1. Semiconductor Electronics Division, National Institute of Standards and Technology, 100 Bureau Drive, MS 8120, Gaithersburg, Maryland 20899-8120, National Eye Institute, National Institutes of Health, 31 Center Drive, MS 2510, Bethesda, Maryland 20892-2510, and Analytical Chemistry Division, National Institute of Standards and Technology, 100 Bureau Drive, MS 8394, Gaithersburg, Maryland 20899-8394