Electrical thermography allows maintenance and reliability teams to identify abnormal heat patterns without interrupting normal equipment operation. By detecting temperature differences across connections, conductors, protective devices, and other components, infrared inspections can reveal developing problems before they cause equipment damage or unplanned downtime.
However, a thermal camera does not measure surface temperature directly. It detects infrared radiation and calculates an apparent temperature based on several inputs, including emissivity, reflected temperature, distance, atmospheric conditions, and the transmission properties of any infrared inspection window in the optical path.
Of these variables, emissivity is one of the most important and one of the most frequently misunderstood. An incorrect emissivity setting can produce misleading temperature readings, particularly when inspecting shiny metals and other low-emissivity materials commonly found in electrical equipment.
Understanding how emissivity affects infrared measurements helps thermographers collect more repeatable data, recognize measurement limitations, and make better maintenance decisions.
What Is Emissivity in Thermal Imaging?
Emissivity describes how efficiently a surface emits infrared energy compared with an ideal emitter at the same temperature.
Emissivity is expressed as a value between 0 and 1. A value close to 1 represents a highly efficient infrared emitter, while a value closer to 0 represents a poor infrared emitter.
Most nonmetallic, painted, coated, oxidized, or weathered surfaces have relatively high emissivity. Smooth, polished, and reflective metals typically have low emissivity.
An ideal theoretical surface, known as a blackbody, has an emissivity of 1.0. Real materials have lower emissivity values, and those values may change depending on surface condition, viewing angle, temperature, wavelength, oxidation, contamination, and finish.
In electrical thermography, emissivity determines how much of the radiation detected by the camera comes from the target itself. A high-emissivity surface emits a strong thermal signal. A low-emissivity surface emits less infrared energy and reflects more radiation from surrounding objects.
This is why a shiny electrical connection can appear warmer or cooler than it actually is. The camera may be detecting reflected energy from the thermographer, nearby equipment, overhead lighting, the sky, a furnace, or another heat source rather than radiation emitted by the component.
Why Emissivity Is Important for Infrared Inspections
Thermal cameras convert detected infrared radiation into temperature values. To make this calculation accurately, the camera must account for how efficiently the target surface emits infrared energy.
When the emissivity setting matches the surface being inspected, the calculated temperature is more likely to represent the actual surface condition. When the setting is incorrect, the resulting measurement can be significantly inaccurate.
Incorrect readings can lead to several problems:
- A developing electrical fault may appear less severe than it really is.
- A healthy component may appear unusually hot.
- Similar components may appear to have different temperatures because their surface finishes differ.
- Maintenance teams may assign the wrong priority to a repair.
- Trending data may become unreliable.
- Inspection results may be difficult to reproduce.
- Temperature-based alarm thresholds may be applied incorrectly.
Emissivity becomes especially important when a maintenance decision depends on an absolute temperature measurement. A thermographer comparing similar components under similar loads may still detect meaningful thermal differences, but precise temperature measurement requires greater control over emissivity and other measurement variables.
For this reason, electrical thermography should evaluate both temperature and thermal pattern. The location, shape, distribution, and relative severity of heating often provide valuable diagnostic information even when the exact surface temperature is uncertain.
How Thermal Cameras Interpret Emissivity
A thermal camera detects infrared radiation within its operating wavelength range. The camera then estimates the target temperature using the amount of radiation received and the measurement parameters entered by the user.
The detected radiation may include:
- Energy emitted by the target
- Energy reflected from surrounding objects
- Energy emitted or absorbed by the atmosphere
- Energy transmitted through an infrared window, lens, or other optical material
The emissivity setting helps the camera estimate how much of the detected energy originated from the target itself.
For a high-emissivity target, most of the detected energy comes from the surface. Reflected energy generally has less influence on the result.
For a low-emissivity target, the surface emits less infrared radiation and reflects more of its surroundings. The camera must therefore separate a relatively weak emitted signal from a potentially significant reflected component. This makes the calculated temperature more sensitive to incorrect camera settings and environmental conditions.
Other settings may also affect the measurement, including:
- Reflected apparent temperature
- Target distance
- Relative humidity
- Atmospheric temperature
- Window transmission
- Viewing angle
- Focus
- Measurement range
- Camera calibration
Emissivity should never be treated as the only variable that matters. Accurate thermography requires the thermographer to consider the complete measurement environment.
High vs Low Emissivity Surfaces
High-emissivity surfaces generally provide more stable and reliable infrared measurements. These surfaces emit a strong thermal signal and are less affected by reflected energy.
Examples of relatively high-emissivity surfaces include:
- Electrical tape
- Matte paint
- Rubber insulation
- Plastics
- Oxidized metals
- Corroded surfaces
- Ceramic components
- Dirt or contamination
- Purpose-designed emissivity labels or coatings
Low-emissivity surfaces emit less infrared energy and act more like mirrors in the infrared spectrum. Their apparent temperature may change as the camera angle or surrounding environment changes.
Examples of low-emissivity surfaces include:
- Polished aluminum
- Shiny copper
- Bright busbars
- Stainless steel
- Galvanized metal
- Chrome-plated components
- Clean metallic terminals
- Reflective equipment enclosures
Surface condition can be just as important as the underlying material. For example, newly polished copper may have very low emissivity, while oxidized or weathered copper may have a substantially higher emissivity.
A material should therefore not be assigned an emissivity value based only on its name. The thermographer must consider the actual surface being inspected.
Common Problems Caused by Low Emissivity Surfaces
Low-emissivity surfaces create several challenges during electrical inspections.
Reflected Thermal Energy
A shiny component may reflect heat from nearby objects. The reflected pattern can look like a genuine hot spot even though the component itself is not overheating.
A thermographer standing in front of reflective equipment may even see a thermal reflection of their own body. Moving to a different angle may cause the apparent hot spot to move or disappear.
Underestimated Temperatures
If a low-emissivity target is measured using a high emissivity setting, the camera may report a temperature that is lower than the actual surface temperature.
This can make a serious electrical problem appear less urgent than it is.
Unstable Measurements
Small changes in viewing angle, distance, background temperature, or camera position can produce noticeable changes in the displayed temperature.
This instability makes it difficult to compare readings across repeated inspections.
False Temperature Differences
Two components operating at the same temperature may appear different if they have different surface finishes. For example, a painted connection and an adjacent polished connection may display different temperatures even when their actual temperatures are similar.
Poor Trending Data
Condition-based maintenance programs depend on repeatable measurements. If the emissivity value, inspection angle, load, distance, or measurement location changes between inspections, the resulting trend may reflect inspection inconsistency rather than a genuine change in equipment condition.
Misclassification of Fault Severity
Maintenance teams may use temperature rise, component comparisons, or established alarm criteria to prioritize repairs. Inaccurate emissivity settings can cause a fault to be placed in the wrong severity category.
How to Improve Thermal Imaging Accuracy on Low Emissivity Surfaces
Low-emissivity surfaces require a controlled and repeatable inspection method. Several techniques can improve measurement confidence.
Use a High-Emissivity Reference Target
Where permitted, apply a known high-emissivity material to the component. Suitable options may include:
- Matte thermal paint
- High-emissivity labels
- Electrical tape rated for the application
- Purpose-designed emissivity targets
- Approved coatings
The camera can then measure the reference target rather than the reflective metal itself.
Any material applied to electrical equipment must be suitable for the operating temperature, voltage environment, surface, and equipment design. It must not interfere with clearances, cooling, insulation, maintenance, or equipment operation.
Compare Similar Components
Comparative thermography can be useful when inspecting similar components under similar loads and environmental conditions.
Examples include comparing:
- Three phases of the same circuit
- Identical breakers
- Similar fuse connections
- Matching cable terminations
- Parallel conductors
- Repeated components in identical equipment
The surfaces, loads, and inspection angles should be comparable. A difference in surface finish can otherwise create a false thermal difference.
Change the Viewing Angle
Reflections often change when the camera moves. If a suspected hot spot shifts, disappears, or changes shape as the viewing angle changes, it may be reflected energy rather than actual heating.
The thermographer should avoid extreme viewing angles because apparent emissivity generally decreases as the angle becomes more oblique.
Control Reflected Apparent Temperature
For more accurate quantitative measurements, reflected apparent temperature should be estimated and entered into the camera.
This setting represents the effective temperature of the surroundings reflected by the target. It can be particularly important when the target has low emissivity or when the surrounding environment is much hotter or colder than the component.
Standardize the Inspection Method
Record and repeat the following wherever practical:
- Camera model
- Lens
- Emissivity setting
- Reflected apparent temperature
- Target distance
- Inspection angle
- Measurement location
- Equipment load
- Ambient conditions
- Infrared window identification
- Window transmission value
- Focus and image composition
Standardization improves the value of historical comparisons and reduces variation between thermographers.
The Role of Thermal Paint and Emissivity Reference Materials
Thermal paint and other reference materials create a surface with a known or more predictable emissivity. They can provide a consistent measurement point on reflective electrical components.
Common reference materials include:
- Matte black paint
- High-temperature coatings
- Electrical tape
- Adhesive emissivity labels
- Factory-applied thermal targets
- Permanent high-emissivity markers
These materials can improve repeatability because the camera measures the reference surface rather than the exposed metal.
However, not every paint, label, or tape has the same emissivity. The material must be evaluated for the camera’s wavelength range and expected operating conditions. Its temperature rating, adhesion, durability, electrical suitability, and chemical resistance must also be considered.
Temporary reference materials can be useful during controlled testing, while permanent targets are generally better for repeated condition monitoring. A permanent target helps ensure that future inspections are performed at the same location using a similar surface.
When reference materials are used, inspection procedures should document:
- The material type
- Its expected emissivity
- The installation location
- The date of application
- Its temperature rating
- Any signs of damage, contamination, or deterioration
A reference target should be replaced if its surface condition changes enough to affect its thermal characteristics.
Best Practices for Emissivity in Electrical Thermography
A reliable emissivity procedure should be part of the organization’s written electrical inspection program.
Recommended practices include:
- Identify the target material and its surface condition.
- Use a verified emissivity value whenever quantitative measurement is required.
- Avoid assuming that every component has an emissivity of 0.95.
- Use high-emissivity reference targets on reflective components where appropriate.
- Account for reflected apparent temperature.
- Keep the viewing angle as close to perpendicular as practical.
- Avoid measuring through ordinary glass, plastic covers, or unknown materials.
- Confirm the transmission value of any infrared inspection window.
- Use the same measurement point during repeated inspections.
- Record the equipment load and operating condition.
- Maintain consistent camera settings.
- Capture both thermal and visual images.
- Document uncertainty when surface properties prevent an accurate measurement.
- Use thermal patterns and comparative data alongside absolute temperatures.
- Provide appropriate thermography training for inspection personnel.
No single camera setting will provide accurate measurements for every surface. The procedure must reflect the materials and conditions found in the specific facility.
Emissivity Best Practices for Electrical Inspections
Electrical systems contain a mixture of metals, insulation materials, coatings, polymers, ceramics, and painted surfaces. A practical inspection strategy must account for these differences.
Before beginning an inspection, the thermographer should review the equipment design and identify likely measurement targets. Reflective conductors, busbars, terminals, and connection hardware may require reference materials or comparative inspection methods.
During the inspection:
- Confirm that the equipment is operating under a representative load.
- Verify that the camera is focused correctly.
- Inspect the overall thermal pattern before placing measurement markers.
- Compare similar components whenever possible.
- Look for changes in apparent temperature when the viewing angle changes.
- Measure a high-emissivity target when one is available.
- Record the emissivity and reflected temperature settings.
- Account for the transmission value of any infrared window.
- Document limitations that may affect the result.
- Escalate uncertain findings for further investigation.
The thermographer should avoid reporting a highly precise temperature when the measurement conditions do not support that level of certainty. A reading shown to one decimal place is not necessarily accurate to one decimal place.
Common Materials and Their Emissivity Values
Emissivity values vary by surface finish, temperature, wavelength, oxidation, contamination, and measurement method. The following values are approximate examples rather than universal settings.
| Material or Surface | Approximate Emissivity Range |
|---|---|
| Black electrical tape | 0.90 to 0.97 |
| Matte black paint | 0.90 to 0.98 |
| Painted metal | 0.80 to 0.95 |
| Rubber insulation | 0.90 to 0.98 |
| PVC insulation | 0.85 to 0.95 |
| Porcelain or ceramic | 0.85 to 0.95 |
| Oxidized copper | 0.60 to 0.90 |
| Heavily oxidized aluminum | 0.30 to 0.80 |
| Weathered or corroded steel | 0.60 to 0.90 |
| Stainless steel, oxidized | 0.30 to 0.80 |
| Bright copper | 0.02 to 0.10 |
| Polished aluminum | 0.02 to 0.10 |
| Polished stainless steel | 0.05 to 0.20 |
| Galvanized steel | 0.20 to 0.40 |
These ranges demonstrate why generic material tables should be used carefully. A polished surface and an oxidized surface made from the same metal can behave very differently.
When measurement accuracy is important, the thermographer should verify the emissivity under the actual inspection conditions or measure a known reference target attached to the component.
How Emissivity Supports Predictive Maintenance Programs
Electrical thermography is commonly used within predictive and condition-based maintenance programs to identify abnormal heating before it develops into a failure.
Accurate and repeatable emissivity settings improve these programs by allowing teams to:
- Compare equipment condition over time
- Recognize meaningful temperature changes
- Prioritize corrective work
- Evaluate the effectiveness of completed repairs
- Identify recurring connection problems
- Support risk-based maintenance decisions
- Reduce unnecessary intrusive inspections
- Build more reliable asset histories
The value of trending depends on measurement consistency. If one inspection uses an emissivity setting of 0.95 and another uses 0.30, the apparent temperature may change even if the equipment condition remains the same.
For trend data to be useful, the inspection procedure should standardize the target, surface condition, camera settings, window, distance, angle, load, and environmental conditions as much as practical.
Temperature should also be considered alongside other information, including:
- Load current
- Equipment history
- Fault type
- Component criticality
- Ambient temperature
- Similar component temperatures
- Previous inspection results
- Ultrasound findings
- Partial discharge data
- Maintenance records
Thermography is most effective when it contributes to a broader condition-based maintenance process rather than being treated as an isolated activity.
Infrared Inspection Challenges in Industrial Environments
Industrial facilities present several conditions that can affect infrared measurement accuracy.
Changing Equipment Loads
Electrical heating often changes with current. A connection may appear normal under a light load but become significantly hotter as demand increases.
Inspection reports should record the equipment load whenever possible.
Reflections from Nearby Heat Sources
Furnaces, steam lines, motors, lighting, sunlight, and other hot objects may reflect from metallic equipment surfaces.
These reflections can create false indications, especially on polished conductors and enclosures.
High Ambient Temperatures
A warm environment can reduce the visible temperature difference between a fault and its surroundings. It can also affect reflected apparent temperature and camera operating conditions.
Outdoor Inspections
Sunlight, wind, rain, humidity, and changing sky conditions can influence surface temperatures and reflected energy.
Outdoor electrical equipment should be evaluated with particular care because the cold sky can create strong infrared reflections from low-emissivity surfaces.
Contamination and Surface Changes
Dust, oil, corrosion, oxidation, moisture, and process residue can change surface emissivity. A component’s apparent temperature may therefore change even when its actual temperature has not.
Restricted Viewing Angles
Enclosure design, barriers, and equipment placement may force the thermographer to inspect from an oblique angle. This can reduce apparent emissivity and make reflective measurements less reliable.
Unknown Barriers
Ordinary viewing glass, clear plastic, mesh, guards, and protective covers may block or alter infrared radiation. A material that is transparent to visible light is not necessarily transparent to infrared energy.
Inconsistent Access
Opening electrical enclosures can expose personnel to electrical hazards. It may also change the equipment’s thermal state by allowing airflow across internal components.
Purpose-designed infrared inspection windows can provide a consistent inspection point while the enclosure remains closed.
Best Practices for Accurate Thermal Imaging Measurements
Accurate thermal imaging depends on more than emissivity alone. A complete measurement process should address the following factors.
Focus
A poorly focused image spreads thermal energy across multiple pixels and can reduce the reported peak temperature. Images should be focused before measurements are recorded.
Spatial Resolution
The target must cover enough detector pixels for an accurate temperature measurement. A component that appears visible in the image may still be too small to measure accurately.
The thermographer should understand the camera’s instantaneous field of view and measurement field of view.
Distance
Increasing the distance reduces the number of pixels covering the target. The camera should be positioned close enough for the measurement target to fill the required measurement area.
Viewing Angle
Measurements should be taken as close to perpendicular to the target as practical. Extreme angles increase reflection and can reduce apparent emissivity.
Atmospheric Conditions
Humidity, dust, steam, gases, and long measurement distances can affect infrared transmission. These effects become more important over longer distances or in harsh environments.
Reflected Apparent Temperature
The camera should account for the effective temperature of the surroundings reflected by the target, particularly when measuring low-emissivity surfaces.
Equipment Load
The inspected equipment should be operating under a load that is sufficient to reveal load-related heating. The load should be documented for future comparison.
Camera Calibration
Thermal cameras should be maintained and calibrated according to the manufacturer’s recommendations and the organization’s quality procedures.
Inspection Consistency
Repeated inspections should use the same targets, routes, camera positions, settings, and operating conditions wherever practical.
Qualified Interpretation
A thermal image does not automatically identify the cause of a problem. Findings should be interpreted by personnel who understand thermography, electrical systems, loading, failure modes, and applicable safety procedures.
How IR Inspection Windows Improve Electrical Thermography Accuracy
Infrared inspection windows provide a permanently installed access point that allows thermal data to be collected while an electrical enclosure remains closed.
This can improve inspection repeatability in several ways.
Consistent Inspection Position
A fixed window gives thermographers a defined inspection location. The same internal targets can be viewed during each inspection, reducing variation in camera position and angle.
Reduced Environmental Disturbance
Opening an enclosure can alter airflow and component temperature. Inspecting through a window allows the equipment to remain in its normal operating state.
Better Access to Internal Components
Properly selected and positioned windows provide a direct view of critical connections, conductors, breakers, busbars, and other inspection targets.
Repeatable Inspection Routes
Each window can be identified within the inspection route and associated with specific internal targets. This helps different thermographers collect comparable data.
Support for High-Emissivity Targets
During equipment design or installation, high-emissivity reference targets can be placed on critical internal components within the window’s field of view. This gives thermographers a consistent surface for quantitative measurements.
Reduced Need to Open Energized Equipment
An infrared inspection window allows qualified personnel to collect thermal data without removing covers or opening enclosure doors solely to obtain a direct line of sight.
The thermographer must still account for the window’s transmission characteristics. Every infrared window absorbs and reflects some infrared energy. The camera or reporting process should use the manufacturer’s documented transmission value for the window, camera wavelength, and relevant measurement conditions.
The window must also be kept clean and inspected for damage. Dirt, moisture, coatings, or deterioration on the optic can affect transmission and measurement accuracy.
Emissivity and window transmission are separate settings. Emissivity describes the target surface, while transmission describes the amount of infrared energy passing through the window. Adjusting emissivity is not a substitute for entering or compensating for the correct window transmission value.
Improving Inspection Accuracy with Proper Emissivity Settings
Proper emissivity settings begin with understanding the target. The thermographer should identify the material, evaluate its surface condition, and determine whether the inspection requires an absolute temperature or a comparative assessment.
For high-emissivity targets, temperature measurement is generally more stable. For low-emissivity targets, the thermographer should use a verified reference material or carefully account for reflected energy and other measurement variables.
A practical process includes:
- Identify the target and its surface condition.
- Determine whether the surface is reflective.
- Use a known high-emissivity target where possible.
- Enter the appropriate emissivity value.
- Estimate reflected apparent temperature.
- Enter the correct infrared window transmission value when applicable.
- Focus the camera and confirm adequate spatial resolution.
- Record the load, distance, angle, and environmental conditions.
- Compare the result with similar components and historical data.
- Document any uncertainty or inspection limitation.
The goal is not simply to produce a temperature number. The goal is to collect defensible information that supports an accurate assessment of equipment condition.
Frequently Asked Questions
What is emissivity in electrical thermography?
Emissivity is a measure of how efficiently an electrical component’s surface emits infrared energy compared with an ideal emitter at the same temperature. It is expressed as a value between 0 and 1.
High-emissivity surfaces emit strong infrared signals and are generally easier to measure accurately. Low-emissivity surfaces emit less energy and reflect more radiation from their surroundings.
Why is emissivity important for accurate electrical thermography?
A thermal camera uses emissivity to calculate surface temperature from detected infrared radiation. An incorrect emissivity setting can cause the displayed temperature to differ significantly from the actual temperature.
Accurate emissivity settings are especially important when measuring polished metals, busbars, terminals, and other reflective electrical surfaces.
What should emissivity be set to during an infrared inspection?
The emissivity setting should match the surface being measured. There is no single setting that is correct for every electrical inspection.
A value around 0.95 may be appropriate for electrical tape, matte paint, many plastics, or other high-emissivity surfaces. It is usually not appropriate for clean, polished metal.
The thermographer should use a verified value for the actual surface or measure a known high-emissivity reference target.
How is emissivity used in infrared thermography?
The thermal camera uses emissivity as part of its temperature calculation. The setting helps the camera estimate how much of the detected radiation was emitted by the target and how much was reflected from the surrounding environment.
Emissivity works together with other measurement inputs, including reflected apparent temperature, distance, atmospheric conditions, and window transmission.
Does emissivity affect thermal imaging accuracy?
Yes. Emissivity can have a major effect on quantitative temperature accuracy.
The effect is greatest on low-emissivity surfaces because reflected energy represents a larger portion of the radiation detected by the camera. High-emissivity surfaces are generally less sensitive to reflected radiation and incorrect environmental settings.
What are the common emissivity mistakes during electrical inspections?
Common mistakes include:
- Leaving the camera set to 0.95 for every target
- Using a material table without considering surface finish
- Measuring polished metal as though it were a painted surface
- Ignoring reflected apparent temperature
- Confusing emissivity with infrared window transmission
- Comparing surfaces with different emissivity values
- Measuring from extreme viewing angles
- Changing settings between inspections without documenting them
- Reporting precise temperatures despite uncertain measurement conditions
How do you measure emissivity for electrical equipment?
One practical method is to place a known high-emissivity reference material on the component, allow it to reach the same temperature as the surface, and compare the apparent temperature of the exposed surface with the reference target.
The camera’s emissivity setting can then be adjusted until the exposed surface reading corresponds with the reference measurement.
This process should only be performed using materials and methods that are suitable for the electrical equipment and operating environment.
Which electrical materials have low emissivity?
Low-emissivity electrical materials commonly include:
- Bright copper
- Polished aluminum
- Stainless steel
- Galvanized steel
- Chrome-plated hardware
- Clean busbars
- Shiny terminals
- Reflective enclosure surfaces
Oxidation, corrosion, paint, dirt, and coatings can increase the apparent emissivity of these materials.
How do high and low emissivity surfaces affect thermal inspections?
High-emissivity surfaces emit stronger infrared signals and usually provide more stable temperature measurements.
Low-emissivity surfaces emit less energy and reflect more of their surroundings. Their apparent temperature can therefore change with camera angle, background temperature, and environmental conditions.
Thermographers should use reference materials, comparative methods, and controlled settings when inspecting low-emissivity targets.
What are the best practices for improving emissivity in electrical thermography?
Best practices include:
- Use permanent high-emissivity reference targets where practical.
- Match the camera setting to the actual target surface.
- Account for reflected apparent temperature.
- Use consistent inspection angles and distances.
- Record camera settings and operating loads.
- Compare similar components under similar conditions.
- Correct for infrared window transmission.
- Avoid relying on absolute temperatures when measurement uncertainty is high.
- Standardize inspection routes and procedures.
- Train personnel in both thermography and electrical fault interpretation.
Conclusion
Emissivity has a direct effect on the accuracy and repeatability of electrical thermography. High-emissivity surfaces generally provide strong, stable infrared signals, while polished and reflective metals can produce misleading readings because they emit less energy and reflect more of their surroundings.
Reliable inspections require more than entering a generic emissivity value into the camera. Thermographers must consider the target material, surface condition, reflected apparent temperature, viewing angle, load, distance, focus, environmental conditions, and the transmission characteristics of any infrared inspection window.
High-emissivity reference targets, standardized inspection routes, documented camera settings, and properly positioned infrared windows can all improve measurement consistency. When these practices are incorporated into a condition-based maintenance program, thermal data becomes more useful for identifying developing electrical problems, prioritizing maintenance, and evaluating equipment condition over time.
