Most mis-ordered optics fail on one of three numbers. Diameter (Dia) is the physical size of the glass — it must match the lens holder bore, typically within ±0.1 mm. Edge thickness (ET) is the thickness at the rim — it controls how the lens seats, how clamping force distributes, and how heat conducts into the mount. Focal length (FL) is the optical working distance — it defines your spot size, depth of field, and nozzle standoff.
Measure before you order
| Spec | How to Measure | Watch Out For |
|---|---|---|
| Diameter | Calipers across the glass edge | 18 vs 19 vs 20 mm look alike — measure, don't guess |
| Edge thickness | Calipers at the rim, not the center | 2 mm vs 3 mm ET changes seating and beam path |
| Focal length | Machine manual, or old lens marking | Quoted in inches or mm — 2" = 50.8 mm |
| Orientation | Meniscus/plano-convex: curved side UP | Inverted lenses focus poorly and run hot |
The numbers the label doesn't show
Two lenses with identical Dia/ET/FL can still perform very differently. Bulk absorption class determines thermal lensing at power — laser-grade ZnSe absorbs a fraction of what economy material does. Coating quality sets both transmission and lifetime. And ET consistency across a batch determines whether every replacement drops in at the same focus height, or whether your operators re-teach the machine each swap. Specify all three visible numbers and demand certs for the invisible ones.
Adapted and expanded from an article originally published on the American Photonics blog.