Optical Specifications Explained: Surface Quality, Flatness, and Coating—Which Matters Most?
optics news
7/5/20261 min read
When requesting a quotation for custom optical components, engineers often focus on three key specifications:
Surface Quality (e.g. 60-40, 40-20, 20-10)
Surface Flatness (e.g. λ/4, λ/10)
Optical Coating (AR, HR, BBAR, etc.)
But which one has the greatest impact on optical performance?
The answer depends on your application.
Surface Quality – Controls Scatter
Surface quality refers to scratches and digs left during manufacturing. Poor surface quality increases light scattering and can reduce laser damage resistance.
Typical applications:
60-40: General imaging and industrial optics
40-20: Precision imaging and standard laser systems
20-10 or better: High-power laser and scientific applications
Choosing tighter scratch-dig specifications than necessary can increase manufacturing cost without improving system performance.
Surface Flatness – Controls Wavefront Accuracy
Flatness measures how closely an optical surface matches an ideal plane.
For many imaging systems, λ/4 provides excellent performance. More demanding applications such as interferometry or precision laser systems may require λ/10 or better.
Higher flatness improves wavefront quality but also increases manufacturing complexity and inspection requirements.
Optical Coating – Controls Light Efficiency
Even a perfectly polished optic can lose performance without the right coating.
A properly designed AR coating reduces reflection, increases transmission, and improves image contrast or laser efficiency. However, coating selection should always match the operating wavelength, angle of incidence, and substrate material.
Which Specification Should You Prioritize?
There is no universal answer.
Concerned about stray light? Focus on surface quality.
Need high wavefront accuracy? Focus on flatness.
Looking for maximum transmission? Focus on coating design.
The best optical component is not the one with the highest specifications—it is the one with the right specifications for the application.
At Positive Optics, we help engineers optimize optical specifications based on performance requirements rather than unnecessary cost, supporting custom lenses, windows, mirrors, prisms, and optical filters.