Large spot size laser vibrometry insensitivity occurs in 1-D vibrations only

An envelope of two approximations forms a plane wave coherent laser vibrometry calculation set that indicates mechanisms of measured and simulated spectral "reduction." Path length differences modulate large spot size continuous wave laser return leading to structural mode sensing (not a random signal issue). Calculations for sine swept and multi-modal approximations show vibrating rectangular plates constrained on all edges have return that varies substantially with low frequency vibration modes, providing modal recognition and ID not available with 1-D modes (strips or bars).