The rectangular cold shield effect in 180 element HgCdTe linear array detector was analyzed. The field of view ( FOV ) for each element of a 180 element linear array and its background limited performance under 300K background radiation were calculated as well. The actual performances of each element for the 180 element linear array detector were compared with the theoretical calculations. The results show that the performances of the 180 element long wavelength infrared linear array detector developed by the authors have approached the theoretical background limitation at room temperature.