To do this Levy adapted a technique used by solar astronomers in which a black disk is placed in the focal plane of the telescope. The size of the disk is chosen so that it exactly matches the Sun's image in the focal plane, allowing astronomers to observe the delicate structure of the corona. Levy hit upon the idea of splitting a mirror or visor into an array of optical components. He then placed a layer of a photosensitive material, such as indium tin oxide, in the focal plane of each component. As light intensity increases, the layer becomes optically thick. However, only those segments being illuminated by light from bright objects get dark, and the rest of the image can pass through the array. Such a device could have applications in rear-view mirrors, or cars, or in binoculars.