Transparent aluminum was a construction material far stronger and much lighter than its predecessor, plexiglass. A one-inch thick sheet of transparent aluminum, measuring sixty feet by ten feet, was capable of withstanding the pressure of 18,000 cubic feet of water, which could be used in place of a six-inch thick sheet of plexiglass.
Dr. 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 Plexiglas 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 were 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")