Iridescence

 

I am fascinated by light and colour, and, the more time I spend photographing birds the more I appreciate the incredible complexity and beauty existing within these two phenomena.  In my opinion light is what makes or breaks a photographic image.  It easily trumps subject, composition and technical accuracy.  As a photographer I am constantly studying light, its qualities, direction and intensity; its effect on subjects and the vast array of subtle hues and tones it forms. 

 

To see colour you must have light.  Light is electromagnetic radiation transmitted in waves or particles at different wavelengths and frequencies which together form the electromagnetic spectrum.  Visible light is the section of the spectrum, between Ultraviolet (UV) and Infrared (IR), which the human eye can perceive.  When visible light is refracted through a prism you see the familiar rainbow as the white beam is broken into component wavelengths which our eye perceives separately as colours. 

Colour is both a property of light and an artefact of the perceiving eye.  Objects do not inherently have colour, rather, they reflect and absorb light in varying wavelengths, depending on their physical properties.  It is the light reflected off objects that our eye perceives as colour.  If all wavelengths of visible light are reflected we see white.  If all wavelengths of visible light are absorbed, we see black. If, an object reflects only the wavelength and frequency of yellow light and absorbs all others, our eye will see the object as yellow. 

Birds are extremely colourful creatures.  It is their feathers which reflect the light we see as colours, so it is interesting to look closely at the physical properties of feathers to see how and why they absorb and reflect light.  It happens in two main ways.

Colour Created by Pigments

 The first way birds create colourful plumage is through the use of pigments which can be either made within the bird’s body or extracted from plants eaten by the bird.  Pigments are molecules which selectively absorb and reflect different wavelengths of light, thus creating colour.  This process happens at the atomic level where the electrons of a pigment molecule occupy different energy levels.  Certain wavelengths of light have energy levels which correlate to that of the electrons. 

                                                                                    Source; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/   

                                                                                    Source; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/   

When a photon of light hits a pigment molecule, these wavelengths of light are selectively absorbed, as their energy is captured and transferred to electrons elevating them to a higher energy level.   A well known example of this is the chlorophyll pigment contained in plants.  Chlorophyll molecules selectively capture the energy which drives photosynthesis from blue and red wavelengths of light, while reflecting green wavelengths.  Hence plants appear green.

There are four main groups of pigments used by birds to create colour.  The first group is melanins which create dark black or brown colouration.  Melanin is made within the bird’s body and is an extremely strong molecule which is also used to add structural strength to flight feathers.  This is the reason many birds have dark wing feathers, especially on the tips, as seen in the Osprey image below.

The second group are carotenoids which create orange, red or yellow colouration.  Carotenoids are made by plants and obtained by birds when they eat plant material or other animals that have ingested plant material or algae. Carotenoids are reason that flamingos are pink.  These birds eat blue green algae and brine shrimp which contain the pigments.  Once digested enzymes in the liver break down the carotenoids into pigment molecules which are then deposited in the bill, feathers and legs.  

The third group are porphyrins which are created in the birds body by modifying amino acids.  Porphyrins create red, brown, pink and green colours and all fluoresce bright red under UV light .  Birds can see a greater range of the electromagnetic spectrum than humans.  They have the ability to see ultra violet (UV) light and also are able to discern smaller differences in colour and tone than humans.  Many owl species use porphyrin pigments in their plumage and are able to perceive UV light.

 
                                                             Source; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/   

                                                             Source; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/   

 

The forth group are psittacines, which are found only in parrot species, are made within the bird’s body and create vivid red, orange or yellow colouration.   Mixtures of different pigments can create an almost infinite array of colours and shades. 

Structural Colouration

Structural colouration is the second way birds create colourful plumage. Under this method it is the physical structure of the feather and it's interaction with light which create the colours we see.  In birds, blue colouration and iridescence are always created structurally.  Blues are created by tiny particles of keratin or air cavities which overlay a dark layer of melanin pigmentation.  These particles are tiny, smaller than the wavelength of red light and, therefore, can only affect the smaller wavelengths of visible light which get scattered in all directions and appear blue.  The other colours are absorbed by the underlying melanin.

                                                                                           Source; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/    

                                                                                           Source; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/    

 Iridescence is created by microscopic keratin structures or layers located on the feather barbules.  These structures create thin films which interfere with light causing diffraction as the light enters and leaves the surface.  Constructive interference causes light waves to combine so their crests and troughs are in synch magnifying the reflected colour.  Destructive interference causes light waves to combine so the crests and troughs cancel each other out.  Changes in the angle of view, shifts the proportion of constructive and destructive interference which causing the iridescence to change colour.  

                                                                      Source; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/   

                                                                      Source; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/   

Images of Iridescent Birds

Over the past year I have taken thousands of photographs of birds and learned an enormous amount about them.  Iridescence is something I always look for because it is interesting to see and photograph and in the right light adds a stunning dimension to bird images.  Here are just a few images from my bird photo library that display iridescence.  

All diagrams in this journal entry are derived from the following website; https://academy.allaboutbirds.org/how-birds-make-colorful-feathers/       Here is the link to the full article;