A vision block is a variation on the venerable periscope
, a device for seeing above an obstacle
. The difference in a vision block is that the entire periscope is made of bulletproof glass
with mirrored facet
s instead of mirrors suspended in a hollow
The reason for the solid construction is that a vision block is used in an armored personnel carrier or tank so that the crew can see out of the vehicle. The solid glass makes it more difficult to damage. They are usually deployed around the rim of the vehicle hatches.