Here is a nice explanation: Blog George Timmermans - George timmermans.
Is it possible to keep the sensor itself in the shade and have it look outside without glass in between ?
Do you want it to capture direct sunlight ? I don't know if the sensor can take that, probably not.
For normal light are domes available that a specified for example to 10% transparent for visible light.
For UV light you need a special UV dome, for example this one: http://www.eoc-inc.com/genicom/UV%20sensor%20probe%20air%20outdoor.htm. It says "Quartz dome".