  Ghost-imaging experiments correlate the outputs from two photodetectors: a
high spatial-resolution (scanning pinhole or CCD camera) detector that measures
a field which has not interacted with the object to be imaged, and a bucket
(single-pixel) detector that collects a field that has interacted with the
object. We describe a computational ghost-imaging arrangement that uses only a
single-pixel detector. This configuration affords background-free imagery in
the narrowband limit and a 3D sectioning capability. It clearly indicates the
classical nature of ghost-image formation.
