A new method is described for measuring the exposure threshold of laser thermal imaging materials exposed by pulses from a laser with a Gaussian radial spatial profile. This method, termed the maximum spot method, involves placing the material in the focused beam of a repetitively pulsed laser and exposing a series of spots of different sizes by moving the material through and beyond the beam focus. The exposure threshold can be deduced by knowing the radius of the largest exposed spot and the pulse energy. The method is demonstrated on a laser ablation transfer film (Lasermask™) and a direct imaging film developed by Presstek, Inc., for computer-to-plate imaging applications. It is shown that the new method, which is convenient and quick, gives the same results with fewer sources of experimental errors as conventional threshold measurement methods. The convenience of the new method permits systematic studies of the dependence of exposure properties on material properties or laser imaging conditions. As an example, the imaging threshold of Lasermask is measured as a function of laser pulse duration from 10⁻¹² to 10⁻⁴ s.