Surface Accuracy for Optics

In the field of precision optics, whether you are a design or manufacturing engineer, a tremendous number of technical specifications exists that define optical performance. When designing components for imaging, sensing, or laser systems, understanding how optical specs are defined and interrelated is critical. Terms like waves, fringes, power, nanometers, and clear aperture often appear together, but they describe very different aspects of an optic’s functionality. The following describes how they compare and why they matter. 

At the core of optical performance is surface accuracy, which determines how closely an optic matches its intended shape. We previously covered flatness discussing RMS vs. Peak-to-Valley measurements, so if you are interested, read that blog here.  

“Waves” refer to deviations in surface form relative to a wavelength of light, typically using 632.8 nm (the wavelength of a helium-neon laser) as the reference. For example, a specification of λ/10 means the surface varies no more than one-tenth of that wavelength or 63.28 nm.

This unit is intuitive for optical engineers because it directly relates how light interacts with the surface—but it can feel abstract outside that context. 

“Fringes” are closely related to waves and come from interferometric testing. One fringe typically represents half a wavelength (λ/2) of deviation. When you see “2 fringes,” that equates to one full wave of surface error. 

Fringes are often used during manufacturing and inspection because they visually represent error patterns, making them practical for diagnosing issues in real time. 

For higher precision—or when clarity matters across disciplines—surface accuracy is often expressed in nanometers (nm). This removes ambiguity tied to wavelength assumptions.

While surface accuracy describes small-scale deviations, power refers to the overall curvature of the optic. Power is a measurement of curvature on the surface of an optic and differs from the radius of curvature in that it applies to the micro-scale deviation in the spherical shape of a lens. 

Opticians specify power in fringes and measure it using an interferometer. One fringe of power corresponds to a slight change in the radius of curvature across the surface.  

If you are not familiar with the principles of interferometry and how light is used to measure microscopic surface deviations in an optical component, please refer to our previous technical article on interferometry. 

Even if an optic has excellent surface irregularity (low wave error), excessive power can degrade system performance—especially in imaging systems where focus and distortion are critical. 

• Power = large-scale shape error
• Irregularity = small-scale surface error  


Both must be controlled independently. For more information on lens specifications and terms head to our blog series on lenses.  
 


Scratch and dig specifications as outlined under MIL-PRF-13830 define the allowable cosmetic imperfections on optical surfaces using two numbers (e.g., 80-50, 60-40, 40-20, 20-10, 10-5). The first number represents the allowable scratch designation, which is based on a standardized visual comparison method rather than a direct physical scratch width measurement. The second number specifies the maximum allowable dig diameter (such as pits, bubbles, or inclusions) in hundredths of a millimeter. For example, a 40-20 specification allows digs up to 0.20 mm in diameter. Scratch and dig are also defined under ISO10110, and you can learn more with the excellent resource available here. 

Optical scatter refers to the unintended redirection of light caused by microscopic surface imperfections, contamination, subsurface damage, or material inhomogeneities within an optical component. This occurs when light hits microscopic roughness and is deflected in unwanted directions, leading to a loss of signal strength or "stray light". Scatter is a critical performance metric because even extremely small surface irregularities can deflect light away from its intended path, reducing image quality, contrast, and overall system efficiency. It is often caused by defects like scratches and digs.

Reducing optical scatter requires tight process control throughout manufacturing, including careful material selection, optimized polishing techniques, and careful quality control. In many high-performance optical systems, achieving low scatter is just as important as maintaining transmitted wavefront accuracy or cosmetic surface quality.  

Clear aperture specifies the portion of the optic that meets all required specifications and is guaranteed to perform across that range. It is usually expressed as a percentage of the optic’s diameter (e.g., 90% clear aperture) or as a defined dimension. 

• Edge defects can occur in manufacturing
• Mounting hardware may obscure portions of the optic
• Performance is only guaranteed within the clear aperture 


Designers must ensure their system only uses this qualified region—otherwise, performance can degrade unexpectedly. 

Balancing these specs is where manufacturing expertise becomes critical. Tighter tolerances often increase cost and complexity, so aligning specifications with actual system requirements not over-specifying an important factor.  

 

Optical specifications are deeply interconnected, but each serves a distinct purpose in defining performance. In the end, the best optical component isn’t the one with the tightest parameters, it’s the one with the right specs for the application. Reach out to our sales representatives today to get started on your next optical need.