The Complete Guide to Anti-Reflective Coating on Watch Glass

Anti-reflective coating, abbreviated as AR coating, is a microscopically thin layer applied to the surface of a watch crystal. Its function is to reduce the amount of light reflected off the glass and increase the amount of light that passes through it. The practical result is a dial that reads clearly under bright lighting conditions rather than reflecting the environment back at the viewer.

Without AR coating, a sapphire crystal reflects approximately 8% of incident light per surface — meaning up to 16% of available light is reflected back rather than reaching the dial on an uncoated two-surface crystal. With a high-quality AR coating, that reflection can be reduced to under 1% per surface. The difference between a coated and uncoated crystal is immediately visible outdoors or under direct artificial light.

AR coatings are used across optical instruments of all types: camera lenses, eyeglasses, binoculars, telescopes, and microscopes all use the same fundamental principle. In the context of watches, the coating serves a dual purpose: practical legibility improvement and aesthetic enhancement, since an AR-coated crystal appears nearly invisible and allows the dial to present itself without the visual competition of reflections.

AR coatings work through destructive interference. The coating is applied at a specific thickness, typically one-quarter of the wavelength of visible light (approximately 100 to 150 nanometres). When light hits the coated surface, part of it reflects off the top of the coating and part reflects off the glass beneath. These two reflected beams travel slightly different distances and arrive back at the viewer out of phase with each other. Out-of-phase light waves cancel each other out, reducing the total reflection.

This is not a surface treatment that absorbs light — it is a physics effect that redirects reflected light energy so that it cancels itself. The same energy that would have created a visible reflection instead passes through the glass toward the dial.

Because the interference cancellation works most effectively for a specific wavelength range, AR coatings are designed to target the peak sensitivity range of human vision, approximately 550 nanometres, which corresponds to green-yellow light. This produces the characteristic faint blue or purple tint visible when looking at an AR-coated surface at an angle — a reliable quality indicator showing the coating is working correctly.

A watch crystal has two surfaces: the outer surface exposed to the environment and the inner surface facing the dial. AR coating can be applied to either or both.

Single-sided coating on the inner surface is the most common approach at the mid-range price tier. It reduces the most visually significant reflection — the one between the crystal and the dial — while leaving the outer surface uncoated, which offers better resistance to fingerprints and smudges.

Double-sided coating, applied to both surfaces, provides maximum reflection reduction. It is used by premium manufacturers and delivers noticeably better dial clarity. However, the outer AR coating is more susceptible to fingerprint marks and requires gentle cleaning with a soft cloth rather than anything abrasive.

Söner applies AR coating to all sapphire crystals across the range. The coating specification allows the dial to read clearly across the full range of lighting conditions the watch is likely to encounter in daily wear, from bright outdoor sunlight to indoor artificial light.

The application process takes place in a vacuum chamber using physical vapour deposition (PVD) — the same process used to apply PVD gold-tone coatings to watch cases. The crystal is cleaned to remove all contaminants, placed in the vacuum chamber, and heated to a controlled temperature. The coating material, typically magnesium fluoride or silicon dioxide, is vaporised and deposited onto the crystal surface at a precisely controlled thickness.

The thickness control is critical. A layer that is too thick or too thin will not achieve the correct destructive interference cancellation. Multiple layers of different materials are often applied in sequence, each targeting a slightly different wavelength range, to broaden the effective spectrum of reflection reduction across the full visible range rather than a single wavelength.

After deposition, the crystal is inspected for uniformity and then sealed with a protective topcoat. On the outer surface, this is often a hydrophobic or oleophobic layer that repels water and oils, making the crystal easier to clean and more resistant to fingerprint marks in daily use.

The AR coating is more delicate than the sapphire crystal beneath it. Sapphire is the second-hardest natural material after diamond and will not scratch in normal use. The AR coating, however, is a thin deposited layer and can be degraded by abrasive cleaning or chemical exposure over time. Gentle care preserves it indefinitely. For guidance on overall watch maintenance, see our guide to daily wear and care.