end up with a gleaming silver, as on fish scales.
(Fish illustration by Patricia Ferrer Beals Ferrer+Beales, copyright held by the artist)
Although the colours may change as you look at your subject, their position relative to each other will not alter. Look at a peacock feather, preferably an actual specimen feather. In some lights the area around the eye looks bronze, in other lights it looks pink. Likewise, the eye will look azure blue or bright grass green as you tilt it in the light. However, the relationship between the two areas of changing colour, their edges and margins, will not alter.
(Peacock feather by Christina Brodie)
For more detailed infromation on the physics behind iridescence take a look at the Microscopy U website.
When it comes to illustrating iridescence, it necessarily has to be about tricks to fool the eye. By painting with pigments, no matter how bright or glorious, one cannot reconstruct the interfering or helicoidal ridged layers. We have to use pigments to convince our brains that our illustrations are in fact iridescent.
The main trick here is to be accurate and bold in observing areas of brightest colour and of the darkest darks. The transition between these two extremes is far more swift than within areas of bright pigmentation as they become darker in a shadow. Also, if there are two different irridescent colours on one animal the transition from one to another is less abrupt than with different areas of pigmented colour. Keep the edges of the bright areas soft, and be sure these brightest areas are startling and your darkest darks really intense.
Iridescent beetle, gradual colour changes, Pigmented beetle, abrupt colour changes,
abrupt change from brights to darks. gentler change from bright to dark areas.
When you come to the lightest areas be sure to observe them carefully; although they may be the lightest and brightest part of the specimen they may well not be white. Iridescent highlights are often characterised by not being white.
Copy the shapes of the areas of light and those of shadow as accurately as possible. Keep your brightest areas clean and crisp in colour. I use Doctor Martin inks which, though fugitive, are extremely vivid (for more on technique please refer to my earlier blog.)
I have also used mica mixed in with paint to confer iridescence; with only partially successful results.
An illustration, remember, is a flat 2D image so whether or not the shiny paint particles glimmer as you change your viewpoint is only relevant to someone with the physical painting in front of them.
Useful colours to consider using when painting iridescence are pinks near bright greens, and cold yellows proximal to greenish blues.
Good luck! Many thanks to the excellent illustrator and tutor Sarah Morrish for suggesting this topic, and to Trudy Nicholson and T. Britt Griswold whose article in the excellent “The Guild Handbook of Scientific Illustration” edited by Elaine Hodges has largely informed this week’s blog.
(NB All illustrations, unless stated otherwise, are by Lizzie Harper)