Long-Wave-Pass Infrared Filters: Key Checks For Harsh Outdoor Thermal Imaging Deployments

A thermal camera image can appear clear during commissioning, but once fog develops—which can reduce contrast as well—ideally with a wet protective window or the camera operating over a large temperature range. The cause is not darkness itself: passive thermal cameras do not detect visible light, but thermally emitted infrared radiation. Before replacing a filter, engineers have to disentangle environmental losses from optical assembly issues.

BoDian Optical Provide Optical Filters for Thermal Detection and industrial Monitoring Long-wave-pass products offer various regions where different wavelengths are transmitted, making it possible for designers to assess the spectral characteristics of a filter with regard to both the optical path and detector on their camera. These checks save teams from falling into the trap of believing a filter is an antidote to any image issue.

What Threatens a Thermal Camera Image in Harsh Outdoor Conditions?

Record what changed: distance, weather, surface temperature, lens condition, or camera settings. This helps prevent unnecessary replacement.

Atmospheric Loss from Fog, Rain, and Humidity

Water vapor absorbs portions of infrared radiation, while droplets in fog and rain scatter and absorb energy along the viewing path. As distance increases, weaker target signals may reduce thermal imaging contrast. For thermal imaging in fog and rain, assess the scene and viewing range before blaming the filter. A long-wave-pass element cannot restore radiation lost between the target and the camera.

Reflections, Condensation, and Window Contamination

If the protective windows are wet and dirty, incoming radiation can be attenuated or scattered. A further issue with shiny metal surfaces is that reflected objects may also appear to be hotspots better than the surface temperatures actually are nearby. Test if the thermal camera changes image after checking out the window or shifting perspective. For background on filtering and image formation, see this earlier article on infrared filters and low-light imaging.

Temperature Swings and Mechanical Stress

Outdoor housings expand and contract as temperatures change. Vibration or an over-tightened mount can also disturb optical alignment. First inspect mounts, seals, and alignment; then assess the filter if problems persist.

Which Long-Wave-Pass Filter Specifications Matter Most Outdoors?

A filter should be chosen from the detector response and system transmission requirements, not simply from its model number. The infrared long-wave-pass filter range includes several products that should be assessed against different camera architectures.

Cutoff Alignment with Detector Response

For infrared long pass filter selection, overlay the detector sensitivity curve with the proposed filter’s transmission curve. The listed high-transmission region of ILP8200 is 9000–14000 nm, whereas ILP7700 is listed at 7900–11000 nm. If a detector depends on wavelengths beyond 11 μm, ILP7700 may remove useful signal. If it relies heavily on 8–9 μm, ILP8200 deserves closer examination.

Transmission and Out-of-Band Blocking

According to the published ILP8200 specification, they purported a peak transmittance (Tp) of 80% at the least. It also states that in the blocking region from 1500–7100 nm, average transmittance is no worse than 1%. These are different statistics of transmittance, namely, peak not an average across the working band. Reminder: For blocking infrared filter transmission for thermal cameras, ask the supplier to share the entire measured curve, including transition region and actual required stop-band wavelengths.

Substrate Compatibility and Optical Geometry

The ILP8200,ILP7700 and ILP10600 products listed use Ge substrates. The lens and protective window are associated with substrate choice. Before approval, verify clear aperture and thickness in mounting clearance angle of incidence One geometry will not predict another assembly of a spectral curve measured. The spectral curve takes one geometry into account while not being able to predict a different assembly.

Which BoDian Long-Wave-Pass Filter Fits Your Outdoor Imaging System?

The three filters below are alternatives for different spectral requirements, not a ranking by image quality. A better thermal camera image depends on how well each candidate fits the detector, optics, and intended task.

ILP8200 for Selected LWIR Imaging Systems

The ILP8200 Infrared Long Wave Pass Filter can be evaluated for security monitoring, industrial temperature measurement, and thermal radiation detection where its specified transmission region matches the detector response and optical system requirements.It is a useful starting candidate for a compatible LWIR system, provided the optical budget and detector response do not require substantial transmission below 9 μm.

ILP7700 for Systems Requiring Shorter LWIR Coverage

ILP7700 includes a specified 7900–11000 nm high-transmission range where Tp ≥90%. That might be a fit for designs with useful detector response in the shorter segments of LWIR spectrum. Note that the listed peak value, by itself, does not guarantee improved sensitivity of your system. Engineers should determine how much radiation the combined detector and optics would lose at wavelengths longer than 11 μm.

ILP10600 for Specialized Long-Wavelength Detection

There was also a high-transmission range listed from 11000–14000 nm for ILP10600, together with Tp ≥80%. It can become relevant whenever a system is defined about that longer-wavelength cutoff. It should not be blindly swapped into a lower-band camera without confirming the detector response and that sufficient signal remains for whatever contrast (or measure) task you seek.

How Should Engineers Test and Maintain Filters Before Outdoor Deployment?

A supplier specification is the starting point, not the final acceptance test. Thermal imaging filter testing should consider both the filter itself and its effect on the assembled module.

Spectral Verification Before Installation

Obtain measured transmission measurements through the proposed part and compare to approved spectral limits Optical Experimental Measure 3 FTIR the Spectrum and equipped with an BoDian Optical measurement calibration instrument, including Cary 5000/Cary7000 testing equipment. The test method, wavelength coverage and reporting format should be confirmed by the buyers themselves Make sure to check the test method, wavelength range and reporting format.

Outdoor Validation Under Representative Conditions

Be sure to test the camera at set target temperature and distance, before introducing any outdoor variables. Perform checks again after exposure to the moisture, temperature change and vibration expected in use. When evaluating changes in the thermal camera image, keep detector settings constant Set-up pass/fail limits Starting from Now record the conditions under which results were obtained.

Inspection and Maintenance to Reduce Long-Term Risk

Check the dirty window moinsture and scratch on outer windows; check sealsand mounts, when you are cleaned internal lenses. Articles to be coated should comply with the approved requirements for cleaning surfaces and substrates. This helps set benchmark inspection records rather than promising a random maintenance period. A broken housing seal or a loose mount will not be fixed with the replacement filter.

What Should Buyers Confirm with BoDian Optical Before Ordering?

The most useful buying request outlines the entire optical issue. A part number by itself does not tell the supplier if a different cutoff, package size or installation arrangement is needed for your camera. Just a part number does not let the supplier know that it requires another cutoff, package size or installation arrangement in relation to the camera.

Custom Filter Design and Prototype Support

BoDian Optical claims that it can manufacture customized wavelengths, sizes & frame designs and work from customer drawings or samples and prototype-to-production specifications. For custom infrared filters for thermal cameras, send detector response data, passband and blocking targets, clear aperture, quantity, and the expected outdoor conditions.

Coating Capabilities and Quality Documentation

Its manufacturing and service information release lists Helios 800 and OTFC-1300 coating equipment, as well as optical measurement instruments. That defines what process and resources you have for inspection, not similar outdoor qualifications of each ILP model. Create a check list of what spectral report, dimensional record and environmental test evidence can be provided for the contender part being suggested

Technical Consultation and Contact

Prior to approving a sample, determine what will be measured and how acceptance of that measurement will be decided. This can include the detector curve, optical drawing-digitization values, operating angle for direction of incoming flux, expected exposure to temperature and moisture in sophisticated applications & expertise on target quantities. Rather than selecting from perfect transmittance, it allows for a meaningful comparison between catalog models and an optimized coating.

Conclusion

Requires detection on whole signal path for stable outdoor thermal images. These include identification of environmental and mechanical defects, matching the long-wave-pass filter with detector results, verifying an assembled camera prior to release. These steps provide a transparent basis for procurement teams to assess the performance of thermal camera images and sample acceptability. This provides a more definitive basis for sample assessment and thermal image performance evaluation by procurement teams.

If your team is comparing ILP models or has an unresolved image-loss problem, share the detector response, required wavelength range, package drawing, and planned test conditions through the contact page. These details help establish whether an existing filter fits the design or whether a different coating and sample evaluation are required.

FAQ

Q: Can a long-wave-pass filter improve a thermal camera image in heavy fog?

A: It can help define the detector’s received spectral range, but it cannot undo atmospheric absorption or scattering. First check distance, weather, window condition, and detector response.

Q: Is a higher peak transmittance always better for outdoor thermal imaging?

A: No. Compare transmission across the wavelengths the detector actually uses, along with blocking and losses elsewhere in the optical path. A peak value alone is insufficient.

Q: What should be checked before purchasing a custom infrared filter?

A: Provide detector response, passband and blocking requirements, substrate and dimensions, operating angle, environmental conditions, and an agreed method of testing the finished camera.