  The construction of low mass vertex detectors with a high level of system
integration is of great interest for next generation collider experiments.
Radiation length images with a sufficient spatial resolution can be used to
measure and disentangle complex radiation length $X$/$X_0$ profiles and
contribute to the understanding of vertex detector systems. Test beam
experiments with multi GeV particle beams and high-resolution tracking
telescopes provide an opportunity to obtain precise 2D images of the radiation
length of thin planar objects. At the heart of the $X$/$X_0$ imaging is a
spatially resolved measurement of the scattering angles of particles traversing
the object under study. The main challenges are the alignment of the reference
telescope and the calibration of its angular resolution. In order to
demonstrate the capabilities of $X$/$X_0$ imaging, a test beam experiment has
been conducted. The devices under test were two mechanical prototype modules of
the Belle II vertex detector. A data sample of 100 million tracks at $4\,
\mathrm{GeV}$ has been collected, which is sufficient to resolve complex
material profiles on the $30\,\mu$m scale.
