Long-Wavelength and Superlinear Photoinitiation in Vat Photopolymerisation: Action Spectra, Photon Upconversion, Curing Contrast and Qualification
Segurola, Juan
2026-08-27 · Informe · Versión 1.1
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Long-wavelength photoinitiation is often presented as a direct route to deeper penetration and lower scattering. That interpretation is incomplete. A visible or red photosystem may be a conventional one-photon Type I or Type II system whose reaction remains sublinear with irradiance, an upconversion-particle system that locally emits shorter-wavelength photons, or a triplet-fusion system whose active-state generation is superlinear over a defined intensity range. These mechanisms require different evidence. The source spectrum must be combined with dynamic absorption, absorbed- photon rate, excited-state transfer, active-species yield, oxygen or inhibitor history, local intensity exponent, conversion and feature response. This review distinguishes direct visible sensitisation, photoredox coinitiator systems, lanthanide- upconversion routes, homomolecular triplet fusion and triplet-triplet-annihilation photopolymerisation. Source-specific demonstrations range from visible Type II hydrogel printing and low-intensity red DLP to NIR-assisted polymerisation and TTAP at continuous-wave intensities. A qualification matrix is proposed around an operational action spectrum, an effective intensity exponent and an application-specific curing-contrast test. The central conclusion is that wavelength extension and superlinearity are independent design variables: longer wavelength can improve optical access without improving confinement, while a superlinear photosystem can improve contrast only within the irradiance, oxygen and concentration regime in which the nonlinearity is demonstrated.