Reproducibility limits of working-curve characterisation in vat photopolymerisation: a methodological perspective

The working-curve measurement, introduced by Jacobs in 1992, remains the foundational characterisation of resin behaviour in vat photopolymerisation additive manufacturing. The two fit parameters of the Jacobs model - the depth of light penetration Dp and the critical exposure for gelation Ec - are routinely reported on commercial datasheets and in the peerreviewed literature. Recent interlaboratory studies coordinated by the National Institute of Standards and Technology indicate that these parameters are reported with substantial dispersion across laboratories examining the same resin: a 2024 study reported Dp varying by up to a factor of seven and Ec varying by up to a factor of seventy across twenty-four participating laboratories, and a follow-up study under controlled bandpass-filtered light sources reduced this dispersion substantially. A parallel publication extended the Jacobs model analytically to broadband sources, and separate single-laboratory studies in Progress in Additive Manufacturing report intensity-dependent working-curve fit parameters. These observations support the reading that Dp and Ec, as currently measured, are coupled to the spectral characteristics of the light source, the irradiance at which the measurement is performed and the procedural choices of the measuring laboratory, rather than behaving as material constants in the strict sense. This Perspective synthesises the published evidence on working-curve variability, enumerates the methodological dimensions implicated and discusses reporting considerations that the available data support. The synthesis is restricted to vat photopolymerisation processes that share the Jacobs framework and does not propose a new standard or protocol.