Kind of hard to see any nav light or strobe light through this lens! The culprit was the failed strobe bulb, but the real cause was a buildup of oil and exhaust residue inside the assembly. Standard practice on Van’s aircraft is to mount the tail/rear strobe light on the bottom of the rudder, right where all the oil and exhaust residue tend to accumulate. If the assembly is not sealed tightly when mounted, the oil and exhaust residue will find its way into the light assembly, contaminating the strobe tube and causing premature failure. Regular cleaning is a good preventative measure. Be careful not to touch the strobe flash tube with your finger, as that can leave contaminants on the bulb, shortening its life.