LiDAR Wavelength & Materials: 905, 940 & 1550 nm
Knowledge Center · LiDAR & Sensing Guide
LiDAR Wavelength & Materials
A practical guide to selecting optical windows and domes for 905 nm, 940 nm, and 1550 nm sensing systems.
Choosing a LiDAR cover begins with the operating wavelength, but it does not end there. Material grade, optical path length, coatings, scan geometry, and the environment determine how much useful light reaches the target and returns to the detector.
This article explains how to screen BK7-type glass, fused silica, sapphire, optical polymers, and silicon for a specific sensing application—and how to turn that shortlist into a clear, testable component specification.
Compare Optical Materials1. 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
| Design factor | 905 nm | 940 nm | 1550 nm |
|---|---|---|---|
| Typical context | Pulsed ranging and scanning LiDAR | Active depth sensing, proximity sensing, and some LiDAR architectures | Selected pulsed and coherent LiDAR architectures |
| Detector considerations | Silicon-based detectors can be used. | Silicon-based detectors can be used; actual sensitivity depends on device design. | InGaAs is a common detector choice; conventional silicon photodetectors are unsuitable for direct detection at this wavelength. |
| Cover priorities | Transmission across source tolerance, scan angle, and temperature | Transmission of the full cover stack, including colorants and surface treatments | Grade-specific absorption, coating performance, and polarization behavior |
| Key misconception | A material transmitting at 905 nm automatically meets the full sensor specification. | A cover qualified at 905 nm automatically meets the 940 nm requirement. | 1550 nm automatically guarantees longer range or eye safety. |
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
| Material family | 905 / 940 nm | 1550 nm | Main verification requirement |
|---|---|---|---|
| N-BK7 / BK7-type crown glass | Candidate | Candidate | Specified grade, thickness-dependent internal transmission, and finished-part performance |
| H-K9L optical glass | Candidate with supplier data | Candidate with supplier data | Actual material certificate and spectral data; substitution for another glass requires approval |
| Fused silica | Candidate | Candidate | Grade, hydroxyl-related absorption, homogeneity, and optical path length |
| Sapphire | Candidate | Candidate | Crystal orientation, polarization effects, coating design, and mounted strength |
| Optical PC / PMMA | Selected grades are candidates | Grade- and thickness-dependent; qualify from spectral data | Resin, colorants, processing, hardcoat, aging, and complete-stack transmission |
| Optical silicon | Unsuitable as a transmissive protective cover | Candidate where visible transparency is not required | Material absorption, resistivity, high-index reflections, and a coating designed for 1550 nm |
“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:
α is the absorption coefficient and L is the path length in the material, expressed in compatible units. This approximation does not account for surface reflection, scattering, or interference.
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.
| Illustrative single-pass cover transmission | Two-pass contribution, T² | Loss over those two passages |
|---|---|---|
| 90% | 81.00% | 19.00% |
| 95% | 90.25% | 9.75% |
| 98% | 96.04% | 3.96% |
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 Tout × Treturn. 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.
| Coating approach | When to evaluate it | What to verify |
|---|---|---|
| Single-band AR | One defined sensing band | Performance across source tolerance and the full local incidence-angle range |
| Dual-band AR | Two separated operating bands | Minimum transmission in both bands, not just two nominal center wavelengths |
| Broadband AR | Multiple channels or a wider continuous band | Required band coverage, polarization behavior, and any tradeoff in peak performance |
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
- Freeze the spectral envelope: Record all required sensing and imaging bands, including source variation.
- Model the cover: Define surface shape, thickness, pupil locations, scan range, and local incidence angles.
- Screen substrates: Obtain grade-specific optical and mechanical data at the intended thickness.
- Allocate losses: Separate allowable absorption, reflection, scatter, and clipping.
- Design the coating: Include wavelength, polarization, angle, and exposed-surface durability.
- Test prototypes: Check transmission, beam deviation, crosstalk, and range bias in the assembled sensor.
- 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
| Required information | What to include |
|---|---|
| Wavelength | Nominal value, required passband, source tolerance, and temperature or sweep range |
| Optical layout | Transmitter and receiver footprints, field of view, incidence-angle range, and polarization |
| Material | Manufacturer and grade, or approved alternatives with required material data |
| Geometry | 2D drawing, thickness, clear aperture, tolerances, and mounting datums; dome radii and concentricity where applicable |
| Performance | Finished-part minimum or average transmission, measurement conditions, beam deviation, and wavefront limits where relevant |
| Surface treatment | Coated faces, band and angles, surface quality, cleaning and durability requirements |
| Production | Prototype quantities, expected annual demand, delivery target, and inspection documents |
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
Which is better for LiDAR: 905 nm or 1550 nm?
Neither is universally better. Compare source and detector architecture, permitted emission, target response, background light, optical losses, cost, and required operating conditions. The cover must be selected to match that system decision.
Can BK7-type glass be used at 1550 nm?
Yes, it is a candidate. Verify the particular grade’s internal transmission at the actual path length and the finished optic’s coated performance. Do not treat material data at a nearby wavelength as an exact finished-part guarantee.
Is UV-grade fused silica unsuitable for 1550 nm?
No. Some UV fused silica optics are used at 1550 nm. Evaluate the actual grade, absorption, thickness, and coating; the grade label alone is insufficient.
Can the same AR coating serve 905 and 940 nm?
Potentially, if its design band covers both sources and their tolerances over the required angles and polarization states. A coating specified only at 905 nm should not be assumed to satisfy 940 nm.
Why is silicon used in a 905 nm detector but not as a 905 nm cover?
A detector must absorb photons to generate a signal, whereas a protective cover must transmit them. Silicon absorption is useful for the detector and undesirable for that cover.
Does a black sensor cover block LiDAR light?
Not necessarily. Some engineered materials block visible light while passing a sensing band. Ordinary black plastics or printed inks may also block the sensing light, so qualify the exact formulation and complete stack.
Is sapphire always preferable outdoors?
No. Abrasion resistance is only one requirement. Consider coating durability, crystal orientation, impact loads, mounting stress, cost, and the intended maintenance method.
Is 1550 nm automatically eye-safe?
No. Wavelength alone does not establish laser safety. The complete product’s accessible emission and operating conditions require the applicable safety assessment.
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
