Tuesday, 13 December 2016

Birds and Light - Polarization

General Principles
While I have researched and written about lots of complex aspects in the realm of light, up until now I have avoided one of it's fundamental characteristics, namely Polarization.  Light exhibits characteristic of both particles and waves.  Waves oscillate as we all know, and we are already very familiar with wave oscillation as we experience it all the time, be it a wave propagating across the surface of water, or along a rope as it is moved up and down (or from side to side).  A light wave which is simply oscillating like this is said to be Linearly Polarized.  Natural sunlight is Unpolarized.  The easiest way to understand what this means is to imagine many ropes oscillating, not in unison along one axis, but in many different random axes, i.e. vertically, horizontally and all angles in between.  Light waves can also rotate as they move forwards through space and time, as illustrated below.  This is referred to as Circularly Polarized.  We need to understand these three simple concepts before we can proceed. 

For a nice overview of polarization I recommend this video by Eric Mickelsen.

For a more complex explanation, or if you are having difficulty sleeping, can I recommend this video (HERE).The next piece to understand is that light can move between these states of polarization when it comes in contact with different types of matter.  The common mechanisms by which this happens are by scattering, reflection and refraction as discussed in the video above.

Polarization by Scattering
When sunlight hits our atmosphere the shorter bluer wavelengths of light are scattered (Rayleigh scattering) giving rise to the appearance of a blue sky and related phenomena.  In doing so unpolarised sunlight is transformed into linearly polarized sky-light.  This is all very neatly explained in this online article (HERE).  The landscape photographers out there will already be aware that a polarized filter may be placed over the camera lens to boost the saturation of the blue sky in a landscape image, simply by blocking some of the glare.  This method is a direct, practical example of the use of polarization in photography.  Could we find other uses for it?

Polarization by Reflection and Refraction
When unpolarized light hits a non-metallic surface the light which reflects back off that surface is polarized as neatly explained by Eric Mickelsen in his video.  The extent to which polarization occurs depends on the material and the angle of incidence.  For example a water's surface when viewed at a shallow angle appears very reflective, with a high degree of glare as the extent of linear polarization is large.  Fisherman use glasses with polarized filters to block out this glare and peer through the water.  HERE are some more examples of this filter in use in photography.  Interesting to note that metallic surfaces, though very reflective, do not tend to reflect polarized light.  Rather, the reflected light from metallic surfaces is unpolarized.  You can read more about it along with the other applications referred to above at this LINK.

Birds and Polarized Light
It's now well accepted that birds use a combination of magnetic fields and polarized light, together perhaps with landmarks to navigate during migration.  In this intriguing recent study (HERE) it was found that birds become disoriented if light polarization is disrupted.  Other animals including bees can see polarized light and also use it for navigation.  Perhaps most intriguing of all, humans too possess a very subtle ability to see polarized light using only our eyes.  The phenomenon is referred to as Haidinger's brush.  Nonetheless, a much easier way to experience light polarization is with the aid of a polarized filter or polarizer.  Here is another nice online video showing different types of light polarization experiments using polarizers.  It includes a demonstration of the all-important blue sky-light polarization, considered so important to the birds and the bees in navigation.

Polarization and Bird Photography
As indicated above, polarized filters play a useful role in landscape photography.  This works by reducing the glare from the sky, thus increasing the intensity and thus the saturation of the blue sky in the image.  Similarly a polarized filter can be used to isolate unwanted glare and reflection from other objects such as surface reflection on water or reflection from a waxy surface on leaves.  It could also be used to help reduce glare on the surface of a bird, such as its bareparts, and to a lesser extent, its feathers.

Like a lot of aspects of natural lighting phenomenon we can see spatial and temporal variation during the day and throughout the seasons.  Taking advantage of what we observe about the polarization of sky-light is there a way we can improve our capture of birds against the sky, either with the use of a polarized filter or a more judicious selection of angle to point the camera relative to the sun?  Of course bird's don't readily cooperate with the photographer when it comes to choosing a flight path.  But if a bird is routinely circling an area, knowing the best place to position oneself relative to the sun and polarized sky-light may be an advantage.

 At sea, as I have already mentioned, the glare of the sky off the water can create an added difficulty for the capture of detail on seabirds passing by.  I don't know if I have yet figured out the ideal location to position myself on a boat during a pelagic on a sunny day!  But I might try using a polarized filter next time to see if it brings some useful results, or at least helps me find the best place to sit and wait for that lucky fly-past!

Photographing birds on snow and ice might seem like a similar case in point but in fact light reflection off snow is different and it's polarization may be more variable.  Like a sea which is rough, snow and ice crystals are multifaceted with more complex reflection, refraction/transmission of light taking place.  That said anything that reduces glare, even slightly may be worth a try.

The downside of using a polarizer is it's overall impact on exposure.  Polarizers dramatically cut the available light reaching the sensor, which can be a big disadvantage in bird photography.

This has been a brief overview of polarization.  I can see myself returning to various aspects of this in due course.