Surface Figure and Irregularity
Figure is usually stated in waves at 633 nm, with power and irregularity called out separately. A lambda/4 flat is routine for many shops. Lambda/10 and tighter require more careful polishing, controlled environment, better test optics and more iterations between polish and measurement.
Cost rises because each cycle consumes machine time and increases the chance of edge roll-off or a damaged surface. Ask whether the customer needs figure over the full diameter or only the clear aperture, since that choice changes what you can achieve and how you inspect.
- Check the test wavelength and whether figure is peak-to-valley or RMS
- Confirm whether power is included or excluded from irregularity
- Verify which reference flat or test plate is required
Scratch-Dig and Surface Quality
Scratch-dig under MIL-PRF-13830B is a visual comparison against standards, with numbers such as 80-50, 60-40, 40-20 or 20-10. Tighter grades increase inspection time, rework and rejection, especially on soft or hygroscopic materials. Laser optics may require much tighter surface quality than imaging optics, and the ISO 10110-7 method states imperfections differently.
Make sure the print says which standard applies and whether it covers the clear aperture only. Be careful with coated surfaces, since coating can reveal or add defects that polish inspection did not catch.
Centration, Wedge and Parallelism
For lenses, centration is the alignment of optical and mechanical axes, often expressed as wedge angle or beam deviation. Tight centration calls for edging on a centering machine, with measurement through autocollimator or similar equipment. For windows and plates, parallelism and wedge matter for fringes and beam steering.
These tolerances add operations rather than just polish time, and a missed spec can make a lens unusable after edging. Check whether the tolerance is achievable with your equipment or requires outside processing.
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Thickness, Diameter, Radius and Clear Aperture
Thickness tolerance affects how much stock remains after generating and polishing, and tight tolerances raise the number of measurement passes. Radius tolerance depends on test plates or interferometer measurement, and a custom radius may need new tooling. Diameter tolerance affects edging and mounting.
A large clear aperture relative to diameter leaves little room for bevels and edge defects. When a customer asks for tight values everywhere, ask which ones are functionally required. Many prints carry tolerances copied from a previous design, and a short conversation can save both sides a lot of cost.
Using This in Estimating
Build a simple tolerance-to-cost table from your own history: for each material and process, record the typical yield and hours at each figure and scratch-dig grade. Then your estimator can see at a glance what a tolerance change does. If your volume is mostly standard catalog optics with fixed grades, you may not need a table at all.
The table matters when custom prints vary widely and quotes depend on one person's experience.
Frequently asked questions
What does scratch-dig 60-40 mean?
It is a surface quality grade under MIL-PRF-13830B. The first number relates to the apparent width of the largest allowed scratch compared with a standard, and the second to the maximum dig diameter in hundredths of a millimeter. 60-40 is a commercial grade, while 20-10 is precision and much harder to produce.
Which optical tolerance is the most expensive?
It depends on the part, but tight surface figure combined with tight scratch-dig usually dominates because both drive polishing cycles and scrap. On lenses, centration and aspheric form can also be major drivers. The reliable way to find out is to compare actual hours and yield on past jobs with similar tolerance classes.
How can I reduce optics cost without losing performance?
Review the print with the customer and relax tolerances that do not affect function: figure outside the clear aperture, scratch-dig on non-critical surfaces, or edge and bevel specs copied from older designs. Offering a standard material or a stock blank can also cut cost and lead time.
If you want estimators to see historical yield and hours for the same tolerance class while they quote, talk to Netray about an on-prem assistant over your ERP and inspection records.