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The horizontal axis represents y sub a, and the vertical axis represents x sub a on the aperture plane A. Three equally spaced arrows represent the aperture illumination behind the aperture plane. A rectangular aperture on the aperture plane has length l sub y and width l sub x. A small rectangle of length d y sub a and width d x sub a is inscribed, inside the plane A. An observation plane O is placed, at a distance z from the plane A. The planes A and O are parallel to each other. The horizontal axis and the vertical axis of the plane O represent the y-axis and the x-axis respectively. A point Q is located, at the plane O which shows radiation of the rectangular area of length d x and width d y. The x-axis makes an angle phi with the line that connects the origin and the point Q. The distance z makes an angle theta with the line that joins the origin of the plane A and the point Q.