You are most likely to find the green light with shades on the far side of a pedestrian crossing with a safety island (i.e. a two-part crossings, most often offset to each other), as illustrated below (the
o are the traffic lights):
_____o------____ <- Shaded light
. . >>> Traffic goes this way
_____|=====o____ The safety island
. . <<< Traffic goes this way
___------o______ <- Shaded light
Due to road layout and light configurations, sometimes it is only safe to cross half of the road (up to the safety island). The shades ensure that you do not mistake the green light on the island as the signal to cross the entire road - you can only be sure that it is a green light (but not a broken light, which the general highway code on crossing a uncontrolled crossing kicks in) once you are on the island.
Similar shades are installed on traffic lights for vehicles to reduce the risk of drivers running into an intersection thinking it is a green, when the light meant for the driver is showing red.
I do not have a written source to back up what I said above, though I was told this by my driving instructor when I learnt to drive in London a few years ago.