Optics is the part of a product where a reasonable-looking drawing gets quoted at ten times what you expected. A lens is not just a shape — it is a shape held to a fraction of a wavelength, in a material with a specified refractive index and dispersion, with a surface smooth enough to scatter almost nothing, coated to control reflection, and mounted so none of that shifts with temperature or a drop. Specify it right and a lens costs a few dollars. Over-specify one number and the same lens costs three hundred.
Glass or polymer: the first decision
| Glass | Polymer (acrylic, PC, COP) | |
|---|---|---|
| Volume economics | Cost falls slowly with volume; each part is ground and polished | Tooling-heavy, then pennies per part |
| Scratch resistance | Excellent | Poor without a hard coat |
| Thermal stability | Very good; low dn/dT | Focus shifts noticeably with temperature |
| Weight | 2–4 g/cm³ | ~1.2 g/cm³ |
| Aspheres and free-form | Expensive (molded or diamond turned) | Nearly free once the tool exists |
| Index range | 1.45–2.0, hundreds of glass types | 1.49–1.64, a handful of options |
The practical rule: below a few thousand units, or where scratch, heat, or high index matter, use glass. Above that, and where the optic can be integrated with a mechanical part, polymer wins decisively — which is why every phone camera and nearly every LED luminaire uses molded plastic optics. The two most common candidates are compared in acrylic versus polycarbonate; for optics, acrylic has better clarity and lower birefringence, polycarbonate is tougher and takes more heat, and cyclic olefin polymer beats both at a higher material cost.
How glass optics are actually made
Traditional spherical lens production is a sequence: blank the glass, generate the curve with a diamond wheel, grind out subsurface damage, polish with a pitch pad and cerium oxide slurry, center the lens so its optical axis matches its mechanical edge, then edge to diameter. Polishing is where surface quality is won, and it is bought in machine hours. Two other routes matter: precision glass molding presses heated preforms into ceramic molds, making glass aspheres economical above roughly 10,000 units at a mold cost of $20,000–$60,000 per surface, and magnetorheological finishing deterministically corrects residual figure error, which is how the tightest surfaces get made.
Diamond turning
Single-point diamond turning cuts optical surfaces directly with a natural diamond tool on an ultra-precision lathe. It produces aspheres, off-axis conics, and free-form surfaces in one setup, with form accuracy under a micron and roughness in the 3–10 nm range. It works on aluminum, copper, nickel-plated steel, germanium, and all the optical polymers — but not on glass, which graphitizes the diamond. It has two roles: making small quantities of custom optics directly, and cutting the mold inserts for polymer molding. Every molded asphere in a consumer product traces back to a diamond-turned insert.
Injection molding optics
Optical molding is conventional injection molding with the tolerance band tightened until it hurts.
- Tooling is a different class of object. Diamond-turned, polished, nickel-plated inserts in a thermally controlled mold base. Expect $15,000–$50,000 for a simple single-cavity optical tool.
- Cycle times are long. Thick lenses need slow packing and cooling, so a lens may cycle in 60–120 seconds where a trim part cycles in 20.
- Wall thickness discipline is absolute. Thick centers with thin edges produce sink and shrink voids — the mechanisms in injection molding defects hit optics twice as hard, because a distortion invisible in a bracket destroys an image.
- Birefringence. Frozen-in stress makes plastic optically anisotropic, ruining polarization-sensitive designs. It comes from gate placement and packing pressure, so the mold designer is part of the optical design team.
- Integrated features. A molded lens can carry its own mounting flange, datums, and baffles, eliminating a mechanical assembly. This is the biggest cost advantage of polymer optics and most teams underuse it.
Coatings
An uncoated air-glass surface reflects about 4% of the light. Two surfaces per element, six elements, and a third of your signal is gone — returning as ghost images and veiling glare.
- Anti-reflection. Single-layer magnesium fluoride cuts reflection to about 1.3% at one wavelength and is cheap. Multilayer broadband AR gets under 0.5% across the visible band; laser V-coats reach below 0.25%.
- Hard coat. Mandatory on any exposed polymer optic. A siloxane hard coat is what lets a molded acrylic window survive a shirt sleeve.
- Mirror coatings. Protected aluminum for general use, silver for higher visible reflectance, gold for infrared, dielectric stacks above 99%.
- Filters. Bandpass, dichroic, IR-cut, and oleophobic top layers. An IR-cut filter is required on essentially every color camera — settle it when choosing the image sensor.
Coating adds cost per surface and, more importantly, lead time — four to eight weeks is normal.
Specifying surfaces without blowing the budget
Three numbers do most of the work on an optical drawing, and each has a price curve.
- Surface figure, in waves at 632.8 nm. Commercial is 1–2 waves, precision λ/4, high precision λ/10. Each step down roughly doubles polishing time.
- Surface roughness, in nanometers RMS. Commercial polish is 2–5 nm, laser-grade below 1 nm. Roughness controls scatter, so it matters enormously in imaging and hardly at all in an illumination lens — the logic of specifying surface finish applies with steeper coefficients.
- Scratch-dig, per MIL-PRF-13830B. 80-50 commercial, 60-40 typical, 40-20 precision. A cosmetic inspection spec, and the one most often over-tightened out of drawing habit.
Add centration (usually 1–3 arcminutes), clear aperture (85–90% of diameter, not 100%, or you pay for perfect edges nobody uses), and index and Abbe tolerance.
When a catalog lens beats a custom one
Most of the time. Stock lenses cover an enormous range of focal lengths, diameters, and coatings, ship the same week, cost $20–$200, and come with measured data. A custom element means NRE, months of lead time, and a minimum order. Go custom only when the packaging envelope will not accept a stock part, when volume repays the tooling, or when you need a free-form or integrated-mount geometry no catalog carries. Even then the winning architecture is usually hybrid: stock elements doing the imaging, one custom molded element doing the packaging-driven job — the same off-the-shelf versus custom tradeoff that governs the rest of the bill of materials. Illumination is the clearest case: most optics in an LED lighting product are catalog TIR lenses until volume justifies a molded array.
Projects House designs and sources optical assemblies — layout and tolerancing, glass or polymer selection, mold and coating vendor qualification, and the optomechanical mount that holds it in alignment. Send your requirement and volume through our contact form.