  Ghost imaging (GI) is an unconventional imaging method that retrieves the
image of an object by correlating a series of known illumination patterns with
the total reflected (or transmitted) intensity. We here demonstrate a scheme
which can remove the basic requirement of knowing the incident patterns on the
object, enabling GI to non-invasively image objects through turbid media. As an
experimental proof, we project a set of patterns towards an object hidden
inside turbid media that scramble the illumination, making the patterns falling
on the object completely unknown. We theoretically prove that the spatial
frequency of the object is preserved in the measurement of GI, even though the
spatial information of both the object and the illumination is lost. The image
is then reconstructed with phase retrieval algorithms.
