Structured illumination used for optical sectioning. The light source is masked by a regular grid, which casts a pattern of stripes on the in-focus object plane. Three images of the thick object are acquired, shifting the grid by one-third repeat between each acquisition. Each image is the sum of contributions from the in-focus plane, which is shadowed by sharply defined stripes, plus blurred out-of-focus planes without distinct stripes. The square root of the sum of the squared difference images removes the out-of-focus blurred light, giving the in-focus plane.