1. Start with the Complete Optical System

The wavelength of a LiDAR system is coordinated with its source, detector, measurement method, target response, and operating environment. A protective cover should support that architecture rather than drive wavelength selection on its own.

Ask three questions before comparing materials: What light must pass? Over which angles and aperture must it pass? What loads and environmental exposure must the component withstand?

The selections below are engineering screening guidance. They are not guaranteed transmission values or a universal ranking of materials.

2. Comparing 905 nm, 940 nm, and 1550 nm

Hamamatsu identifies silicon and InGaAs detector families for these sensing bands. The cover material is a separate selection: silicon’s usefulness as a detector at 905 nm does not make a silicon plate transparent at 905 nm. [1]

None of these wavelengths alone establishes range, weather capability, or laser safety. Those are properties of the complete system under defined conditions.

3. Specify an Operating Band, Not Just a Nominal Wavelength

A label such as “905 nm” identifies the nominal source wavelength. It does not fully describe manufacturing tolerance, spectral width, temperature-related shift, or—in a frequency-swept system—the tuning range.

• Source envelope: Define the lowest and highest wavelengths that must be transmitted under all intended operating conditions.
• Receiver band: Include the passband and tolerances of any receiver filter.
• Multiple channels: Identify whether the same cover must transmit visible imaging, illumination, and ranging light.
• Angle and polarization: State the local incidence-angle range and the relevant polarization states.

A broadband cover may be appropriate for several channels, while a narrowband receiver filter may still be needed to suppress background light. These components have different jobs and should have separate specifications.

4. Optical Material Selection Matrix

“Candidate” means suitable for further evaluation, not approval for every geometry or application. A catalog transmission range does not establish a minimum transmission at a particular wavelength. Relevant supplier references are listed below. [2]–[6]

5. BK7-Type Glass and H-K9L: Define the Actual Grade

BK7-type crown glass is worth screening for visible and near-infrared windows and domes. Do not reject it for 1550 nm merely because that wavelength lies beyond the visible spectrum.

For example, SCHOTT’s N-BK7 datasheet lists internal transmittance of 0.992 at 1530 nm for a 10 mm path and 0.980 for a 25 mm path. These are material data at 1530 nm, not guaranteed finished-component transmission at 1550 nm. Surface reflection and coating performance must be evaluated separately. [2]

N-BK7 and H-K9L should not be treated as identical contractual specifications. For a proposed substitution, compare the supplier’s refractive-index data, internal transmission, homogeneity, thermal properties, and inspection requirements. Verify whether the optical model and coating design need updating.

RFQ wording: State the manufacturer and grade when mandatory. If equivalents are allowed, define the properties that must match and require approval before manufacture.

6. Fused Silica: Grade and Path Length Matter

Fused silica is often considered when low thermal expansion and optical quality are important. Its infrared absorption depends on composition and manufacturing history, including hydroxyl content; “fused silica” alone is therefore an incomplete purchasing specification. [3]

At the same time, “UV-grade” does not automatically mean unsuitable for 1550 nm. Thorlabs offers UV fused silica optics with 1550 nm coatings, illustrating that the actual grade and loss budget matter more than the broad marketing label. [7]

• Request a transmission curve around the complete operating band.
• Confirm the sample thickness and whether the curve excludes surface reflection.
• For a thick dome or a tight absorption budget, request grade-specific absorption information.
• Do not transfer optical-fiber requirements directly to a millimeter-thick cover; the propagation lengths differ substantially.

If a drawing uses a trade name or grade designation such as JGS1, obtain the actual supplier specification rather than assuming universal near-infrared performance from the name.

7. Sapphire: Surface Durability with Optical Tradeoffs

Sapphire is a useful candidate when abrasion resistance is a priority. It can transmit at the three wavelengths discussed here, but its crystalline optical properties require attention. Supplier specifications identify sapphire’s birefringence as well as its transmission behavior. [4]

Define crystal orientation relative to the component and the optical path. Aligning a central ray with the crystal optic axis does not remove all polarization effects for an entire wide-angle ray bundle. This is particularly relevant to coherent or polarization-sensitive sensing.

For domes, examine the real ray directions through the crystal rather than assuming that a material orientation described for a flat window transfers unchanged to the curved design.

Surface hardness also does not determine impact survival or pressure rating. Edge quality, thickness, support conditions, flaws, and assembly stresses must be included in the mechanical design. The exposed coating may have a different wear limit from the sapphire beneath it.

8. PC and PMMA: Qualify the Entire Molded Cover

Optical polymers can support lightweight covers and shapes suited to molding. Their suitability depends on the exact resin and manufacturing process, not simply on whether the plastic looks clear.

Covestro publishes sensor-oriented polycarbonate grades, including dark materials designed to transmit near-infrared light. Such data demonstrate why appearance and sensing-band transmission must be specified independently. They do not establish the performance of every PC or PMMA grade. [5]

• Color formulation: Identify the approved resin and color code; a visually similar substitute can have different spectral absorption.
• Thickness: Measure the final optical path, including thicker zones in a molded part.
• Processing: Evaluate stress birefringence, surface form, flow-related defects, and beam distortion.
• Surface stack: Include hardcoat, AR treatment, printed layers, and any heater or adhesive in testing.
• Aging: Recheck optical performance after the agreed UV, humidity, temperature, and cleaning exposures.

At 1550 nm, insist on transmission data around the actual operating band. A 905 nm qualification does not establish suitability at 1550 nm.

9. Silicon: A Conditional Option for 1550 nm

Optical silicon is used for infrared windows; Edmund Optics lists a typical application region beginning around 1.2 μm for its silicon window family. This supports evaluating it at 1550 nm, while excluding it as a transmissive cover for 905 or 940 nm. [6]

Its high refractive index makes surface reflection a major consideration. Confirm absorption for the specified material grade and path length, and design the AR coating for the required wavelength and angles.

A stock silicon window with a coating specified only from 1.65 μm upward is not automatically qualified at 1.55 μm. The substrate transmission band and coating band are separate requirements.

Silicon also blocks a visible camera view through the same aperture. A combined visible-camera and 1550 nm sensor may therefore need a different shared cover or separate optical openings.

10. Separate Internal, Component, and Round-Trip Transmission

Internal transmission describes propagation through the material, excluding surface reflection. Component transmission includes the finished optic’s surfaces and coatings under stated measurement conditions. System throughput also includes apertures, filters, alignment, contamination, and other components.

For a simple absorbing material, an engineering approximation is:

For one pass through a two-surface optic, neglecting multiple reflections and scatter, T ≈ (1 − R₁)(1 − R₂)exp(−αL). Each R is the surface reflectance at the actual wavelength, incidence angle, and polarization.

These are calculated examples, not Felix Glass product specifications. T² applies when outgoing and returning light each experience the same single-pass transmission. Otherwise, use T_out × T_return. The calculation does not predict detection range by itself.

11. Match the AR Coating to the Wavelength and Geometry

A coating must be designed for the substrate, operating band, incidence angles, and polarization. “AR coated” without those conditions is not a complete performance requirement.

A sensor’s scan angle is not necessarily the angle of incidence on a dome. The local surface normal changes with position, so the coating requirement should come from ray tracing the actual part geometry.

Specify the inner and outer surfaces independently when their exposure or optical conditions differ. If hydrophobic treatments or protective overcoats are added, qualify the complete stack. An AR layer cannot recover light already absorbed in the substrate.

12. A Practical Material-Selection Workflow

1. Freeze the spectral envelope: Record all required sensing and imaging bands, including source variation.
2. Model the cover: Define surface shape, thickness, pupil locations, scan range, and local incidence angles.
3. Screen substrates: Obtain grade-specific optical and mechanical data at the intended thickness.
4. Allocate losses: Separate allowable absorption, reflection, scatter, and clipping.
5. Design the coating: Include wavelength, polarization, angle, and exposed-surface durability.
6. Test prototypes: Check transmission, beam deviation, crosstalk, and range bias in the assembled sensor.
7. Repeat after exposure: Validate the chosen environmental and cleaning conditions before approving production.

Compare candidates under the same measurement conditions. A thin polished coupon tested at normal incidence is not directly comparable to a finished curved cover measured across a scan.

13. How to Specify a LiDAR Window or Dome for Quotation

Example specification structure: “Finished-part single-pass transmission ≥ [required value] over [wavelength band], at [local AOI range], for [polarization], across [clear aperture], at [test temperature].” Replace every bracketed field with a project requirement; this is a wording template, not a default acceptance limit.

Agree on test geometry and sampling before ordering. If a coating witness sample is used, establish how its results relate to the actual curved component.

14. Frequently Asked Questions

15. Discuss Your LiDAR Optical Material Requirements

Felix Glass supports custom precision optical component projects, including optical windows and domes. Send your drawing, operating wavelength, scan geometry, environmental requirements, and quantities to discuss material and manufacturing options.

If the substrate is not yet fixed, identify your optical loss budget and mechanical priorities. Proposed grades, coating performance, tolerances, inspection methods, and delivery conditions should be confirmed for the specific project.

Dongguan Felix Glass Co., Ltd.
Email: james@felixglass.com