Crystal IS Inc., a Watervliet, N.Y., supplier of single-crystal aluminum nitride (AlN) substrates, announced it has received a $750K one-year funding extension from the Defense Advanced Research Projects Agency (DARPA), under its Semiconductor Ultraviolet Optical Sources and RF/Microwave/Millimeter-wave Technology programs, for the continued development of native AIN technology for use in ultraviolet emitters and radio frequency transistors.   . . .   Essex Corp. announced it completed the acquisition of Computer Science Innovations, of Melbourne, Fla., on April 30 (See Photonics.com news, 4/22/04   . . .   The International Society for Optical Engineering (SPIE) reports that a record 4,977 attended The Defense & Security Symposium 2004, held April 13-15 in Orlando, Fla. There were 3080 conference attendees, 812 exhibit-only visitors, 1085 sales reps from 289 exhibitors (up 43 percent) and 1820 technical presentations. Topics included infrared technology and applications, 3-D laser radar, unmanned ground vehicles, helmut-and head-mounted display, millimeter-wave imaging and biometric technology.