Over the next several weeks, we are publishing a series of blog posts that dive into each chapter of our 11 Things You Should Know About Infrared Windows guide. This series unpacks the technical details, practical tips, and real-world considerations that every maintenance professional needs to know when specifying, installing, and using IR windows. If you want the full picture right away, you can download the complete eBook here.
Infrared inspection windows have transformed the way electrical inspections are performed, making them safer, faster, and more reliable. But to get truly accurate results, maintenance teams need to understand one critical factor: transmittance. If you don’t account for how much infrared energy actually passes through a window, you could be misled by temperature errors as large as 30%.
What Is Transmittance?
Transmittance refers to how much infrared radiation passes through an inspection window and reaches the thermal imager. Every material, whether crystal or polymer, filters energy to some degree. The type of material, its thickness, and even the camera angle all affect how much signal comes through.
For example:
- Crystal materials can vary in transmission rates depending on thickness. Thicker crystals allow less energy through.
- Material differences matter. Some, like sapphire, don’t work well with long-wave cameras, while zinc selenide is suitable across bands but costs more.
- Camera angle also plays a role, with even a 30° tilt causing measurable error.
Knowing these factors and compensating for them helps ensure accuracy.
Why Infrared Transmittance Matters in Thermal Inspections
Infrared transmittance determines how much infrared energy from a target passes through an infrared window and reaches the thermal camera. Because the window sits between the camera and the component being inspected, some of the infrared energy may be absorbed or reflected by the window material rather than reaching the detector.
This matters because thermal imagers calculate temperature based on the infrared radiation they receive. If the transmission loss through the window is not properly accounted for, the camera may report a temperature that is lower or otherwise different from the actual target temperature.
For condition-based maintenance programs, consistency is particularly important. Maintenance teams often compare thermal images collected months or years apart to identify changes in equipment condition. Using known transmittance values and consistent camera settings helps ensure that changes in recorded temperature represent changes in the asset rather than changes in the measurement process.
Accurate transmittance compensation therefore supports more reliable trending, better maintenance decisions, and greater confidence when determining whether an electrical connection or component requires further investigation.
Accuracy Depends on Compensation
Thermal imagers don’t “see” heat directly. Instead, they measure the infrared radiation that reaches the sensor. That signal includes emissions from the target, reflections from the environment, and transmission losses from the window.
By entering the correct transmission coefficient into the camera or analysis software, thermographers can offset these losses and calculate real surface temperatures. Failing to adjust for transmission can cause significant discrepancies. In one test, a window with 50% transmission caused the apparent temperature to read almost 12°C lower than the true value.
Factors That Affect Infrared Transmittance
Infrared transmittance is not simply a fixed property shared by every infrared window. Several variables determine how effectively infrared energy passes through the optic.
- Window material. Different materials transmit different portions of the infrared spectrum. A material that performs well in one wavelength range may perform poorly in another, which is why the window material must be compatible with the spectral response of the thermal camera being used.
- Material thickness. Increasing the thickness of some infrared optical materials can reduce the amount of energy transmitted through them. This makes the construction and thickness of the optic relevant when determining transmission values.
- Wavelength. Transmittance varies across the electromagnetic spectrum. A window should therefore be evaluated within the wavelength range used by the specific thermal imaging system rather than treated as having one universal transmission value.
- Viewing angle. The angle between the thermal camera and the infrared window can influence transmission and measurement accuracy. Wherever possible, inspections should be performed from a consistent viewing position and within the manufacturer’s recommended viewing angles.
- Window condition. Contamination, physical damage, chemical exposure, surface deposits, or deterioration of the optic may alter its infrared transmission characteristics over time.
- Environmental conditions. Atmospheric temperature, reflected apparent temperature, and the temperature of the window itself can influence radiometric measurements and should be considered when accurate temperature measurement is required.
Understanding these variables helps thermographers establish inspection procedures that can be repeated consistently.
Practical Methods for Compensation
Most cameras can’t directly adjust for transmission, but their software can. A simple field workaround is calculating a “compensated emissivity” by multiplying the target emissivity by the window transmission rate.
Example: If the target emissivity is 0.90 and the window transmittance is 0.55, then:
0.90 × 0.55 = 0.495.
Set the camera to 0.495, and you’ll offset both emissivity and transmission losses.
This kind of compensation keeps readings consistent, especially when using the same window over multiple inspections.
Monitoring Transmission Over Time
Transmission can degrade as windows age or are exposed to tough environments. Crystals, for instance, are vulnerable to both stress and chemical absorption. Research shows calcium fluoride windows can degrade noticeably within just two years, while polymer-based windows remain far more stable. That’s why it’s critical not only to measure transmission once, but to track it over time as part of routine maintenance.
Infrared Transmittance vs Emissivity: What’s the Difference?
Infrared transmittance and emissivity both affect thermal measurements, but they describe two different behaviors.
Emissivity describes how effectively the surface of the target itself emits infrared radiation compared with an ideal blackbody at the same temperature. Highly emissive materials emit infrared radiation effectively, while shiny or reflective materials typically have lower emissivity and can be more difficult to measure accurately.
Transmittance, by contrast, describes how much of that infrared radiation passes through another material, such as an infrared inspection window, before reaching the camera.
Think of the measurement path as two separate stages:
Target → Infrared window → Thermal camera
The emissivity of the target affects the infrared energy leaving the target. The transmittance of the window affects how much of that energy subsequently reaches the thermal camera.
Both values therefore matter. A thermographer could correctly compensate for the emissivity of a component but still obtain an inaccurate temperature measurement if the transmission loss through the infrared window is ignored.
In some measurement procedures, the target emissivity and window transmittance can be combined into an equivalent value.
For example:
Target emissivity: 0.90
Window transmittance: 0.55
0.90 × 0.55 = 0.495
The correct method ultimately depends on the capabilities of the thermal camera and analysis software being used.
The Coffee Cup Test: A Simple Field Check
One practical way to measure transmission is the “Coffee Cup Test.” Using a warm cup of water with a known emissivity target, thermographers can compare readings with and without the window in place. Adjusting the transmission coefficient until both values match provides an accurate rate.
Performing this test at installation, and again during regular maintenance, ensures accuracy as windows age. It also helps teams catch degradation before it impacts critical inspection results.
Common Mistakes When Measuring Infrared Transmittance
Small inconsistencies in inspection technique can create misleading temperature differences. Some of the most common mistakes include:
- Assuming the manufacturer’s nominal transmission value always represents actual field conditions.
- Treating transmittance as identical across every infrared wavelength.
- Using a transmission value intended for a different window material or model.
- Ignoring target emissivity when compensating for window transmission.
- Changing the viewing angle significantly between inspections.
- Measuring through a dirty, damaged, or contaminated optic without checking its condition.
- Comparing readings collected through a window with readings taken directly from a target without compensating for the window.
- Using different camera settings during successive inspections.
- Assuming the window’s transmission characteristics can never change.
- Failing to document the transmittance value used during the inspection.
Another mistake is treating a thermal image as an absolute representation of temperature without considering the variables involved in radiometry. Thermal cameras calculate temperature from detected infrared radiation, so emissivity, reflected energy, transmission, environmental conditions, and camera configuration can all influence the result.
Best Practices for Maintaining Infrared Transmittance Accuracy
Reliable thermal inspections depend on repeatability. Establishing a consistent procedure makes it easier to compare results from one inspection to the next.
Start by determining the transmittance of the installed infrared window using the manufacturer’s information or an appropriate field measurement method. Where practical, record the value with the asset information so future inspectors do not have to determine it again from scratch.
Maintain consistency by:
- Using the same camera settings whenever possible.
- Recording target emissivity and window transmittance.
- Inspecting from approximately the same position and viewing angle.
- Keeping the infrared optic clean according to manufacturer recommendations.
- Checking windows for contamination, damage, or deterioration.
- Periodically verifying transmission when environmental exposure or aging could affect the optic.
- Using a calibrated thermal camera when quantitative temperature measurements are required.
- Documenting inspection conditions so future measurements can be compared correctly.
Standardized measurement procedures are especially important when thermal data is being trended. A five-degree change means very little if one inspection was performed using different compensation values or significantly different conditions.
One common method for determining transmittance is to compare a known infrared radiation source with and without the window in the measurement path. The Coffee Cup Test described earlier in this article provides a practical field approach based on the same underlying principle.
Applications of Infrared Transmittance Across Industrial Facilities
Infrared transmittance becomes important anywhere a thermal camera measures equipment through an intervening infrared-transparent material. In industrial facilities, this is particularly relevant to electrical condition monitoring.
Infrared inspection windows can provide a repeatable viewing point for equipment including:
- Switchgear
- Switchboards
- Motor control centers
- Transformers
- Distribution panels
- Bus connections
- Circuit breakers
- Fuse connections
- Drives and control equipment
- Other energized electrical assets
The principle extends beyond electrical inspections. Infrared-transparent optics are also used in applications involving process equipment, furnaces, research systems, manufacturing equipment, and other environments where thermal radiation must travel through a protective barrier before reaching an infrared detector.
In every case, the same principle applies: the camera can only measure the infrared energy that reaches its detector. When another material sits between the target and the camera, its transmission characteristics become part of the measurement.
For maintenance teams, understanding this relationship helps separate genuine equipment temperature changes from measurement errors and supports more dependable condition-based maintenance decisions.
The Takeaway
Infrared windows improve safety and efficiency, but accuracy depends on understanding and compensating for transmittance. By tracking transmission rates, applying proper compensation, and monitoring window condition over time, thermographers ensure inspections remain both precise and reliable.
Dive Deeper Into IR Windows
There’s more to know, plus all the technical info you need to make informed decisions about safer inspections. Read our free Ebook: 11 Things You Should Know About Infrared Windows.
Frequently Asked Questions About Infrared Transmittance
What is infrared transmittance?
Infrared transmittance is the proportion of infrared radiation that passes through a material. In an infrared inspection window, it describes how much infrared energy from the target travels through the window and reaches the thermal camera.
Transmittance may be expressed as a percentage or as a decimal between 0 and 1. For example, 60% transmittance can also be represented as 0.60.
Why is infrared transmittance important for thermal inspections?
Infrared transmittance affects the amount of energy reaching the thermal camera. If some of the radiation is absorbed or reflected by an infrared window and that transmission loss is not compensated for, the camera can calculate an inaccurate target temperature.
Accurate compensation is particularly important when technicians are collecting quantitative temperature measurements or comparing results over time.
What does 100% infrared transmittance mean?
A theoretical transmittance of 100%, or 1.0, would mean that all infrared radiation within the relevant wavelength range passes through the material without being absorbed or reflected.
Real optical materials generally introduce some degree of transmission loss. For thermography, the important value is the actual transmittance of the material within the wavelength band used by the thermal camera.
What factors affect infrared transmittance?
Infrared transmittance can be influenced by the material used, its thickness, infrared wavelength, viewing angle, surface condition, contamination, temperature, and environmental exposure. The spectral response of the thermal camera is also important because different cameras operate within different infrared wavelength bands.
These variables are why the transmittance value used during an inspection should correspond to the actual window and thermal imaging system.
How is infrared transmittance measured?
One method involves measuring a stable infrared radiation source without the window and then repeating the measurement with the window positioned between the source and thermal camera. Camera settings are adjusted or the results are compared to determine the transmission loss introduced by the window.
The Coffee Cup Test described in this article provides a practical field method using this comparison principle.
Can infrared transmittance change over time?
Yes. Depending on the material and operating environment, transmission performance can potentially change because of contamination, physical damage, chemical exposure, moisture, surface degradation, or changes to the optic itself.
For this reason, infrared windows should be visually inspected and transmission should be periodically verified when conditions indicate that performance may have changed.
What materials affect infrared transmission?
Many materials interact differently with infrared radiation. Materials used for infrared optics can include polymers and crystalline materials such as calcium fluoride, sapphire, and zinc selenide. Their usefulness depends partly on the wavelengths they transmit and the thermal imaging equipment being used.
Materials that appear transparent to visible light are not necessarily transparent to infrared radiation, and materials that appear opaque to the human eye may transmit certain infrared wavelengths.
What is the difference between infrared transmittance and emissivity?
Emissivity describes how efficiently a surface emits infrared radiation. Transmittance describes how much infrared radiation passes through a material.
When inspecting through an infrared window, both can affect temperature accuracy. The target has an emissivity value, while the window has a transmittance value. Both variables must be considered when accurate quantitative temperature measurement is required.
Can infrared radiation pass through all materials?
No. Whether infrared radiation passes through a material depends on the material and the wavelength of the radiation.
A material may transmit some wavelength ranges while absorbing or reflecting others. This is why infrared window materials must be selected to match the wavelength range of the thermal camera.
How does infrared transmittance affect temperature measurement accuracy?
Lower transmittance means less infrared energy from the target reaches the camera. If the thermal imaging system assumes more energy is being transmitted than actually reaches the detector, the resulting calculated temperature can be inaccurate.
Thermal imaging systems that provide a transmission setting allow the thermographer to enter the appropriate value so the system can compensate for the infrared window.
This is particularly important when temperatures are being used for trending, alarm thresholds, severity assessments, or maintenance decisions rather than simply identifying relative hot spots.
