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Chapter 13:
Eight Single Optical Elements as Imaging Objectives
13.1 Introduction A single optical element is often adequate to meet the image quality expected, especially from an infrared system, if the field to be covered is moderate. To provide a reference for a quick evaluation and comparison, eight different singlet configurations are presented in the form of a summary matrix of the previous subjects covered. This matrix will indicate whether a single element is sufficient for the task at hand or if a multi-element objective is required. To make matters manageable and simple, it has been assumed that the object is located at infinity and the aperture stop is placed at the objective. To avoid unpleasant surprises, it is recommended to always check first the size of the diffraction blur formed by the element. This is especially important for applications in the infrared region where the wavelengths are five to ten times longer than in the visible spectrum. 13.2 Diffraction Limit The diameter of the diffraction blur, which contains 83.9% of the energy of an imaged object point, is B diffr =2.44λ(f∕#), where λ is the wavelength and f∕# the relative aperture or “speed” of the objective. This diffraction blur is also known as Airy disk, named after Lord Airy, the British mathematician and astronomer. 13.3 Eight Chosen Configurations The eight chosen single elements, as possible imaging objectives, are pictured in Fig. 13.1. Aspheric lens #1 has a concentric second surface with respect to the focal point. Therefore, rays entering the lens parallel to the optical axis are not refracted at that surface. Aspheric lens #2 has been shaped to eliminate spherical aberration and coma.
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