The present study considers the absorption field in an externally-irradiated bench-scale Solar Simulator Photo-CREC Water Reactor with 9.8 1 of irradiated volume. This photo reactor consists of an annular slurry region surrounded by four curved and equally spaced reflector units. Each reflector unit consists of a polished metal reflector surface containing of two 15W UVA lamps. Each pair of lamps can be independently turned "on" or "off", generating different absorption fields within the annular region. Irradiance measurements were obtained at different axial and angular locations and for different external irradiance conditions and photocatalyst loadings. Experimental irradiance data was compared to Monte Carlo (MC) simulations accounting for: (a) Lambertian emission at the lamp surface, (b) specular and ideally diffuse reflection, refraction and absorption at all interfaces and (c) wavelength specific absorption and scattering coefficients. This MC model includes a Henyey-Greenstein (H-G) phase function with a "g" scattering parameter of 0.68. This H-G phase function was first reported in Valades-Pclayo et al. (2014b), using symmetric irradiance and a smaller scale annular reactor unit. This fully predictive model shows good agreement with experimental irradiance data in an ample range of conditions studied in non symmetrically irradiated units. It is thus concluded that the proposed MC approach as implemented by Valades-Pclayo et al. (2014b) is a reliable predictive tool to scale-up externally and unevenly irradiated photoreactors, as is the case in solar irradiated units. (C) 2014 Elsevier Ltd. All rights reserved.