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This paper aims to explore a new technique for structural damage identification using cubic spline interpolation. The method is based on the interpolation of modal rotations measured with shearography, making use of the analytical derivative of the spline to compute the modal curvature, which is known to be very sensitive to damage. As a means of reducing noise and measurement uncertainty propagation to a minimum, an expression for an optimal spatial sampling is derived. Furthermore, a baseline-free damage factor, allied with an optimal sampling, is also introduced. The proposed identification method is validated using experimental data of a beam. Using a damage localisation quality index, a comparison between the present method and one using finite differences is carried out, showing that the differentiation of spline interpolation leads to better damage identifications. The results obtained with the proposed approach show robustness and consistency in the localisations. Additionally, the hurdles of identifying small and multiple damage are tackled with the proposed method, yielding a good performance.