Peoptix - Infrared Optics & Custom Optical Components
Peoptix - Infrared Optics & Custom Optical Components
Single-point diamond turning (SPDT) holds a distinct place in optical manufacturing: one pass of a diamond tool on an ultra-precision lathe yields a mirror-grade surface - nanometre roughness, sub-micron form accuracy - with no grinding or polishing. For aspheric and freeform infrared elements it turned an "impossible" process into a mature one.
Where It Works - and Where It Fails
The diamond edge is single-crystal carbon. At the cutting point it reacts with certain elements and dulls within minutes. That single fact draws the material map:
| Suited to SPDT | Why | Avoid | Why |
|---|---|---|---|
| Aluminium, copper, electroless Ni-P | No carbon affinity; mirror finish directly | Steels and nickel alloys | Carbon diffuses; edge fails in minutes |
| ZnSe, ZnS, chalcogenide glass, CaF₂, silicon | Cut in the ductile regime | Titanium alloys | Chemically too active |
| Optical plastics (PMMA, COC) | Soft; ideal mould masters | Most ceramics | Brittle fracture dominates |
A Tool Is an Optical Element
The nose radius sets the theoretical peak-to-valley of the tool-mark profile (f²/8R; f = feed per revolution, R = nose radius). Edge defects above roughly 50 nm print straight onto the workpiece, and crystal orientation can swing tool life by half. Selecting the tool is optics, not just machining.
Tool Marks Are an Optical Issue
The spiral path leaves periodic grooves. In visible and ultraviolet systems those grooves behave like a diffraction grating - stray rings, ghost images, lost energy. In the infrared the same marks are harmless - one reason SPDT excels there. Short-wavelength parts may still need MRF or ion-beam post-polish.
Figure - Periodic grooves act as a grating: destructive in visible/UV, harmless in IR.
Judge the Surface with More than Ra
A single Ra value hides periodic error: a surface at Ra 2 nm with one 50 nm groove is still scrap for a laser. Reliable assessment stacks the evidence:
| Metric | What it tells you |
|---|---|
| Ra / Rq | Average and RMS smoothness |
| PV | The worst point on the aperture |
| PSD | Periodic error across spatial frequencies |
| Scatter / MTF | What the optical system actually sees |
Bottom Line
SPDT is a system - material, tool, parameters, environment, measurement - where a 0.1 °C drift or 0.1 µm of wear is amplified into the result. The machine sets the ceiling; the process sets the consistency. That discipline is how we produce our ZnSe, ZnS and chalcogenide lenses, windows and domes.
