In this paper, a new method to detect small targets in infrared images is presented. The proposed method is based on the optical responses of infrared photodetectors, mainly point spread function (PSF). The effects of PSF on gray level and shape of the targets is used to distinguish the targets from background clutter. Also, a novel operator called Lapalacian of Point Spread Function (LoPSF) is proposed to identify location and size of the infrared targets. Since detecting the variable-size targets in infrared search and track systems (IRST) is of critical importance, unlike previous methods of target detection, the proposed method can robustly detect variable-size targets. The proposed algorithm is applied on a set of real infrared images which are captured under different conditions such as target size, target distance, background clutter etc.. Simulation results demonstrate the effectiveness and validity of the proposed method in comparison with the other conventional methods.