- Virtual surfaces of four semi-arid soils (regs, crusts, playas and sands), used to predict their bidirectional reflectance in the optical domain, are discussed in the paper. These surfaces are defined by the horizontal and vertical semi-axes of equal-sized opaque spheroids simulating their particles or aggregates. The spheroids are regularly dispersed in a net of squares of a given side length, on a freely sloping plane. They are absorbed into the ground with their tops projecting to a given height above the slope. The material of the vertical structure is additionally defined by its refractive index. Virtual surfaces determined in this way were used as part of the input data to a geometrical model, which predicts the soil surface normalized reflectance NR, expressed as the ratio of the total radiance of the simulated surface viewed from the off-nadir direction to the radiance viewed from the nadir.