Infrared Imaging Principles
Infrared imaging converts invisible infrared radiation into an image that reveals differences in a scene. Depending on the wavelength band and application, those differences may come from reflected illumination, thermal emission or both.
For engineers and equipment manufacturers, understanding these principles is the starting point for specifying infrared lenses, protective windows, domes and coatings. A detector can only use the radiation that the complete optical path delivers.
1. What Is Infrared Imaging?
An infrared imaging system collects radiation from a scene, forms an image on a compatible detector and converts the detector response into electronic image data. Infrared wavelengths lie beyond the red end of the visible spectrum.
The term covers more than thermal cameras. Near-infrared and short-wave infrared systems commonly image reflected light, while mid-wave and long-wave infrared cameras are widely used to image thermal emission. Selecting a band therefore begins with a practical question: what physical difference must the camera detect?
Key principle: Infrared imaging is a broad category. Thermal imaging is one part of it, and quantitative temperature measurement requires additional calibration and compensation.
3. NIR, SWIR, MWIR and LWIR Explained
Common infrared imaging bands and practical uses
| Band | Typical working range | Common signal source | Example applications |
|---|
| NIR | Approximately 0.75–1.0 µm in many silicon-camera applications | Reflected ambient or active illumination | Machine vision, illuminated night imaging |
| SWIR | Often 0.9–1.7 µm with standard InGaAs detectors; extended systems cover other ranges | Usually reflected light at ordinary scene temperatures | Material sorting, moisture contrast, semiconductor inspection |
| MWIR | Commonly 3–5 µm | Thermal emission, with possible reflected contributions | Industrial process observation, hot-object imaging, specialized thermal systems |
| LWIR | Commonly 8–12 µm or approximately 8–14 µm | Thermal emission, with reflected background contributions | Building inspection, equipment monitoring, thermal observation |
Band boundaries vary between conventions and detector technologies. Specify numerical wavelengths rather than relying only on “SWIR” or “LWIR.” A camera operating from 8 to 12 µm does not require exactly the same optical specification as one operating from 8 to 14 µm.
SWIR can also detect thermal emission from sufficiently hot objects. The distinction between reflected-light imaging and thermal imaging depends on temperature and signal strength as well as wavelength. See technical reference [1].
4. Why Objects Emit Infrared Radiation
Objects above absolute zero emit thermal radiation. For an ideal blackbody, Planck's law describes how spectral radiance varies with wavelength and temperature. Raising temperature increases emission and shifts the peak toward shorter wavelengths.
Wien's displacement law gives the peak of the wavelength-based blackbody spectrum:
λmax ≈ 2898 / T, where wavelength is in micrometers and temperature is in kelvin.
At 300 K, approximately 27°C, the peak is about 9.7 µm. This helps explain the usefulness of LWIR for observing objects near room temperature. It does not mean that all their radiation occurs at 9.7 µm or that another band cannot detect them.
Real surfaces depart from ideal blackbody behavior. Their spectral emissivity, surroundings and viewing geometry affect the radiation measured by a camera.
5. Emissivity, Reflection and Temperature Measurement
Emissivity describes how strongly a surface emits relative to a blackbody at the same temperature and wavelength. It depends on material, finish, wavelength, angle and sometimes temperature.
For an opaque surface, a simplified spectral model is:
Lsurface = εLblackbody(T) + (1 − ε)Lbackground
This model assumes an appropriate effective background radiance and omits atmosphere and intervening optics. Semitransparent targets require a transmitted-radiation term as well.
A polished metal surface may reflect a warm person or nearby machinery, creating an apparent hot area. Accurate thermography therefore needs appropriate emissivity and reflected-background settings, together with compensation for the atmosphere and any protective window. Window temperature can also matter. See references [2] and [3].
6. Atmospheric Transmission and Working Distance
The atmosphere does not transmit every infrared wavelength equally. Water vapor, carbon dioxide and other gases absorb selected wavelengths. The commonly used MWIR and LWIR bands overlap useful atmospheric transmission regions, but these regions are not perfectly transparent.
Humidity, path length, rain, fog, aerosols and temperature conditions influence the available signal. Infrared imaging may improve visibility in some conditions, but “seeing through fog” is not a universal capability.
For a short-range inspection station, component transmission and focus may dominate. For longer paths, atmospheric attenuation and target contrast become increasingly important. Choose the band against the intended environment and distance, not a generic transmission chart alone.
7. Detector Types and Their Optical Requirements
Photon detectors
Photon detectors generate an electrical response through interactions between incident photons and the detector material. InGaAs is common in SWIR systems. Other semiconductor detector technologies serve MWIR and LWIR applications; many high-performance thermal photon-detector systems require cooling.
Thermal detectors
Uncooled microbolometers detect radiation through a small temperature-related change in the sensing element, typically read as a resistance change. They are widely used in LWIR cameras.
The detector determines more than the spectral band. Pixel pitch, active area, acceptance angle and packaging constrain lens focal length, f-number, image circle and mechanical clearance. The optic and detector must be designed as a compatible system.
8. How Infrared Lenses Control Image Quality
A lens determines field of view, focus and the transfer of scene detail to the detector. For an ideal rectilinear system focused near infinity:
Horizontal field of view ≈ 2 arctan(sensor width / (2 × focal length))
As an illustrative calculation, a 640-pixel-wide detector with 12 µm pixel pitch has a width of 7.68 mm. With a 25 mm focal-length lens, its horizontal field of view is approximately 17.5°. Actual distortion and usable image area can change the result.
The f-number is focal length divided by entrance-pupil diameter. A lower f-number generally delivers more irradiance from an extended scene, subject to transmission, vignetting and detector acceptance. Thermal sensitivity specifications must be compared at stated optical conditions [4].
Resolution also depends on aberrations, diffraction, focus, sampling and modulation transfer function (MTF). For an ideal circular aperture, the Airy-pattern first-zero diameter is approximately 2.44λN. At 10 µm and f/1, it is about 24.4 µm; this is a diffraction scale, not a stand-alone camera resolution rating.
9. Why Protective Windows and Domes Matter
A protective window or dome shields the camera from its environment while transmitting the required radiation. It remains part of the optical system even when it has no intended focusing function.
- Absorption and reflection reduce the radiation reaching the detector.
- Surface error and transmitted wavefront error can reduce image quality.
- Wedge, tilt and mounting stress can introduce beam deviation or image changes.
- Thickness and position affect optical path length and can shift focus in a converging beam.
- Dome curvature and alignment can introduce field-dependent aberrations.
- Window self-emission can influence thermal measurements.
Evaluate the complete assembly at its actual field angles and operating temperatures. A room-temperature transmission measurement alone does not demonstrate imaging performance or measurement accuracy.
10. Optical Materials Must Match the Imaging Band
Material considerations for infrared optical components
| Material | Relevant use | Selection consideration |
|---|
| Fused silica and suitable optical glasses | NIR and selected SWIR applications | Grade and absorption bands matter; ordinary visible optical glass is not a practical LWIR camera window. |
| Sapphire | Durable windows for suitable NIR, SWIR and portions of MWIR | Check the long-wavelength absorption edge against thickness and temperature; not a standard 8–14 µm window. |
| Silicon (Si) | Selected SWIR and MWIR components | Verify grade, absorption and coating; do not assume suitability across the full 8–14 µm band. |
| Zinc selenide (ZnSe) | Infrared lenses and windows, including thermal imaging | Specify actual thickness, surface protection and coating performance. |
| Zinc sulfide (ZnS) | Infrared protective windows and imaging components | Standard and multispectral grades differ; visible transmission and scatter requirements need explicit review. |
This is a selection overview, not a guaranteed transmission specification. Request data for the actual material grade, part thickness, coating and operating conditions. Material-family characteristics are discussed in reference [5].
11. AR Coatings, Protective Coatings and Throughput
Anti-reflection coatings reduce surface reflection within a specified spectral and angular range. High-index infrared substrates can lose substantial energy at uncoated interfaces, making coating design especially important.
At normal incidence, an ideal lossless air-to-material interface has reflectance R = ((n − 1) / (n + 1))². A hypothetical refractive index of 2.4 gives approximately 17% reflection at one uncoated interface. This is an illustrative interface calculation, not a material datasheet or complete-window transmission value.
For a first throughput budget, multiply component transmission values at the same wavelength: τsystem(λ) ≈ τwindow(λ) × τlens(λ) × τfilter(λ). Multiplying unrelated broadband averages can be misleading because the spectra may have different shapes.
DLC or other protective coatings may improve environmental durability in suitable designs, but their compatibility and optical loss require evaluation. Specify each surface separately, including wavelength range, angle of incidence, polarization if relevant, and whether transmission is an average or a minimum requirement.
13. How to Specify an Infrared Optical Component
Provide the following information when requesting an engineering review or quotation:
- Application, imaging band and whether quantitative temperature measurement is required.
- Detector format, pixel pitch, field of view and working distance where relevant.
- Component type, material preference, drawing revision and 3D model if available.
- Dimensions, thickness, clear aperture, mounting features and tolerances.
- Surface quality, surface figure or transmitted wavefront requirement, with test wavelength.
- Coating requirements for each face and complete-part transmission acceptance criteria.
- Operating temperature, humidity, pressure, cleaning method and environmental exposure.
- Prototype quantity, production quantities, inspection documents and delivery target.
For example, a request for a protective window for an 8–12 µm camera should identify the window thickness, clear aperture, maximum ray angle, required band-average or minimum transmission and mounting environment. “High infrared transmission” alone is not a measurable acceptance criterion.
14. Frequently Asked Questions
Is infrared imaging the same as thermal imaging?
No. Infrared imaging includes reflected-light methods such as many NIR and SWIR systems. Thermal imaging primarily uses emitted radiation to visualize scene differences.
Can a thermal camera see in complete darkness?
A passive thermal camera can operate without visible illumination because objects emit thermal radiation. Useful image contrast still depends on the scene, detector and optical system.
Can a thermal camera see through ordinary glass?
Typical LWIR cameras cannot image a scene through ordinary window glass. They primarily detect radiation associated with the glass surface and reflections. Some NIR and SWIR systems can image through suitable glass within its transmission band.
Can thermal imaging see through walls?
A conventional thermal camera does not see directly through a wall. It can reveal surface-temperature patterns that may result from conditions behind the wall, such as insulation differences or heat flow.
Does a lower NETD mean better temperature accuracy?
No. Lower NETD indicates finer thermal sensitivity under comparable conditions. Absolute accuracy also depends on calibration, surface emissivity, reflected radiation, atmospheric effects and intervening optics.
Is MWIR always better than LWIR?
No. The better choice depends on target temperature, environment, detector technology, range, image requirements, size, power and cost. Band selection must follow the application.
Can I add an infrared window without changing calibration?
Do not assume so. The added window changes transmission and may contribute its own thermal emission. Radiometric systems may require window compensation and validation or recalibration in the final configuration.
Does an 8–15 µm material range guarantee an 8–15 µm imaging system?
No. Usable system response depends on the detector, coating, material thickness, atmosphere and acceptance threshold. A nominal material range is not a guarantee of flat transmission or system sensitivity.
15. Discuss Your Infrared Optics Requirements with Felix Glass
Felix Glass focuses on custom precision optical components for integration into customer systems. For an infrared window, lens or dome project, share the operating band, drawing, environmental conditions and required quantities so the proposed material, coating and inspection requirements can be reviewed together.
Component specifications should support the complete camera design. Transmission, imaging quality, durability and temperature-measurement performance are separate requirements and should be verified accordingly.
Request a Custom Infrared Optics Review
Send your PDF drawing and STEP file, if available, together with your wavelength range and prototype or production quantities.
Discuss Your Project
Email: james@felixglass.com
Dongguan Felix Glass Co., Ltd.
Continue Reading in the Infrared Optics Guide
Related topics in this series: Germanium (Ge) Lens Guide; ZnS Infrared Window Explained; IR Transmission Range Guide (8–15 µm); Infrared Optical Material Comparison.
Technical References
- Edmund Optics: What Is SWIR? — SWIR imaging characteristics and detector bands.
- FLIR: How Does Emissivity Affect Thermal Imaging? — emissivity and reflected background radiation.
- Teledyne FLIR OEM: Setting Up Radiometry — environmental parameters and window compensation.
- Teledyne FLIR OEM: Lens F-number, Camera Performance and NEdT — optical conditions in thermal-sensitivity comparisons.
- Edmund Optics: Infrared Capabilities — infrared material selection overview.
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