Subwavelength structure coating
Canon developed SWC (Subwavelength structure coating) for its EF lens system in order to reduce unwanted reflections. It was first introduced in December 2008 with the EF 24mm f1.4L II USM lens. We look at why this new coating was developed and how it works.
Taken from July-September 2009 issue of EOS magazine
Reflections occur when there is a large change in the refractive indices of the materials in a light wave’s path. It follows that if it were possible to vary gradually the refractive index of one material to the refractive index of the next material there would no sharp refractive index change to cause reflections.
Canon has developed subwavelength structure coating (SWC) on this premise. When used on lenses with large angles of incidence/exit (such as those with small radii of curvature), glare and ghosting, which would not be corrected with conventional coating technology, are significantly reduced.
SWC is a layer of material that varies its refractive index from 1.033 (air) to that of glass (approx. 1.6) and thereby minimises reflections (see illustration below).
The anti-reflective properties are not reliant on particular geometry and therefore are minimally affected by angle of incidence. Furthermore, because the coating removes the cause of a reflection rather than suppressing it, most of the light that hits the lens is transmitted, further improving lens performance.
Structure of SWC
In the 1960s it was discovered that a moth’s eye is extremely good at capturing available light. The surface of a moth’s eye is covered in nanometre scale undulations that function as a variable refractive index layer to minimise reflections and therefore maximise the amount of captured light.
The SWC takes this idea and refines it. Canon has developed a technique for forming microscopic wedge-shaped structures on a lens surface that are smaller than the wavelengths of visible light. There is no
single air-surface boundary, and so there is no sudden change in the refractive index. Less light is reflected, which means that more light is transmitted through the lens.
By arranging wedge-like structures of the lens with the points upwards (away from the glass), the refractive index gradually increases as you move from the tips of the wedges to the bases. This has the effect of channelling the light from the tips of the wedges down towards the lens.
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