Grayscale and Spatial Dose Modulation in Vat Photopolymerisation: Local Conversion, Surface Shape and Property Gradients

Grayscale vat photopolymerisation uses spatially varied exposure to produce an intended field of conversion, surface height or material response. The digital image is only the first link in a chain. Grayscale code, irradiance, exposure dose, local reaction history, conversion, network state and mechanical property are not interchangeable. Projector nonlinearity, oxygen inhibition, absorption, scattering, diffusion, heat, accumulated layer exposure, shrinkage and post-treatment can make the response nonlocal, nonlinear and history dependent. Equal nominal dose delivered through different intensity-time pairs may not be equivalent, and a conversion gradient does not uniquely establish a modulus, toughness or residual-monomer gradient. This review establishes a qualification framework for intentional spatial dose modulation in vat photopolymerisation. It requires code-to-irradiance calibration, reciprocity testing, spatially resolved conversion and property measurements, separation of intended gradients from unintended geometry bias, and prospective validation on withheld spatial patterns. Direct grayscale-DLP studies demonstrate functionally graded materials, reaction-diffusion behaviour, polymer blends, organogel-to-thermoset transitions, shape-memory structures, stress-oriented property design and machine-learning-assisted optimisation. These examples establish feasibility but remain bound to their chemistries, post-treatments, apparatus and load cases. Dose-zoning studies further show that cross-layer accumulation can govern surface and internal features. No universal grayscale-to-dose curve, conversion-to-property map, minimum gradient length, modulus range, surface-height response or reciprocity rule follows. Qualification must preserve the complete material, optical, thermal and spatial state.