  Digital holographic microscopy is an emerging potentially low-cost
alternative to conventional light microscopy for micro-object imaging on earth,
underwater and in space. Immediate access to micron-scale objects however
requires a well-balanced system design and sophisticated reconstruction
algorithms, that are commercially available, however not accessible
cost-efficiently. Here, we present an open-source implementation of a lens-less
digital inline holographic microscope platform, based on off-the-shelf optical,
electronic and mechanical components, costing less than $ 190. It employs a
Blu-Ray semiconductor-laser-pickup or a light-emitting-diode, a pinhole, a
3D-printed housing consisting of 3 parts and a single-board portable computer
and camera with an open-source implementation of the Fresnel-Kirchhoff routine.
We demonstrate 1.55 {\mu}m spatial resolution by laser-pickup and 3.91 {\mu}m
by the light-emitting-diode source. The housing and mechanical components are
3D printed. Both printer and reconstruction software source codes are open. The
light-weight microscope allows to image label-free micro-spheres of 6.5 {\mu}m
diameter, human red-blood-cells of about 8 {\mu}m diameter as well as
fast-growing plant Nicotiana-tabacum-BY-2 suspension cells with 50 {\mu}m
sizes. The imaging capability is validated by imaging-contrast quantification
involving a standardized test target. The presented 3D-printable portable
open-source platform represents a fully-open design, low-cost modular and
versatile imaging-solution for use in high- and low-resource areas of the
world.
