Light transmittance through sea ice is affected by surface cover and ice optical properties in the vicinity of the measurement. We present three-dimensional Monte Carlo simulations of light propagation in sea ice to derive upper bounds on the lateral spread of light. Our results give guidance on equipment design and on the possibility of using one-dimensional light transfer models to describe transmittance. Rules were derived for simple cases of optically homogeneous slabs. In the absence of absorption, 10% and 90% of the flux detected under optically thick, homogeneous ice are incident on the surface within a radius of less than 0.3 and 2.0 times the ice thickness, respectively. Any increase in optical thickness or absorption in the ice will reduce these radii. For example, the wavelength-dependent absorption of ice results in a 20% reduction at 700. nm. Optical anisotropy of the slab was also found to produce potentially significant spatial narrowing of the transmitted light field. In the case of direct sunlight, the photon path is displaced toward the sun relative to the location of the detector. This distortion can reach 1. m or more in optically thick, ponded ice but will be negligible under a surface scattering layer or snow cover. Case studies showed that transmittance of diffuse light in the vicinity of a semi-infinite surface obstruction could be approximated with exponential and error functions. An absorbing cylindrical perturbation of 0.05. m diameter in 1. m thick ice placed 1. m from the point of measurement will absorb less than 1% of the light otherwise registered by the detector. Many results for transmitted light were independent of the mean cosine of the scattering phase function. © 2011 Elsevier B.V..