Virtual surfaces of four semi-arid soils (regs, crusts, playas and sands), used to predict their bidirectional re ectance in the optical domain, are discussed in the paper. These surfaces are de ned 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

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... t above the slope. The material of the vertical structure is additionally de ned 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 re ectance NR, expressed as the ratio of the total radiance of the simulated surface viewed from the oV-nadir direction to the radiance viewed from the nadir.