CMM for Optics: Where It Fits and How to Choose One

Picture this. A batch of lens mounts comes back from the machine shop, the bores look fine by eye, and two weeks later the assembled optics start drifting off axis. The part was never the problem. Nobody checked the mount geometry in three dimensions before it went into the lens barrel. That gap is where a coordinate measuring machine for optics earns its place on the floor.

A CMM, short for coordinate measuring machine, measures the 3D position of surfaces, edges and mounting features. In plain terms, it touches or scans a part and records where each point sits in space, down to the micron level. Think of it as a very precise digital caliper that can walk around a part and map it.

Here is the short answer. A CMM checks lens radius, centre thickness and mount geometry. It does not measure the optical wavefront itself. For that, you still need an interferometer. Use the CMM alongside interferometers and profilometers. Do not treat it as a replacement for either.

Where a coordinate measuring machine for optics fits in the workflow

Optics work is a chain of steps, and each step can hide a mechanical error that the next step will not catch. The CMM is most useful where the part is still mechanical, before the glass is locked into the final assembly. Four moments stand out.

• Incoming inspection of blanks and mounts, before polishing starts
• In-process checks on edge thickness, bevels and mechanical datums
• Final verification before assembly and shipment
• Splitting the workload: which measurements belong to the CMM and which belong to interferometry

That last point confuses many new buyers. The CMM tells you where the surface sits and how thick the glass is. The interferometer tells you how light bends off that surface. A CMM can give numbers for surface figure, but those numbers will not mean what you hope. Keep each tool to its job.

Contact probes versus optical probes

A probe is the tip that touches or sees the part. Choosing it is where many shops make their first costly mistake. There are two families, and they suit very different parts.

Touch-trigger and scanning probes are the classic choice. They are accurate on hard, well-defined features like a machined bore or a flat seat. The catch is contact. Drag a stylus across a soft or freshly coated surface and you can leave a scratch that ruins the part.

Optical and non-contact probes avoid that risk. They are safe for coatings and fragile glass. They are also slower, and they struggle with shiny or highly reflective surfaces, because the light they rely on scatters unpredictably. The choice comes down to the part: its material, whether a coating sits on the surface, and how tight the tolerance band is.

Measuring lens radius and mount geometry

The radius of a lens tells you how curved its surface is. Measuring it depends less on the probe and more on how you sample the surface. You collect a cloud of points across the face, then fit a mathematical shape to that cloud. For a spherical surface, the fit is a sphere. For an aspheric surface, the fit is a conic or a higher-order form, which is harder to get right.

Sampling strategy matters more than most people expect. Too few points and the fit wobbles. Too many and you waste inspection time for no gain. A common approach is a dense ring pattern near the edge, where form error tends to show up, plus a handful of points across the centre.

Mount geometry is the other half of the job. You check that the mount seat is concentric with the bore, perpendicular to the optical axis and flat enough to seat the glass without strain. A seat out by a few microns in any of these can tilt a lens enough to matter in a precision system.

How much can you trust the number?

Every measurement carries doubt. The honest way to handle that doubt is an uncertainty budget: a list of every source of error, each with an estimated size, combined into one figure called expanded uncertainty. If that figure is smaller than your tolerance, the CMM is fit for the job. If not, it is not, however impressive the machine looks.

The sources you will usually list are probe calibration, thermal drift, fixturing, the part’s own form error and the software that fits the sphere or conic. Some are easy to estimate from a calibration certificate. Others, like thermal drift, you measure yourself by logging the room over a shift.

Write the budget before you buy, not after. It forces you to ask what accuracy you actually need, and it stops you from paying for a machine far more precise than your parts require.

Keeping the room and the part steady

A CMM is only as stable as the room around it. Temperature is the biggest enemy. Glass and metal expand at different rates, so a glass lens sitting in a metal mount will shift as the room warms or cools. A few degrees over an afternoon can move a result past your tolerance without any obvious sign.

Vibration is the second enemy. A nearby pump, a forklift in the corridor or even foot traffic can make repeat measurements disagree. Isolation pads and a sensible spot on the floor help. Fixturing is the third. If you clamp the part too hard, you bend it and then measure the bend. Design the fixture so the clamping force lands where the part can take it, and verify by measuring the same part with the clamp loosened and tightened.

Choosing a CMM for your optics shop

Start with the parts, not the brochure. Working volume has to fit your largest part with room to spare. Accuracy has to sit comfortably inside your uncertainty budget. The probe options have to match your materials and coatings. The software needs to fit spheres and conics properly, because a generic fitting routine can give a smooth-looking number that is quietly wrong.

Before you sign, ask the supplier a few pointed questions. What is the calibration interval, and who performs it? Can they show a measurement of a reference artefact on the machine you would buy? Which optical fitting routines are included, and are they validated? What service response time can you expect in your country?

Sometimes the right answer is not to buy at all. If your volume is low, or your tolerances are tight but rare, a good service lab with traceable calibration will serve you better. The goal is correct measurements, not owning a machine.

Frequently asked questions

Can a CMM measure optical surface figure directly?

Not really. A CMM gives you the shape of the surface as coordinates, which is useful for radius and form. Surface figure, the fine error that controls how light focuses, needs an interferometer. Use the CMM to check mechanical geometry and the interferometer to check optical quality.

How accurate does a CMM need to be for lens mounts?

It depends on your tolerance. Write the uncertainty budget first, then compare. If the expanded uncertainty is comfortably below the tolerance band, the machine is accurate enough. Buying far more accuracy than the tolerance needs is a common and expensive mistake.

Do optical probes work on coated lenses?

Yes, that is their main strength. Because they do not touch the surface, they will not scratch a coating. The trade-off is speed and sensitivity to reflectance, so test your specific coating before you commit to a measurement plan.

How often should a CMM be calibrated?

Follow the manufacturer’s interval as a starting point, then adjust based on performance. If you see drift between checks against a reference artefact, shorten the interval. Keep a record so you can see the trend over time.

Is a CMM worth buying for a small optics workshop?

Often it is not, at least not at first. Compare your annual measurement volume with the cost of outsourcing and the cost of owning. Many small shops start with a service lab and buy a machine only once their volume justifies it. Run the numbers honestly and let them decide.

If you are weighing this up for your own shop, start with the uncertainty budget and the parts list. Those two documents will tell you more than any spec sheet. For the wider picture on how optical measurement fits together, see optical design and wavefront error. Our guide on preparing optics for surface roughness measurement covers another step in the same chain. For sourcing measurement and metrology services, The MONA is a useful starting point.