Researchers have endeavored over the past several years to overcome this zoom delay, which is primarily limited by the realities of mechanical actuation. Combining a set of “adaptive” lenses, or ones whose focal length can change by directly varying the lens shape or refractive index, can enable optical magnification without any moving parts. It turns out these flexible or liquid-filled lenses also offer zoom capabilities much faster than their mechanical counterparts.
Adaptive lenses rely upon techniques such as electrowetting, liquid crystal actuation, deformation of polymers, and pressure actuation – the technique applied in this work by Santiago et al. Perhaps not surprisingly, such formable lenses have a rich research history dating back to the mid-nineteenth century. Indeed, our own eye’s flexible lens may have been an inspiration to the early pioneers in this field. Unfortunately, since two or more adaptive lenses are needed to significantly magnify an image, our eye’s single flexible lens cannot help us “zoom in”. Instead, a slight bend to its shape will just help us accommodate our focus.
Luckily, adaptive lens technology does not face the same biological limitations. Replacing a camera’s traditional solid lenses with multiple adaptive lenses will allow motion-free magnification adjustment, but only after careful system design. A large amount of modeling is required to understand the interactions between each lens’s changing volume and its radius of curvature, which primarily defines its tunable focal length. These models must be accurate at micrometer scales for clear image formation across a range of zoom settings and colors. While prior work has considered this problem in part, it has yet to extend lens design to the case of a robust, wide-aperture achromatic doublet — one of the most important lens components in a high performance imaging system.
Here, Santiago and team have designed, constructed and tested an adaptive achromatic doublet lens comprised of three flexible membranes and two different fluids. They tune their lens focus by changing each fluid’s pressure via an external actuator. This pressure differential alters the enclosed fluid volume within each membrane, subsequently changing the doublet’s radius of curvatures to define a wavelength-dependent focal length. Their “doublet” design, while clearly more complex than constructing a single flexible volume, helps remove chromatic aberration across the visible spectrum by minimizing the focal length’s color dependence.
More importantly, this work’s modeling pipeline helps to significantly reduce the complexity of how light interacts with multiple flexible volumes. Future researchers who might struggle with the intricacies of multi-adaptive lens design can directly benefit from this well-planned study. A finite-element model combined with straightforward calculation and Zemax ray tracing quickly yields a critical set of parameters — the thickness of the three flexible PDMS membranes used to encapsulate the two liquid volumes — for a given lens system. Armed with such a deterministic model, the team successfully constructs a lens that closely matches their Zemax design across a range of focal lengths (i.e., liquid pressures).
Santiago et al. demonstrate successful achromatization across the visible band (i.e., approximate focal distance matching for blue and red light) for a lens that can shift its effective focal length from 50 to 200 millimeters. This significant focal range should enable plenty of optical zoom for a first-generation adaptive zoom camera and will hopefully cut down those precious seconds between taking a wide-field and close-up snapshot.
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