Protection: Here’s where the light tank will benefit the most from technological innovation. Active protection systems such as the Israeli Trophy and Russian Arena, which use radar antennas to track enemy rockets and missiles and shoot them down with shotgun-like blasts, can protect a small tank just as well as a large one. That means that a modern U.S. light tank could dispense with much of its armor—perhaps keeping just enough to stop 12.7-millimeter heavy machine gun bullets. For larger projectiles, an active protection system would keep the tank crew safe.
http://www.popularmechanics.com/military/weapons/a22337/us-army-new-light-tank/
But active protection won’t protect from, say, a 37mm antitank gun of 1940, with solid steel shot.