Why Tolerances Are a Design Decision, Not Just a Spec Sheet Item
Every optical drawing we receive tells a story. Sometimes the story is precise and well-considered — someone clearly thought through what each tolerance actually needs to be. Other times, every number is set to “±0.01mm” across the board, which tells us the drafter either copied a template or did not have time to think it through.
That matters because tolerances drive cost in ways that are not always obvious. A 40-20 surface quality costs less than 20-10. A λ/4 surface irregularity is easier to hold than λ/10. Centering at 3 arc minutes is routine; 1 arc minute requires more care and more rejected parts. Tightening every number on a lens that will sit behind a field stop is spending money you do not need to spend.
This is not a criticism — it is just how it is. The goal here is to help you understand which tolerances matter for which situations, so you can make informed decisions when you specify your next order.
Diameter and Thickness: Where Tight Tolerances Are Usually Justified
For diameter and center thickness, you often do need tight tolerances — but probably not as tight as you think. Most precision lenses in industrial applications work well with +0.00/-0.10 mm diameter tolerance for diameters up to 50mm, and +0.00/-0.15 mm for larger lenses up to 100mm.
The real question is: what happens if the lens is 0.15mm oversized? If it mounts into a precision-holed housing with a light press fit, that matters. If it sits in a spring clip retainer with 0.5mm clearance, then ±0.2mm is perfectly fine and will cost less.
Thickness tolerances of ±0.05 mm are achievable on most spherical lenses without special processes. If you need ±0.01 mm, tell us — we can do it, but expect a price adjustment and potentially a longer lead time, since some lens centers do not hold ±0.01 mm through the full polishing cycle without in-process metrology.
Surface Irregularity: This One Deserves Real Attention
Surface irregularity — typically measured as peak-to-valley deviation from a perfect sphere at a reference wavelength — is one of the most consequential tolerances for optical performance. A lens with λ/4 irregularity will work fine in most imaging systems but will cause measurable wavefront error in interferometric or laser applications.
Here is a practical breakdown:
| Irregularity Grade | Typical Use Cases | What Happens If You Over-Spec |
|---|---|---|
| λ/2 PV | Illumination, beam guidance, low-power sensing | Unnecessary cost; standard production achieves this easily |
| λ/4 PV | General imaging, machine vision, many laser systems | Good balance for most commercial optics |
| λ/10 PV | Interferometry, precision laser focusing, advanced imaging | Noticeably higher cost; justify it before specifying |
| λ/20 PV | Interferometric cavities, metrology, high-power lasers | Special process control, inspection under interferometer required |
One practical note: when we quote λ/10, we measure it at 632.8nm with a Twyman-Green interferometer. If your laser runs at 1064nm, the effective wavefront error in your system is actually four times better in terms of waves — so a λ/10 @ 633nm lens is λ/40 effective at 1064nm. This is useful context when deciding whether you need to specify at a particular wavelength.
Surface Quality: Matching the Grade to the Intensity
Scratch-dig specifications (e.g., 60-40, 40-20, 20-10) are often overspecified for applications that do not actually require them. The reason people tend to over-specify is understandable — it feels safer to ask for better quality. But here is what actually happens:
In a high-power laser system, surface imperfections absorb energy. A scratch on a lens surface exposed to 10kW/cm² can cause localized heating, which changes the lens refractive index slightly, which distorts the beam, which may eventually crack the coating. At that power level, 40-20 is not acceptable; 20-10 or better is genuinely necessary.
For a lens in a machine vision camera, scratches on the surface scatter a tiny fraction of the visible light that passes through. At typical illumination levels, you will never see the effect. Specifying 20-10 here is not safer — it is just more expensive for no gain.
When in doubt, ask your lens manufacturer what grade they would recommend for your specific application and power density. We are usually happy to advise, and we would rather help you spec it right the first time than have you pay for something you did not need.
Centering and Wedge: Often Overlooked, Sometimes Critical
Lens centering — the deviation between the geometric axis (defined by the outer diameter) and the optical axis (defined by the surface geometry) — is expressed in arc minutes or sometimes arc seconds. Most commercial lenses are centered to 3 arc minutes, which is sufficient for most lens groups and barrel assemblies.
If you are building a lens group where the beam must pass through multiple lenses without tilt, or where the exit pupil position is critical, then 1 arc minute or better matters. We have also seen centering problems show up as ghost images in systems with multiple closely-spaced lenses — not because any individual lens is badly centered, but because the centering errors stack.
For aspheric lenses and steep curved surfaces, centering becomes harder to control and is typically quoted at 3 arc minutes unless specifically addressed in the manufacturing process. If your application requires tight centering on an aspheric lens, discuss it with us before placing the order.
Coating Tolerances: Angle of Incidence and Spectral Shift
Anti-reflection coatings are designed for a specific angle of incidence (AOI). At normal incidence (0°), a BBAR coating might peak at 99.2% transmittance at 550nm. At 15° AOI, the peak shifts to shorter wavelengths by roughly 15–20nm depending on the design. At 30°, the shift is more pronounced and the coating may begin to show polarization dependence.
If your lens will be used at a significant off-axis angle — in a wide-angle lens, a periscope, or a folded optical path — tell us. We can design the coating to peak at a shifted wavelength, or use a design that is more angle-tolerant. Specifying “BBAR 400-700nm” without mentioning angle is fine for near-normal incidence but may not give you the performance you expect if the beam hits the surface at 20° or more.
When to Ask Your Manufacturer
Specifying optics is a collaborative process, especially for custom parts. The drawings we receive that work best usually include not just the tolerances but also a brief description of the end application — something like “this lens is the second element in a 3-element collimation group, working distance 150mm, beam diameter 8mm, 1064nm.” That context helps us flag potential issues before manufacturing starts.
For custom optics or high-volume production, we recommend discussing your drawing with our engineering team before committing to a firm order. We will often identify one or two tolerances that can be relaxed without affecting optical performance, which can meaningfully reduce unit cost.