I spent large portions of today outside filming the sky with a dual-camera rig we set up to try and imitate the polarisation sensitive portion of the insect eye. To get the full sky hemisphere at low resolution, we used a normal (albeit small) camera pointed at a curved mirror – in fact, we used two cameras and two mirrors, because finding a sufficiently small camera that had good UV transmission as well as good visible spectral transmission was prohibitively expensive.
Some background on polarisation as it relates to insect vision: Many insects possess the ability to detect the directional component of light, what we term its polarisation properties. The dorsal rim area, in particular, is strongly sensitive to the direction of polarisation (the phase) of incoming light, and is thought to be used for navigational purposes.
The compound eye of an insect is made up of many ommatidia, which include a lens, cornea, and photoreceptor cells. Each ommatidium has its own ‘preferred’ orientation – this is the direction of polarisation which it responds most strongly to. By comparing signals coming from ommatidia with different preferred orientations, but which view the same area of the sky, the insect can create a ‘map’ of the polarisation properties of the incoming light.
When light originating from the sun hits our atmosphere, it results in a distinctive ‘pattern’ of different polarisation orientations and magnitudes. This pattern can be used as a sun-compass, giving us the ability to detect the position of the sun even on overcast days, or where the sun is invisible due to environmental features, eg. under a forest canopy. Other environment features, like water, or particular kinds of vegetation, can also be distinguished by their polarisation properties.