A tough challenge, but one that Greisukh et al. have fully realized. In this paper they present all plastic achromatic and apochromatic designs that use a diffractive microrelief element. Polymer lenses are used as they are an inexpensive material that can be formed into high performance, highly aspheric elements. However, plastics have limited material options, making color correction more difficult. Greisukh et al. have demonstrated in this paper how the use of a single diffractive surface element can color correct a micro-objective system with a minimal number of elements and polymer lens materials.
An achromatized system for the visible wavelengths is presented made of refractive elements that are all crown-like plastic. Achromatic systems bring two wavelengths into the same focus, typically the maximum and the minimum wavelength of the system with the goal of minimizing the focal shift of the wavelengths in between. An apochromatic system for the visible/NIR is presented that has some crown-like and some flint-like plastic elements. Apochromatic systems bring three wavelengths to the same focus. In this case the third wavelength was chosen in the NIR. This visible/NIR color-corrected system allows for the camera to maintain image quality for daylight and night-vision settings. Both systems use only a single diffraction surface element and meet qualifying standards for cell-phone objectives and CCTV cameras. The authors also detail design paths to improve spherochromatism (the variation of chromatic aberration with color of light) as well as reducing field angles in the image space to improve performance for photodetector arrays.
The incorporation of a single diffraction element into the design demonstrates a clear path to color correction and improved image quality in micro-objectives. The authors have succeeded in the presentation of optical design forms for a growing area that demands high quality and inexpensive solutions in a small package.
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