Key Parameters to Define When Requesting a Custom Optical Lens Design

Introduction

Getting a custom optical lens made is rarely as simple as sending a drawing and waiting for delivery. The gap between what you imagine and what the manufacturer produces often comes down to how well you define the parameters before the design work starts. Miss one critical specification, and you might receive a lens that focuses light at the wrong point, transmits the wrong wavelengths, or fails under thermal stress.

Traditional off-the-shelf lenses force you to adapt your system to whatever is available. That works for prototyping, but production systems usually demand tighter tolerances, specific substrates, or unusual geometries. This article walks through the key parameters to define when requesting a custom optical lens design, in the order a manufacturer needs them. It is written for optical engineers, procurement specialists, and R&D teams who need a practical checklist rather than a theoretical overview.

Key Takeaways

Define the operating wavelength range first, as it dictates substrate selection and coating design.
Specify surface figure and irregularity in waves or fringes, not just in vague terms like "high quality."
Provide clear environmental limits—temperature, humidity, vibration—so the designer can select appropriate materials.
State the required clear aperture explicitly; it directly affects cost and manufacturability.
Include coating requirements with damage threshold values if the lens will handle laser power.

What You Need Before Starting

Before you contact a manufacturer, gather your system-level requirements. You need the optical layout, the detector or sensor specifications, and the mechanical envelope. Without these, the designer cannot determine focal length, back focal length, or diameter.

You also need to know your production volume. A prototype run of five pieces allows different manufacturing methods than a production run of 5,000. This affects whether the lens will be polished individually or generated and polished in batches.

Finally, decide on your quality standards. Familiarize yourself with ISO 10110, the international standard for optical drawing notation, or the American MIL-PRF specifications if you work in defense. These standards give you a common language with the manufacturer. Our main products include optical window,prism, lens, beamsp liters, filter, wedge, blanks and etc., and we also provide customized service according to customer's requests—so having your parameters ready lets us respond faster and more accurately.

Step 1 — Define the Wavelength Range and Substrate Material

What to Do

Start with the spectral band your lens must transmit. Write down the shortest and longest wavelengths, plus the central wavelength if you have one. For laser systems, specify the exact laser line, such as 1064 nm or 532 nm, and the expected power density.

Next, select the substrate material based on that wavelength range. Common choices include:

N-BK7 for visible and near-infrared (350–2000 nm)
Fused silica for UV through IR (185 nm to 2.5 µm) with low thermal expansion
Calcium fluoride (CaF2) for IR applications from 0.13 µm to about 10 µm
Zinc selenide (ZnSe) for CO2 laser optics at 10.6 µm

Why This Matters

The substrate determines the lens's transmission, thermal behavior, and durability. Fused silica, for example, has a coefficient of thermal expansion of about 0.55 × 10⁻⁶/K, roughly one-tenth that of N-BK7. If your system experiences temperature swings, that difference matters. CaF2 transmits well into the infrared but is softer and more fragile than glass, which affects mounting and handling.

Common Mistakes to Avoid

Specifying only "visible light": This is too vague. A lens for 400–700 nm and one for 700–1100 nm may need different coatings and even different substrates.
Ignoring the coating's wavelength range: The substrate may transmit, but an AR coating designed for 400–700 nm will not perform at 1064 nm. Always specify the coating band separately.

Step 2 — Specify Surface Figure, Irregularity, and Surface Quality

What to Do

Surface figure is the deviation of the actual surface from the ideal shape, usually expressed in waves at a test wavelength (often 632.8 nm from a HeNe laser). A typical specification is λ/4 or λ/10. Irregularity, sometimes called "power error," measures the asymmetry of that deviation and is also given in waves.

Surface quality follows the MIL-PRF-13830B standard, written as a scratch-dig number like 60-40 or 20-10. The first number refers to scratch width, the second to dig diameter, both in thousandths of a millimeter.

Why This Matters

Surface figure directly affects wavefront error. For a diffraction-limited system, you typically need λ/10 or better. For imaging systems with relaxed requirements, λ/4 may suffice. Surface quality matters for laser systems because scratches and digs absorb energy and can initiate damage. A 20-10 scratch-dig specification is common for laser optics, while 60-40 is acceptable for many imaging applications.

Common Mistakes to Avoid

Over-specifying without reason: Requesting λ/20 when your system only needs λ/4 increases cost significantly without improving performance.
Confusing surface figure with surface quality: They are separate parameters. You can have a λ/10 surface with a 60-40 scratch-dig, or a λ/4 surface with a 20-10 finish.

Step 3 — Define the Clear Aperture and Mechanical Dimensions

What to Do

The clear aperture is the portion of the lens that must meet the optical specifications. It is always smaller than the physical diameter. State both values explicitly. For example: "Physical diameter 25.0 mm, clear aperture 22.0 mm."

Also specify edge thickness, center thickness, and the radius of curvature for each surface. If you have a mechanical drawing, include the tolerance for each dimension—typically ±0.1 mm for diameter and ±0.2 mm for thickness unless your application demands tighter.

Why This Matters

The clear aperture determines how much of the lens the manufacturer must polish to specification. A larger clear aperture relative to the physical diameter means more precision work and higher cost. Center thickness affects the lens's mechanical strength and its ability to handle thermal stress. Edge thickness matters for mounting—a lens with a sharp edge is prone to chipping.

Common Mistakes to Avoid

Forgetting the bevel or chamfer: A sharp edge is fragile. Specify a chamfer of 0.2–0.5 mm at 45° to protect the lens during handling and mounting.
Not specifying centering error: This is the tilt between the optical axis and the mechanical axis, usually given in arcminutes. For precision systems, specify 1–3 arcminutes.

Step 4 — Specify Coating Requirements and Laser Damage Threshold

What to Do

Decide what coating your lens needs: anti-reflection (AR), high-reflection (HR), or partial reflection. Specify the wavelength range and the maximum allowable reflectance. For example, "AR coating with R < 0.5% at 1064 nm" or "R < 1% average from 400–700 nm."

If your lens will handle laser power, specify the laser damage threshold (LDT) in J/cm² for pulsed lasers or W/cm² for continuous-wave lasers. Industry-standard tests follow ISO 11254 or ISO 21254.

Why This Matters

Coatings are often the limiting factor in system performance. An uncoated N-BK7 surface reflects about 4% of incident light at normal incidence. A good AR coating reduces that to below 0.5%. For a multi-element system with ten surfaces, the difference between coated and uncoated is the difference between 66% and 95% transmission.

The damage threshold depends on the coating material, the deposition process, and the substrate's surface quality. A 20-10 scratch-dig surface with a high-quality coating can handle significantly more laser power than a 60-40 surface with a standard coating.

Common Mistakes to Avoid

Not specifying environmental durability: If the lens will face humidity, salt spray, or abrasion, specify MIL-C-48497 or similar durability requirements. Standard AR coatings may not survive harsh conditions.
Ignoring the angle of incidence: Coatings are designed for a specific AOI. If your lens operates at 30° rather than 0°, say so. The coating design changes.

Step 5 — Provide Environmental and Operational Conditions

What to Do

Write down the operating temperature range, humidity levels, and any exposure to chemicals or mechanical stress. Also note whether the lens will be subjected to thermal cycling or shock.

For high-temperature applications, consider the substrate's thermal expansion and whether the coating can withstand the temperature. For cryogenic applications, specify the operating temperature and ask about material behavior at that temperature.

Why This Matters

Temperature changes alter the refractive index of glass and the physical dimensions of the lens. This shifts the focal length and introduces aberrations. For precision systems, you may need athermalized designs that compensate for these effects.

Humidity and chemicals attack coatings and substrates. Uncoated CaF2 is hygroscopic and degrades in humid environments. Some coating materials absorb water and change their optical properties.

Common Mistakes to Avoid

Assuming room-temperature performance is enough: If your system operates at 80°C, the lens design must account for it. Specify the temperature range explicitly.
Not mentioning vibration or shock: A lens that performs well on a lab bench may fail on a vehicle or in a manufacturing line. Specify the mechanical environment.

Step 6 — Define Quality Standards and Documentation Requirements

What to Do

State which standard your drawings and inspections must follow. ISO 10110 is the most common international standard for optical elements. It uses a series of numbered blocks to specify surface form, surface texture, and material imperfections.

Also specify what documentation you need: inspection reports, coating certificates, material certificates, or test data. If you need serialized traceability, say so.

Why This Matters

Using a recognized standard eliminates ambiguity. ISO 10110 notation is precise—it tells the manufacturer exactly what to measure and what tolerances to apply. Without it, you risk receiving parts that meet your verbal description but not your actual requirements.

Documentation matters for quality control and regulatory compliance. In medical and defense applications, you may need full traceability from raw material to finished part. Medical Instruments rely on high-precision optical components for clear imaging and reliable diagnostic performance, which is why documentation standards are non-negotiable in that sector.

Common Mistakes to Avoid

Skipping the inspection plan: Agree on how many parts will be inspected and what measurements will be taken. A 100% inspection of critical parameters adds cost but may be necessary.
Not specifying packaging: Optical components need proper packaging to survive shipping. Specify foam-lined boxes, individual wrapping, and desiccant if needed.

Step 7 — Review and Iterate with the Manufacturer

What to Do

Share your draft specification with the manufacturer and ask for feedback. A good manufacturer will flag impossible tolerances, suggest alternative materials, or recommend coating changes.

Request a design review or a tolerance analysis. This is a formal process where the manufacturer evaluates how each tolerance affects performance and cost. It often reveals that one parameter can be relaxed to save money without affecting system performance.

Why This Matters

Manufacturing capability varies. A tolerance that one shop can hold routinely may be a stretch for another. The design review catches these issues before you commit to production. It also gives you a chance to see prototype parts and test them in your system.

For laser applications, this step is critical. A Laser Optical System typically uses specific substrates, coatings, or a combination of the two to provide superior performance at specific wavelengths. The manufacturer's experience with similar systems is invaluable.

Common Mistakes to Avoid

Skipping the prototype phase: For complex designs, always order prototypes before full production. The cost of a prototype is small compared to the cost of scrapping a production run.
Changing specifications mid-production: If you must change something, expect cost and schedule impacts. Lock the specification before production starts.

Pro Tips for Success

Use ISO 10110 notation in your drawings: It is unambiguous and universally understood by optical manufacturers. It also makes quoting faster and more accurate.
Specify the test wavelength for surface figure: Most manufacturers use 632.8 nm, but if your application is at a different wavelength, say so. The surface figure requirement scales with wavelength.
Ask for a tolerance sensitivity analysis: This shows which parameters are critical and which can be relaxed. It often reveals that the most expensive tolerance is not actually necessary.
Consider the mounting method: A lens that is glued, clamped, or spring-loaded will experience different stresses. Mention the mounting method in your specification.
Request coating witness samples: These are small pieces coated alongside your lenses. They allow you to test the coating's durability and optical performance without sacrificing a production part.

Frequently Asked Questions

What is the difference between surface figure and surface quality?

Surface figure measures the shape accuracy of the lens surface, expressed in waves or fringes. Surface quality measures cosmetic defects like scratches and digs, expressed as a scratch-dig number. A lens can have excellent surface figure but poor surface quality, or vice versa.

How do I choose between N-BK7 and fused silica?

N-BK7 is cheaper and suitable for most visible and near-infrared applications. Fused silica is more expensive but has lower thermal expansion and better transmission in the UV. Choose fused silica if your system operates in the UV, experiences temperature swings, or needs high laser damage resistance.

What does a laser damage threshold of 10 J/cm² mean?

It means the coating can withstand 10 joules per square centimeter of pulsed laser energy before damage occurs, as tested per ISO 21254. For continuous-wave lasers, the threshold is given in W/cm². Always specify the pulse duration and repetition rate, as these affect the threshold.

Conclusion

The key parameters to define when requesting a custom optical lens design start with the wavelength range and substrate, then move through surface specifications, mechanical dimensions, coatings, environmental conditions, and quality standards. Each parameter interacts with the others—change the wavelength and you may need a different substrate; tighten the surface figure and the cost rises.

This structured approach works because it mirrors how a manufacturer actually designs and produces a lens. It forces you to think through your system requirements before spending money on tooling or prototypes. The result is a specification that is complete, unambiguous, and manufacturable.

Your next step is to draft a preliminary specification using the seven steps above. Even a rough draft helps the manufacturer understand your needs and respond with useful feedback. Then request a design review and prototype. The time invested in defining parameters correctly pays off in fewer iterations, lower costs, and a lens that performs as designed.