## 1 Introduction  
Any task--in which the mechanical components are used--brings its own dynamic signature, in terms of specific spectrum of loadings, the latter being external--as lumped forces or distributed pressures--or internal--as the result of motion laws or dynamics with accelerations and inertia forces. Therefore, it can be said that the specific duty a component, a mechanism or a structure, do fulfil brings a task-dependent excitation, characterised by its specific spectrum and location of energy injection. But, also, that the expected life of each mechanical part depends on both dynamic signature and location of loadings, together with the structural dynamics of the component that give responses to the specific excitation. Structural Health Monitoring (SHM) needs therefore to follow both the structural dynamics and the excitation, as they can change in time and can bring to new expected lives. In this paper, the structural dynamics is explored in one of the most adherent-to-reality approaches that is currently available, especially for lightweight structures, which starts from the broad frequency band optical full-field measurements. In this way, the experiment-based investigation deals exactly with the realisation of the structure, with the specific manufacturing and mounting conditions, with the materials and damping properties and broad band loading levels, while the above properties are hardly modellable in a numerical tool such as a Finite Element Model (FEM), also when properly tuned as in [1; 2; 3], but still with approximation residuals. Instead of running any Experimental Modal Analysis (EMA) as in [4; 5] - also in its Experimental Full-Field Modal Analysis (EFFMA) extension in [3] - the experiment-based _receptances_ - or _displacement_ over _force FRFs_ - are used to avoid any modal identification uncertainty or modal base truncation errors. Thus, _full-field receptances_ - here obtained from SLDV-based _mobilities_, or _velocity_ over _force FRFs_ - retain the best description of the actual operative deformation shapes due to the structural dynamics, especially in the spatial domain, later used to derive the dynamic strains and feed the fatigue life estimations with increased spatial sampling and detail. Due to their intrinsic nature, _full-field receptances_ may easily present different frequency-domain relations between response displacements and excitations, depending on the energy injection locations. In particular, a _complex-valued_ representation of the structural dynamics is ideal to retain all the real characteristics of the test. To exploit such a refined structural dynamics, also the excitation needs to be of a _complex-valued_ nature, as it easily comes from the signal processing of any force cell data in a test: a broad frequency band complete spectrum, with _complex-valued_ parts, to retain all the _complex amplitude_ and _phase_ relations in the frequency range of interest, without simplifications nor assumptions. Already introduced with the aid of _image_-based _receptances_1, here the approach exploits instead the _non-synchronous scanning_ of SLDV to maintain the following advantages, particularly relevant for the structural dynamics of lightweight components: (i) the real structural dynamics comes directly from testing, without the need of any tuning of the numerical modelling technique, e.g. FEM; (ii) the spatially dense grid of measurements overtakes the effort of finding the best location of traditional sensors, e.g. strain gauges; (iii) the measurements are contactless, without any distorting inertial contribution from attached sensors and their cabling. The expected task-oriented fatigue-life mapping, coming from SLDV-based _full-field receptances_, retains therefore the characteristic of the specific realisation of the component under test with good detail in a broad frequency band and spatial domain.