The South Pier Inner Range Light is the tallest operating lighthouse in Duluth. It was built on the South Pier of the Duluth Shipping Canal and was completed in 1901. The tower is a skeletal pyramidal tower with a central column that contains an enclosed spiral staircase. It is topped with a cylindrical watch room and and an octagonal lantern. This lighthouse originally displayed a 4th order fresnel lens (although some sources say it was 5th order lens) which was rotated by a clockworks mechanism. That lens was removed in 1995 and is now on display in the Canal Park Marine Museum, which is adjacent to the Duluth ship canal. The lens currently being used is a modern acrylic fresnel lens which is also rotated to produce a flashing white light.

The purpose of this light was to help ships align themselves to the canal at night. If the white flashing range light can be seen directly above the steady green light of the lighthouse at the end of the same pier, then you know you are approaching the canal straight on. If the two lights are not seen one above the other, then you are not lined up with the canal.

This lighthouse was sold in 2008 and is now in private hands, but it is still active as an aid-to-navigation.

This is a closeup of the original fresnel lens for the Duluth South Pier Inner Range Light. In this shot you can see two of the six flash panels and bulls-eyes. It was mounted on a brass frame and rotated originally by a small clockworks mechanism that was driven by counter weights.

Sources:
http://lighthouse.boatnerd.com/gallery/superior/Duluth-South-Inner.htm
http://www.terrypepper.com/Lights/superior/duluth-s-rear/index.htm
http://www.lsmma.com/visitors/exhibits.html
https://www.unc.edu/~rowlett/lighthouse/mn.htm
http://www.lighthousefriends.com/light.asp?ID=264