Anti-reflective treatment

Anti-reflective treatment is not an aesthetic optional extra: it is a physical response to a physical problem. Sapphire crystal, extremely hard and transparent, has a high refractive index — about 1.77 — which means that without intervention it reflects up to 4% of light at each surface. On a crystal with two faces, you reach 8% loss in readability, visible and annoying.

The operating principle is destructive interference. Extremely thin layers of metal oxides are deposited — magnesium, aluminum, zirconium, depending on the recipe — with thicknesses calibrated to fractions of the wavelength of visible light. Each layer reflects a small amount of light; if the thicknesses are right, the reflections cancel each other out. The result is a crystal that seems almost not to be there: the dial appears in the foreground, sharp.

The number of layers matters, but it is not the only parameter. Three well-calibrated layers can outperform seven poorly executed ones. That said, the specifications circulating — Clemence Munro at eight layers, Porsche Design at seven on both sides, Perception up to ten — still indicate real industrial commitment to reducing residual reflection. Economical single-layer coatings bring reflection to 2–3%; well-executed multi-layer treatments drop below 1%.

The position of the treatment changes everything. Coating only on the inside protects the dial from reflections of the surrounding environment but leaves the external surface exposed to front reflections. Double-sided — inside and outside — eliminates both sources of disturbance, but the external face is vulnerable to scratches; for this reason some manufacturers add a protective layer over the external coating, as in the case of the Porsche Design Chronograph 1. On domed crystals, the coating is more difficult to deposit uniformly: the curvature imposes varying angles during the evaporation or sputtering phase.

Aviation watches have made anti-reflective treatment almost mandatory: in the cockpit, grazing sunlight transforms an untreated crystal into an unusable mirror. On sapphire display casebacks, the treatment also serves to read the movement without having to tilt your wrist at every angle. From the bench, I recognize a poorly treated crystal by the colored residual glare — violet or green — that remains visible even during the day. A good coating leaves only a neutral shadow, almost imperceptible.