Paul Leon Hartman, a pioneering researcher and Cornell University professor emeritus, died at his home in Ithaca, N.Y., on May 20. He was 91. Hartman was one of the first to investigate the use of x-rays generated as a byproduct of high-energy electron accelerators. In the 1950s, he and colleague Diran Tomboulian measured the spectrum of electromagnetic radiation from an electron beam circulating in a synchrotron, work that was background to the eventual use of high-energy x-ray radiation to study such things as the molecular structure of proteins. This work is carried out in the Cornell High Energy Synchrotron Source (CHESS). Hartman also studied far-ultraviolet radiation, optics and solid-state physics.    . . .    Universal Display Corp., of Ewing, N.J., said it has developed a high-resolution, active matrix, transparent OLED (organic light-emitting diode) display using its phosphorescent OLED and transparent OLED technologies along with amorphous silicon backplane technology developed by Jin Jang, a professor at Kyung Hee University in Seoul, Korea. The company said the advance is a step toward using see-through displays in applications such as architectural vision glass, entertainment, medical and industrial products and in helmet shields for military, security, fire and rescue applications. It reported on the technology and demonstrated the 120 x 160 (QQVGA), 200 dots-per-inch monochrome diplay at the Society for Information Display International Symposium, Seminar & Exhibition, held last week in Boston.