Superchromatic null lens for chromatic depth imaging

Chromatic depth imaging (CDI) is a depth-sensitive imaging mode, performed in a high level of chromatic aberration (CA) by which the imaging system produces wavelength-dependent foci along the depth. The CDI system demands a high-CA objective lens but with a minimal level of monochromatic aberrations for the best image quality. Here, we present a method of obtaining a suitable high-CA objective by utilizing a dispersive null lens of virtually zero refractive power. The so-called <i>superchromatic</i> null lens consists of cemented multiple elements in alternation of dispersion characteristics. Our superchromat is a compact component that can be easily added to a conventional imaging system for the implementation of CDI. With a low-cost RGB color image sensor, depth-sensitive imaging capabilities are easily obtained from the RGB image channels by the characteristic focal differences produced by our superchromat. In our study, an optimized lens design of our superchromat was found for boosted CA and suppressed overall aberration. Its imaging performance was validated through computer modeling and experimental evaluations. Finally, by attaching our superchromat on the objective in practice, an ordinary photographic camera and a conventional phase-contrast microscope were successfully reconfigured for extended depth of field and depth sensing. This report includes the basic theory, design optimization, and the performance evaluation of our superchromat, as well as the application techniques for cost-effective CDI implementations.