Point Spread Function Analysis

In the classic optical imaging theory of a lens [3.1, 3.2], discussions on imaging performance of a lens are usually limited to thin objects because conventional optical microscopy provides a two-dimensional (2-D) image of a thin sample. However, confocal microscopy exhibits an optical sectioning property [3.3] which allows one to perform three-dimensional (3-D) imaging of a sample with depth structures. The 3-D imaging property, or the 3-D light distribution near the focus, of an objective lens is also important in laser trapping technology [3.4]. All of these new developments require a better understanding of the performance of a lens along the axial direction. This chapter is to study diffraction properties of a single lens in the focal region under the paraxial approximation introduced in Section 2.4. A discussion on the same topic without this approximation can be found in Chapter 6. There are two ways for analysing the imaging performance of a lens, the point spread function method and the transfer function method. The former, relatively simple in terms of the mathematical skills involved, is based on the image of a single point object and is the topic of the current chapter, while a detailed discussion based on the transfer function method will be given in the next chapter.