1.1 Optical definitions

At present, there is a certain class of microwave radars capable of imaging various types of extended targets. These are usually termed imaging radars. Before giving a definition of a microwave image , we should like to draw the reader's attention to two circumstances. First, a microwave image is always viewed by a radar operator in the visible range, while the imaging is performed in the microwave range. Second, this book considers radar imaging based on a combination of holographic and tomographic approaches. Therefore, we should first recall the basic concepts necessary for the description of imaging by conventional photographic and holographic devices in the visible spectral range.

Let us construct an image of an object (AB) formed by a thin lens (Fig. 1.1) ^[19]. The lens thickness can be neglected, and one can assume that the principal planes of the object AB and its image A B coincide and pass through the lens centre (line M ? N ?). The other designations are the focal lengths HF, HF ?, f, f ? and the distances x, x ? separating the object and its image from the respective focal points F and F ?.

Figure 1.1: The process of imaging by a thin lens

The straight line AA ? connecting the vertices of the object and the image passes through the centre of the lens H. If we draw an auxiliary ray AF intercepting the principal plane at...