  This paper develops a hybrid-size meshing scheme for target-dependent imaging
in Chemical Species Tomography (CST). The traditional implementation of CST
generally places the target field in the central region of laser sensing, the
so-called Region of Interest (RoI), with uniform-size meshes. The centre of the
RoI locates at the midpoint between the laser emitters and receivers, while the
size of the RoI is empirically determined by the optical layout. A too small
RoI cannot make the most use of laser beams, while a too large one leads to
much severer rank deficiency in CST. To solve the above-mentioned issues, we
introduce hybrid-size meshing, for the first time, by reforming the density of
the pixels in the entire sensing region of CST. This development alleviates the
ill-posedness of the CST inverse problem by detailing the target flow field
with dense pixels in the RoI and fully considering the complete physical
absorption model with sparse pixels out of the RoI. The proposed scheme was
both numerically and experimentally validated using a CST sensor with 32 laser
beams using a variety of computational tomographic algorithms. The images
reconstructed using the hybrid-size meshing scheme show better accuracy and
finer profile of the target flow, compared with those reconstructed using the
traditionally uniform-size meshing. The proposed hybrid-size meshing scheme
significantly facilitates the industrial application of CST towards practical
combustors, in which the combustion zone is bypassed by cooling air. In these
scenarios, the proposed scheme can better characterise the combustion zone with
dense meshes, while maintaining the integrity of the physical model by
considering the absorption in the bypass air with sparse meshes.
