Detection systems used for computerized tomography are often approximated by line integrals, despite having non-negligible beam widths due to a finite detector size and a finite acceptance angle. Ways to take into account these beam widths in algorithms for two-dimensional straight-line emission tomography are discussed. It is shown that the full three-dimensional imaging properties of the detection system, including filter functions, can be described in projection space. The relationships with the geometric matrix and étendue, and the application to the Cormack and Iterative Projection-space Reconstruction methods are discussed. Two techniques to compensate for most of the beam-width effects have been developed, which can be combined with many tomography algorithms. The two techniques are demonstrated to improve the quality of tomographic reconstructions of measurements by the bolometer tomography system on the JET (Joint European Torus) tokamak. The strengths and limitations of the methods are discussed.