Spatial resolution of hard-field tomography systems with severely limited data

The term ‘spatial resolution’ frequently appears in scientific literature from numerous research areas, where it is used to characterise the performance of an imaging system or tomographic modality. In this paper we will first investigate the current definitions and understanding of the term in different research areas and provide an interdisciplinary definition. Subsequently we will focus on spatial resolution in the case of in-cylinder near-infra red (N-IR) absorption chemical species tomography (CST) to visualise the distribution of hydrocarbons in a compression ignition engine. The functional stages of tomographic imaging are presented and examples of factors that contribute to the reduction of spatial resolution are given. An experimental method is described in detail to consistently measure the spatial resolution of the CST system that uses a sparse and irregular beam array. The method is based on tomographic reconstruction of an edge feature using a novel angular translation technique and the division of the reconstructed images into a matrix of sectors. Analysis of the collected data for each sector proceeds with plotting the edge spread function (ESF), followed by the line spread function (LSF) and finally the modulation transfer function (MTF). An amplitude threshold is set, and the highest occurring frequency is translated back into the spatial domain to yield a spatial resolution figure for each sector. We believe that the proposed experimental approach can be adapted for other systems that are characterised by a non-isotropic spatial resolution.