  The continuing improvement in quantum efficiency (above 90% for single
visible photons), reduction in noise (below 1 electron per pixel), and shrink
in pixel pitch (less than 1 micron) motivate billion-pixel X-ray cameras
(BiPC-X) based on commercial CMOS imaging sensors. We describe BiPC-X designs
and prototype construction based on flexible tiling of commercial CMOS imaging
sensors with millions of pixels. Device models are given for direct detection
of low energy X-rays ($<$ 10 keV) and indirect detection of higher energies
using scintillators. Modified Birks's law is proposed for light-yield
nonproportionality in scintillators as a function of X-ray energy. Single X-ray
sensitivity and spatial resolution have been validated experimentally using
laboratory X-ray source and the Argonne Advanced Photon Source. Possible
applications include wide field-of-view (FOV) or large X-ray aperture
measurements in high-temperature plasmas, the state-of-the-art synchrotron,
X-ray Free Electron Laser (XFEL), and pulsed power facilities.
