Friday 7 November 2014

Birds and Light - Translucency

When light hits an object it is either absorbed by it, it is all, or in part, reflected off of it, or it all, or in part transmits through it.  In this way objects can be described as opaque (they do not transmit light), translucent (they partly transmit light) or transparent (they fully transmit light).  

Feathers and even the bare parts of birds are translucent.  Their level of translucency is dependent on their thickness and the density and colour of pigmentation.  The amount of light transmitted also depends on the angle and brightness of the light source behind the feather.  For most birds, it is only the flight feathers, remiges (primaries and secondaries) and the rectrices (tail feathers) in flight that are noticeably translucent, because the other feathers tend to hug the body most of the time.

The image above neatly shows how a subtle change in the angle of a bird's wing relative to the position of the observer and sun can totally alter the type of lighting illuminating all or part of it. Direct sunlight, reflecting off of both the upper and under-wing surfaces simultaneously produces a very similar pattern on both the upper and under surfaces of the primary feathers of this Common (Black-billed) Magpie.  Contrast this with the less bright, predominantly blue light from the sky which is transmitted through the translucent primaries to the camera.  The light makes it through the white portion of the feather with minimal disruption, so much so that light can be seen to pass through the white portion of two overlapping feathers.  The light doesn't get through the outer vane or the tips of the primaries quite as well.  The black pigment in these areas is far more effective at blocking the light, though there is the faintest impression of blue light transmission on the inner vane, nearest the feather tips.  Clearly, light passing through feathers can complicate things greatly!  For more on the properties of translucent materials see (HERE).

I have conducted an experiment using pigment targets on white paper in which I illuminated the targets first from in front of the page (creating a reflected image), then from behind the page (creating a transmitted image).  In both cases I created a series of camera exposures from completely underexposed - a fully black image, to completely over-exposed - a fully white image.  The results are presented below and show there is really no major difference in the characteristics of image exposures regardless of whether the light is reflecting off of a surface or transmitting through it, other than that the internal structure of the material is illuminated by light passing through it.

See also HERE

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