Last updated: 2026-08-06 6 min read

How a lamp is modeled has a major effect on the accuracy of a UV fluence calculation. UVSim3D adopts MSSS (Multiple Segment Source Summation), proposed by Liu (2004), as its default model, and also supports Bolton (2003)'s MPSS (Multiple Point Source Summation). This document summarizes how both models validate against experimental data published in the literature. For validation of the underlying numerical method (DOM) against the radiative transfer equation itself, see "DOM model validation".

Model overview

At wavelengths below 300 nm, absorption dominates the medium's behavior and scattering/emission are nearly negligible, so the radiative transfer equation (RTE) simplifies to:

dIλds=−κλIλ\frac{dI_\lambda}{ds} = -\kappa_\lambda I_\lambda

Integrating this gives the Beer-Lambert law, a special solution for an absorption-dominant medium:

Iλ(s)=I0,λ e−κλsI_\lambda(s) = I_{0,\lambda} \, e^{-\kappa_\lambda s}

A lamp's contribution is then computed by combining this with refraction at the lamp → quartz sleeve → water interfaces (Snell's law), reflection (Fresnel's law), and distance-based attenuation (the inverse square law). The two models differ in how the lamp itself is represented as a light source.

  • MPSS: models the lamp as the sum of several discrete point sources. Simple to compute, but tends to over-predict relative to measured values near the lamp.
  • MSSS: models the lamp as a segment (line) source, which represents refraction and reflection closer to reality. It has been recognized as a validated standard model in UV reactor design for over 20 years.

UV disinfection pilot reactor

Reference: Wols, B.A., et al. "Comparison of CFD, Biodosimetry and Lagrangian Actinometry to assess UV reactor performance." Delft University of Technology, 2012.

Test condition: 4 low-pressure UV lamps (Hereaus NNI125 84KL); fluence rate distribution measured across the reactor cross-section at three water UV transmittance (UVT) levels — 72.5%, 79%, and 87%.

Pilot reactor cross-section fluence distribution (254nm)

Cross-section of the 4 lamps (white circles) and the measurement line (1→4). Fluence rate rises locally where adjacent lamps overlap

Cross Line 1-4: simulation vs. experiment

At all three UVT conditions (72/79/87%), the MSSS simulation (solid lines) tracks the experimental values (circles/squares/triangles) closely

Result: including the pattern of elevated fluence rate at the reactor center from overlapping adjacent lamps, all three UVT conditions were reproduced within an average error of 5% or less versus experiment. The attenuation effect from differing UVT levels is also predicted reliably.

Conclusion

  • Beer-Lambert-based MSSS provides sufficient accuracy (average error of 5% or less against literature) for the design stage of absorption-dominant UV reactors, while running roughly 10-15x faster than RTE-based models.
  • MPSS tends to over-predict near the lamp (see the MPSS/MSSS validation document), so MSSS should be the default for design and optimization work. MPSS is the one that conserves absolute radiated power, however.
  • Further reading: MSSS 모델을 활용한 자외선 방사 시뮬레이션 - 상세 분석 (CLEW blog, Korean)