The period of operation immediately following laser switch-on, while a working population inversion is established and thermal gradients are both created and dissipated, may be defined as the transient regime. During this period the distribution of energy within a laser beam is typically in a state of flux. Depending on the device, a steady-state laser output can take several minutes to establish. Military lasers installed on fast strike aircraft are typically operated for short periods of up a few seconds duration, which allows almost no time at all for the laser output to settle. However, knowledge of the beam output characteristics during transient regime operation can be important for laser safety considerations. A regression model based on the work of Cutolo, et al, (1992) enables a measured laser output to be expressed as a linear combination of irradiance patterns based on Hermite-Gaussian transverse electromagnetic modes. The results and effectiveness of such a linear regression model when applied to a production-standard laser output are discussed. The implications of this prototype model, together with further refinement possibilities including the complementary development of a non-linear modelling approach, are also discussed.