William R. Bennett Jr., 78, a physicist who helped develop one of the first lasers nearly 50 years ago, died Sunday of cancer of the esophagus at his home in Haverford.

In 1960, Mr. Bennett, Ali Javan and Donald Herriott built the first gas laser, which generated a continuous infrared beam from a mixture of helium and neon, at Bell Laboratories in Murray Hill, N.J. Mr. Bennett would go on to develop nearly a dozen additional lasers.

The research helped lead to the widespread use of lasers in modern technology, in things such as CD players, supermarket scanners, surgical tools, and weapons navigation systems. The argon laser helped provide an effective treatment for the prevention of blindness in diabetes and remains widely used.

Mr. Bennett became a tenured professor at Yale University in 1962, was named Charles Baldwin Sawyer professor in applied science and physics in 1972, and spent 38 years at the school, becoming an emeritus professor in 1998 and retiring in 2000. He received a bachelor's degree in physics from Princeton University and a Ph.D. in physics from Columbia University.

He is survived by his wife of 55 years, Frances Commins Bennett; a son, Bill; daughters Jean and Nancy; a sister; and five grandchildren.

- Associated Press