1. What Does “8–15 µm Transmission” Mean?

The notation describes a wavelength interval from 8 to 15 micrometers, equivalent to 8,000–15,000 nanometers. It does not state a transmission threshold, material thickness, operating temperature or coating design.

A component that passes a small fraction of radiation at 15 µm can technically show transmission there while remaining unsuitable for a high-throughput application. “Transmits to 15 µm” is therefore not equivalent to “high transmission from 8 to 15 µm.”

The term LWIR is useful for broad classification, but its boundaries vary with convention. Use numerical wavelengths for design, quotation and inspection.

2. Comparing 8–12, 8–14 and 8–15 µm

A wider band can collect more useful signal when the source, optics and detector support it. It can also introduce additional background, absorption, aberration or coating complexity. Bandwidth should follow the measurement or imaging task rather than a preference for the largest number.

For example, extending a window specification to 15 µm offers little benefit to a camera whose response is negligible above 13.5 µm.

3. Why This Region Matters for Thermal Imaging

Objects above absolute zero emit thermal radiation. For an ideal blackbody, the peak of the wavelength-based spectrum follows Wien's displacement law:

λ_max ≈ 2898/T, with wavelength in micrometers and temperature in kelvin.

At 300 K, approximately 27°C, the peak occurs near 9.7 µm. This helps explain why long-wave infrared cameras are useful for observing objects near room temperature.

The peak is not a narrow emission line. Radiation extends across a broad spectrum, and the detected contribution from each wavelength depends on the object, its emissivity, the atmosphere and the instrument. A warmer object shifts the peak toward shorter wavelengths, but can still produce a useful LWIR signal.

4. The Atmospheric Window Is Not Perfectly Transparent

An atmospheric window is a region of relatively low atmospheric absorption. It is not a guarantee of uniform transmission, and its useful boundaries depend on the path and conditions.

Water vapor, ozone and carbon dioxide introduce wavelength-dependent absorption. Ozone has a feature near 9.6–9.7 µm, while carbon dioxide has a strong band around 15 µm [1, 2]. Their impact depends on gas concentration, path length and observing geometry.

This is especially relevant when extending a requirement from 14 to 15 µm. Good transmission through a laboratory optic does not remove absorption in the air between the target and camera.

Short indoor paths, long horizontal outdoor paths and satellite viewing paths are different cases. Use a suitable atmospheric model or measured data for the intended environment; a generic atmospheric plot should not be treated as a predicted field result.

5. Four Different Ranges Must Be Kept Separate

The overlap of nominal bands is a useful initial screen, but the actual spectral curves are more informative. A narrow absorption dip or a gradual detector cutoff may significantly affect a task even when every datasheet lists the same nominal endpoints.

6. Material Selection Across 8–15 µm

Crystran lists a broad nominal range of 0.6–21 µm for ZnSe [3]. This makes it worth evaluating for an extended band, but does not establish a finished-window guarantee. Likewise, a supplier's ZnS range approaching 14 µm does not establish a high-transmission 15 µm design [4].

Silicon illustrates the danger of using endpoints alone. Crystran lists a broad range but also describes its common use in the 3–5 µm band [5]. Its grade-specific spectral structure must be examined before considering a broad LWIR application.

7. How Thickness Changes Transmission

For a homogeneous material, a simplified internal-transmission model is:

τ_internal(λ) = exp[−α(λ)t]

Here α(λ) is the wavelength-dependent absorption coefficient and t is path length in compatible units. The expression excludes surface reflection, coating loss and scattering.

As a hypothetical example, at a wavelength where α = 0.20 cm⁻¹, a 2 mm path gives approximately 96.1% internal transmission and a 5 mm path gives approximately 90.5%. Neither number is complete-part transmission.

Near a material's absorption edge, additional thickness can have a stronger effect because α rises. At oblique incidence, the path through the substrate is also longer. Obtain data for the intended thickness and angles rather than applying a thin-sample curve directly.

Thickness must still satisfy mechanical needs. Optimize optical throughput and structural performance together.

8. AR, BBAR and DLC Coatings Have Their Own Limits

An AR or broadband AR coating reduces reflection over a designed band. “BBAR” does not mean that the coating performs equally well at every infrared wavelength.

Specify the band, allowable reflectance or transmission, angle range and relevant polarization conditions. An 8–12 µm coating should not be accepted for an 8–15 µm requirement without additional evidence.

Protective DLC or hard-carbon layers also affect the spectral response. Review the complete external and internal coating stacks, including absorption and environmental requirements. A coating cannot restore radiation already absorbed inside the substrate.

A normal-incidence witness-coupon result may not represent a curved dome or an oblique field ray. Match the verification method to the optical geometry and define when finished-part testing is required.

9. Detector and Package Windows Can Set the Band Edges

The optical substrate is not always the limiting component. A detector's sensing layer, package window and internal filters may define the effective cut-on and cutoff wavelengths.

FLIR describes one standard microbolometer package in which the front window sets a 7.5 µm short-wavelength cut-on and the detector sets a 13.5 µm long-wavelength cutoff [6]. This is a specific example, not a universal microbolometer range.

Request the spectral responsivity of the actual detector configuration. A nominal range should be accompanied by its definition where endpoint behavior matters.

If the detector has no useful response at 15 µm, extending the external window alone will not create a 15 µm camera. Conversely, a detector capable of that wavelength still requires compatible optics and an appropriate signal model.

10. Calculate Throughput Wavelength by Wavelength

For a simplified optical path, neglecting multiple-reflection coupling:

τ_optics(λ) ≈ τ_window(λ) × τ_lens(λ) × τ_filter(λ)

Use fractional transmission values at the same wavelength. For example, 0.90 × 0.92 × 0.85 = 0.704, or approximately 70.4%. This single-wavelength example does not define a broadband result.

A simplified target-signal calculation is:

S ∝ ∫ L_target(λ) τ_atmosphere(λ) τ_optics(λ) R_detector(λ) dλ

R is detector responsivity defined consistently with the radiance units. The expression omits geometry and additive radiation from the atmosphere, optics and reflected surroundings. These may be necessary for a radiometric model.

Do not normally multiply unrelated broadband averages. Two components with the same average transmission can have different spectral shapes and therefore different combined performance.

11. Average Transmission vs Minimum Transmission

A band-average requirement controls the mean over a stated interval. A minimum requirement controls the lowest permitted transmission within that interval. Neither automatically implies the other.

Consider a hypothetical window with 95% transmission from 8 to 12 µm and 60% from 12 to 15 µm. Its uniformly wavelength-weighted average over 8–15 µm is:

(4 × 95% + 3 × 60%)/7 = 80%

The same window has a 95% average over 8–12 µm. Changing the averaging interval changes the result even though the physical component is unchanged.

Define whether averaging is uniform in wavelength, uniform in wavenumber, or weighted by a source and detector response. These methods generally produce different values.

When a spectral dip would compromise the application, specify both a band average and a minimum, plus suitable measurement resolution. Set achievable limits after material and coating review.

12. How to Read and Verify an FTIR Transmission Report

Fourier-transform infrared spectroscopy can characterize transmission across this region. A useful report identifies the sample and the test conditions, not just a smooth curve.

• Material grade, thickness, coating condition and sample traceability.
• Wavelength or wavenumber axis, transmission units and spectral resolution.
• Angle of incidence, beam size, sample position and collection geometry.
• Background/reference method and purge or atmospheric compensation conditions.
• Measurement temperature, uncertainty and the stated acceptance calculation.
• Whether the test measures the finished part, a witness coupon or a reference sample.

Wavelength and wavenumber are related by λ (µm) = 10,000 / wavenumber (cm⁻¹). Thus, 8 µm corresponds to 1250 cm⁻¹ and 15 µm to about 667 cm⁻¹. Axis direction and sample spacing change under this conversion.

Residual atmospheric absorption can appear when sample and background conditions differ. Investigate unexpected features instead of automatically attributing them to the substrate. Instrument resolution and detector coverage must also be adequate for the specified endpoints.

13. A Practical 8–15 µm Specification Workflow

1. Define the task: imaging, temperature measurement, spectroscopy or another function.
2. Confirm detector response: establish whether the full requested interval contributes useful signal.
3. Define the external path: distance, humidity, temperature and any controlled atmosphere.
4. Choose candidate materials: compare curves at realistic thickness and temperature.
5. Specify coating and angles: include both surfaces and relevant field rays.
6. Set acceptance metrics: distinguish complete-part transmission, band average and spectral minimum.
7. Check integration: account for mounting, focus, thermal emission and calibration.
8. Validate a prototype: use spectral measurements and application-level tests.

Example specification wording: “Finished coated window transmission shall be measured from 8.0 to 15.0 µm at the agreed incidence angle and temperature. Report the uniformly wavelength-weighted band average, minimum transmission and full measured spectrum. Acceptance limits, spectral resolution and measurement uncertainty shall be agreed before manufacture.”

This wording defines the reporting approach; it still needs numerical performance limits and mechanical specifications. An isolated requirement such as “T > 95% at 8–15 µm” leaves too much ambiguity for production acceptance.

14. Frequently Asked Questions

15. Define Your Infrared Band with Felix Glass

For a custom infrared window, lens or dome, share the exact wavelength interval together with the detector information, dimensions, coating requirements and operating environment. These details help establish a specification that can be manufactured and verified.

Felix Glass focuses on custom precision optical components. Material and coating proposals should be reviewed against the finished-part requirements, with feasibility and inspection criteria confirmed before production.

Continue Reading

Other articles in this series: Infrared Imaging Principles; Germanium (Ge) Lens Guide; ZnS Infrared Window Explained; Infrared Optical Material Comparison.

Technical References

1. EUMETSAT Training: Basics of Visible and Infrared Remote Sensing — selective atmospheric absorption and infrared windows.
2. NOAA: The Atmospheric Window — atmospheric absorption and transmission concepts.
3. Crystran: Zinc Selenide — representative material transmission data.
4. Hellma Materials: Zinc Sulfide and Cleartran — thickness- and threshold-defined material ranges.
5. Crystran: Silicon — material characteristics and optical applications.
6. FLIR: Microbolometer Systems Standard Sensor Packages — an example of package-window and detector band limits.

Numerical examples are explanatory calculations, not measured product performance. Use traceable spectral data and agreed test conditions for acceptance.