l elements (DOEs) offer many degrees of freedom in their design, and by using modern lithographic fabrication techniques, it is as expensive to fabricate DOEs with strong aspheric phase functions as it is to fabricate simple Fresnel-band lenses with quadratic phase functions, and so DOEs can be used in some cases instead of more expensive aspheric surfaces. In addition, photolithographic fabrication techniques guarantee a fairly good accuracy about the DOE phase function compared to the more difficult fabrication of well-defined aspheric
surfaces. Diffractive optics with strong dispersion of the opposite sign to the refractive element allow correction of chromatic aberrations, and in this case DOE also provides an additional degree of freedom in the design, since the aspherical term of the phase function also corrects the monochromatic aberrations of the system as long as these are much smaller than the quadratic term of the phase function that corrects for chromatic aberrations.
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