We propose to develop an inexpensive, yet very high quality detector system for protein crystallography, using CMOS (complementary metal-oxide-semiconductor) technology. We will base this effort on an existing CMOS imaging system for X rays: the "RadEye1" manufactured by Rad-icon Imaging Corporation. Although the RadEye1 is very good already, it isn't currently usable as a crystallographic detector. We plan to modify the RadEye1, by making it bigger, lower noise, and with higher readout precision than its current design. Detector systems built with this new CMOS device will be better than those now used by the structural molecular biology community, and will cost less. During Phase I of this proposed project, we will increase the active area of the existing RadEye module 4-fold, by doubling its pixel size from 485m to 965m. Full-scale (50mm x 100mm) prototypes will be fabricated and tested. Second, the RadEye electronics will be redesigned, specifically to incorporate 16-bit digitization and correlated double sampling. We will fabricate small (16pixel x16pixel) prototype devices with the redesigned readout, conduct appropriate engineering tests (gain, sensitivity, noise, linearity, spatial resolution, etc.), and verify that the device satisfies our design specifications. In Phase II, we plan to scale up the newly designed pixel architecture, featuring correlated double sampling and 16-bit readout, in the full-size module. We will then develop a complete detector system, incorporating 12 of these modules in a 2x6 array that covers an area about 20 cm x 30 cm. As part of the Phase II effort, we will develop the cooling system for the detector to reduce dark current to acceptable levels. [unreadable] [unreadable] [unreadable]