Binary optics and their application to imagery Focusing optics

Binary optics are optical components encoded in phase or amplitude by a succession of patterns. Compared with conventional optical components, which use refraction or reflection to modify the direction of light rays, binary components exploit the phenomenon of diffraction, which calls on the wave aspect of light. For a long time, diffraction was considered a limitation (optical systems were said to be in the diffraction limit), but it has gradually been exploited in optical architectures. Initially, it was used to improve the performance of refractive optics combinations (e.g. to correct chromatism), then gradually, with the advent of digital sensors that enable images to be manipulated after acquisition, some research teams are simply considering replacing refractive and reflective optics with diffractive components, in order to create lightweight, inexpensive and compact systems.

Imaging using binary optics is particularly popular in the gamma-ray and X-ray fields, where the material is highly absorbent. This makes it tricky to create refractive lenses that require variations in optical thickness, or to use mirrors where the stacking of reflective dielectric layers can be problematic. The use of binary optics makes it possible to limit radiation absorption while providing a focusing function.

In this article, the binary optical components considered are the only optical "ingredients" for forming an image, with a focus on focusing binary optics whose imaging properties are close to those of conventional optics. The formalism of image formation will first be recalled to introduce the associated merit functions (aperture, transfer function, point spread function...). Several examples of binary focusing optics are then described. For each, the equations used to model them are described, along with examples of imaging applications.

Finally, a comparative table summarizing the different binary optics and their characteristics is presented.

The notion of binary optics is extended in the appendix to include multi-level optics that can be produced with a single photolithography mask, close relatives of the binary optics family.

This article is the first of a two-part series, dealing with conventional aspects of imaging, while the second [E 4 046] explores less conventional approaches based on so-called self-imaging binary optics.

A table of acronyms and notations is provided...