Spatial Cure-State Mapping by Raman and Infrared Spectroscopy

Raman and infrared spectroscopy can map the consumption of reactive groups in photopolymers, but a spectral map is not automatically a map of network state. Each pixel or depth point represents a finite sampling volume shaped by optics, refractive index, absorption, scattering, confocal response, surface contact and data reduction. Reactive-band loss may correlate with conversion while remaining sensitive to band overlap, orientation, baseline, photobleaching and the stability of the chosen internal reference. ATR-FTIR, transmission FTIR and confocal Raman therefore observe different volumes and cannot be merged without a transfer model. This review establishes a measurement framework for spatial cure-state mapping. It defines band-ratio conversion, calibration against independent chemistry, point-spread and depth-response characterisation, uncertainty propagation and rules for interpreting gradients. Confocal Raman is treated as a three-dimensional optical measurement whose apparent depth can be distorted by refractive-index mismatch; infrared methods are separated by transmission, reflection and evanescent-wave sampling. The review proposes an acquisition and validation sequence for surfaces, sections, layers and volumes, together with minimum reporting requirements. Conversion is kept distinct from gel fraction, crosslink density, modulus and residual extractables. The result is a method-specific chemical map with declared spatial resolution and calibration boundaries, not a universal cure-state image.