Objects with low refractive-index contrast are tricky to image under a microscope. They barely alter light passing through them, which makes them nearly invisible in bright-field images. A better approach for such objects is dark-field microscopy. In the technique, a hollow cone of light illuminates a sample. The cone’s inner angle is so large that photons miss the objective lens unless they’re refracted by the object. The result is a bright shape on a dark background—the opposite of a typical microscope image.
Dark-field microscopes need specialized condensers or objectives to shape their light cones. Those elements can be bulky and expensive, and they sometimes compromise the resolution of the final image. Now Cécile Chazot and her coworkers at MIT have developed a substrate that can do the same job with far less equipment. The substrate supports the sample being imaged and generates the illuminating light cone by itself. A normal bright-field microscope collects the light scattered by the sample and forms an image.
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