In Chapter 4 the diffractive lens, including a simple hybrid achromat, was discussed in some detail. In this chapter, we extend our discussion of lens design by introducing the wavefront aberration polynomial and the Seidel coefficients. Although it is not our intention to sneak a lens design text into this book, we do have to provide sufficient information on the expression of aberrations and their correction so that you can gain an appreciation of the power of diffractive surfaces in lens design and the additional degrees of freedom that they provide. The limitations of these approaches are also discussed. The chapter begins with an introduction to the aberration polynomial and its expression for a thin lens. Then pure diffractive designs, including superzone versions, are discussed. This is followed by a number of examples of hybrid lenses. Diffractive surfaces can also be used in the design of systems with reduced thermal sensitivity. The chapter concludes with a short discussion of applications that use microlenses.
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