Searchlight beam test. The top left panel of the figure depicts the purpose of this test. A slanted beam, with direction n, enters a zero opacity box at its base, and emerges from it at the top. The objective is to compare the entering beam profile with the emerging one; both are theoretically the same. The bottom panels show the normalized specific intensity of the beam, as a function of positions x and y, at the bottom of the box (left panel), and at the top of the box (right panel). The top right panel displays sectional views of the beam along x-axis, at z = 0 and y = 3 (red curve), at z = 10 and y = 6.8 (green curve). The blue curve is identical to the the green one, but shifted so that its center fits the center of the red curve. See Sect. 8.1 for explanations.