Anti-Reflective Coatings: The 4% Per Surface Loss That Turns Images Milky

Every air-glass interface is a tiny mirror. Each uncoated glass surface reflects roughly 4% of incident light—the exact value varies with wavelength and incident angle, but 4% is a solid working estimate.

On a filter plate, that reflection creates two distinct problems. First, there's simple intensity loss: light that bounces off the surface never reaches your sensor. Second, and often more damaging, internal reflections between the filter's surfaces degrade image quality in ways that are hard to correct after the fact.

What Bad Reflections Look Like

The visible symptoms are distinctive. Contrast drops as scattered light fills in what should be dark areas. Images take on a "milky" quality, as if someone smeared a thin haze across the frame. In severe cases, ghost images appear—faint duplicates offset from the primary image, caused by multiple bounces inside the glass.

None of these problems are acceptable in machine vision, where image quality directly affects measurement accuracy and defect detection.

How AR Coatings Solve the Problem

Anti-reflective coatings are thin dielectric films, typically around λ/2 in thickness, engineered to cancel reflections through destructive interference. When light reflects from the top and bottom of the coating layer, the two reflections arrive out of phase and cancel each other.

Coatings can be single-layer or multi-layer. Single-layer coatings are effective over a narrow wavelength band and cost less. Multi-layer coatings extend the effective range across broader spectral regions, but the added complexity increases cost. Common coating materials include metal oxides and calcium fluoride (CaF₂).

Finding the Right Trade-Off

For many machine vision applications, a single-layer CaF₂ coating delivers a good balance of performance and cost. It significantly reduces reflection losses and image degradation without the expense of multi-layer broadband coatings.

If your system uses multiple wavelengths or covers a wide spectral range, multi-layer coatings may be worth the investment. The decision depends on how much reflection loss you can tolerate and whether ghost images would meaningfully affect your inspection results.

The Often-Overlooked Requirement

When the filter is the front element exposed to the factory environment, coatings serve double duty. Beyond their optical function, they must also protect the glass surface from the real world.

This means the coating should be nonabrasive—able to survive regular cleaning without scratching—and noncorroding, able to handle humidity, oils, and whatever industrial contaminants your particular environment produces.

In production settings, a filter that optically degrades after six months of exposure costs far more than a premium coating would have. Specify coatings that protect both your photons and your uptime.

This is part of KUPO's technical resource series on optical filters for machine vision. For custom filter solutions, contact KUPO CO. LTD.