Transparent aluminum was a construction material far stronger and much lighter than its predecessor, plexiglass. A 1-inch thick sheet of transparent aluminum, measuring 60' x 10', was capable of withstanding the pressure of 18,000 cubic feet of water, which could be used in place of a 6-inch thick sheet of plexiglass.
Dr. Marcus "Mark" Nichols of the San Francisco-based Plexicorp, acquired the formula for transparent aluminum in 1986 from a mysterious engineer from Edinburgh, known as "Professor Scott." In exchange for the formula, Dr. Nichols provided enough plexiglass to "Professor Scott" to construct a giant whale-tank in the cargo hold of the stolen Klingon Bird-of-Prey. (Star Trek IV: The Voyage Home)
The viewports of the USS Enterprise-D are made of transparent aluminum. In 2367 when an anomaly caused atmospheric decompression in the observation lounge, Data scanned the transparent aluminum in the windows, and found a pattern of transient electrical currents characteristic of subspace distortion. (TNG: "In Theory")