Emissivity in Thermal Imaging: Why It Matters for Accurate Inspections

Infrared inspections are a powerful way to assess the health of electrical and mechanical assets without direct contact, but thermal images are only useful when the readings are accurate. Emissivity is a critical factor in temperature measurement accuracy for thermal imaging, as it directly influences the reliability of the results.

If the emissivity setting is wrong, challenging factors such as low emissivity and reflected temperature can complicate accurate temperature measurement, causing the camera to show a component as cooler or hotter than it really is. In electrical inspections, that error can hide a developing fault, distort temperature comparisons, and lead maintenance teams to make the wrong decision.

What Is Emissivity in Thermal Imaging?

Emissivity is defined relative to a theoretical object called a blackbody, which perfectly absorbs and emits all incident energy. A blackbody has an emissivity value of 1.0, meaning it perfectly absorbs all incident energy and emits the maximum possible emitted energy at a given temperature. Real materials have emissivity values below 1.0 because some of the infrared energy is reflected or transmitted instead of emitted. Emissivity values range from 0 (a perfect mirror) to 1 (a blackbody), with most real materials typically falling between 0.01 and 0.99.

Material emissivity is a crucial parameter in thermal imaging because it determines an object’s ability to emit infrared energy, or thermal radiation. The spectral emissivity of a material can vary with wavelength, surface condition, and temperature, which affects the accuracy of temperature measurements in infrared thermography. While visible light affects how surfaces appear to the eye, thermal imaging relies on the emission of infrared energy, which is governed by the material’s emissivity.

Electrical equipment can contain materials with very different emissivity values. Components inside electrical cabinets may range from about 0.07 to 0.95. That is a wide span, and it explains why a single camera setting cannot be trusted for every target in an electrical enclosure.

Why Emissivity Is Important for Infrared Inspections

Accurate infrared inspections depend on accurate temperature data, and in thermal imaging, emissivity is the single most critical factor for accurate temperature measurement and achieving accurate results. If the camera does not account for the emissivity of the target surface, the calculated temperature may be wrong. This is especially important in electrical thermography because inspections often rely on temperature differences. These temperature differences, often called ΔT values, help identify loose connections, overloaded components, failing contacts, unbalanced loads, or early signs of electrical deterioration.

To accurately measure the true temperature of a surface, the thermographer must account for both emissivity and reflected temperature; otherwise, the apparent temperature shown by the camera may differ from the actual value. Taking temperature measurements requires calibration of the thermal imager to the emissivity of the surface, as different materials emit infrared radiation at different intensities even at the same temperature. When a thermographer sets emissivity incorrectly, readings will not be correct. A shiny busbar, for example, will reflect infrared radiation from its surroundings and could appear hotter than it is. The problem is that the camera is interpreting radiation from a surface that may be reflecting more than it is emitting, so correct settings are essential for accurate temperature readings.

How Thermal Cameras Interpret Emissivity

A thermal camera detects infrared radiation coming from the target area. That radiation can come from three sources:

Reflected radiation, also known as reflected temperature, is thermal radiation originating from other objects, surrounding objects, or even an adjacent object, that bounces off the target surface. This reflected temperature can significantly influence the apparent temperature of the object being measured, especially for low-emissivity surfaces.

Transmitted radiation is energy that passes through the target from a source behind it.

Emitted radiation is the thermal radiation emitted by the object itself. The camera uses this thermal radiation emitted to determine the temperature of the object, but the accuracy can be affected by less influence from emissivity and reflected temperature if not properly compensated.

For most electrical thermography, the thermal radiation emitted by the object is the value that matters most. The camera needs to know how much of the radiation it sees is actually being emitted by the inspected component, as opposed to thermal radiation from other objects or surrounding objects. That is where emissivity settings come in. For accurate temperature measurements, thermal imagers must be calibrated to account for the emissivity of the surface and reflected temperature, since different materials and surface conditions emit infrared radiation at different intensities even at the same temperature.

The relationship between emitted, transmitted, and reflected radiation is described by Kirchhoff’s Law:

ε + τ + ρ = 1

In this formula, ε is emissivity, τ is transmissivity, and ρ is reflectivity. When one value changes, the others are affected. A highly reflective surface has lower emissivity. A high-emissivity surface reflects less of the surrounding environment and gives the camera a cleaner signal from the target itself.

High vs Low Emissivity Surfaces

High-emissivity surfaces tend to give more accurate readings because they emit infrared energy consistently, allowing the object’s actual temperature to dominate the measurement. Many painted, coated, oxidized, non-metallic, or matte surfaces have relatively high emissivity (ε > 0.9), which means they emit energy reliably and provide highly accurate readings. The emissivity of most non-metallic materials decreases as temperature increases, while for most pure metals, emissivity increases approximately proportionally with higher temperature.

Low-emissivity surfaces are harder to inspect. Highly polished metal objects, such as polished copper or aluminum, usually have an emissivity below 0.10, causing them to act like mirrors and reflect thermal radiation from surrounding sources rather than emitting their own heat. As emissivity decreases, the influence of reflected temperature increases, making accurate measurement more challenging. For metallic materials, surface roughness significantly impacts emissivity; for example, wrought iron with a rough surface has an emissivity of 0.94, while polished wrought iron has an emissivity of only 0.28. Roughened or oxidized metallic surfaces can have emissivity values of 0.6 or greater. The surface condition—including whether it is a rough surface or a highly polished metal object—directly affects how much infrared radiation is emitted versus reflected, impacting the accuracy of thermal imaging.

A useful example is two sides of the same object—a pan. One side is aluminum or stainless steel, and the other is coated with Teflon. Both sides are at the same temperature, but due to different materials and surface conditions, the apparent temperature differs in the thermal image. The Teflon-coated side, with higher emissivity, appears hotter, while the lower-emissivity metal side appears cooler because it reflects cooler background temperatures. This demonstrates that even when the same object is at the same temperature, differences in emissivity and surface condition can cause significant variation in how the object radiates heat and how it is detected by the camera.

Common Problems Caused by Low Emissivity Surfaces

Low emissivity is one of the challenging factors that affects temperature measurement accuracy during infrared inspections. It can cause the apparent temperature seen by the camera to differ from the true value, especially when taking temperature measurements on reflective or low-emissivity surfaces.

If the camera emissivity setting is too high for a low-emissivity target, the camera may report a temperature lower than the true surface temperature, reducing accurate temperature measurement. In electrical systems, this can make a hot connection look less serious than it is.

Reflective metal surfaces can give an incorrect reading because they are reflecting the thermographer themselves, heat, or reflected sunlight, further impacting temperature measurement accuracy and leading to errors in apparent temperature.

If two surfaces have different emissivity values, comparing them directly can produce misleading results. One component may look hotter simply because it emits infrared radiation more efficiently, while another may look cooler because it is reflecting its surroundings, making it difficult to accurately measure true temperatures.

Because radiant energy relates to absolute temperature through Stefan-Boltzmann’s Law, measurement errors can grow significantly as temperatures rise.

To accurately measure temperature on low-emissivity surfaces, special techniques are required. Common practices include applying a known high emissivity coating or using a contact thermometer for calibration. To improve temperature measurement accuracy, techniques such as applying a piece of insulating tape or a known emissivity paint can be used to create a higher emissivity surface for thermal imaging.

How to Improve Thermal Imaging Accuracy on Low Emissivity Surfaces

The best way to improve temperature measurement accuracy is to create or identify a high-emissivity measurement point on the target, such as a painted surface with known emissivity. This approach enables more accurate temperature measurement by providing the thermal camera with a reliable surface for taking measurements.

Common methods include applying electrical tape, high-temperature paint, high-emissivity labels, or thermal indicator paint to the inspection target. These materials create a surface with known emissivity, allowing for precise calibration and correction, and making the camera’s temperature calculation more dependable. When taking measurements on low-emissivity surfaces, such as polished metals, it is recommended to apply a piece of insulating tape or paint with a known emissivity and set the thermal imager’s emissivity value to match the tape or paint used. This ensures accurate temperature measurement by compensating for the surface’s properties and reflected temperature.

For electrical equipment, this work should be done when the equipment is open and de-energized. That is the safest time to prepare inspection targets, apply reference materials, and standardize measurement points before future closed-panel infrared inspections.

When using IR windows, it’s important to apply targets during installation while the equipment is deenergized. This means a thermographer will have a target to focus their camera on during inspections.

The Role of Thermal Paint and Emissivity Reference Materials

Thermal paint helps thermographers get more reliable measurements from difficult surfaces by creating a painted surface with known emissivity, which serves as a reference for accurate temperature readings. Using a surface with known emissivity enables precise calibration and correction during infrared thermography, especially when using reference materials or contact methods. Users must either measure the material’s emissivity or reference emissivity tables to ensure accurate thermal camera readings.

One simple field method of creating high-emissivity targets is black electrical tape. Thermo paint also creates a high-emissivity target with the added color change benefit. Thermo Paint from the IRISS Safe-Connect range adds another layer of value because it also changes color at a defined temperature threshold, typically 70°C or 150°F. That means it can support both infrared inspections and quick visual checks. A high-emissivity reference surface means better thermal imaging accuracy, while the visual color change provides an immediate warning that a component needs attention.

Emissivity Best Practices for Electrical Inspections

The following is a list of best practices for collecting correct data with an IR camera as part of a structured inspection program to ensure temperature measurement accuracy:

• Use the correct emissivity setting for the target material whenever possible.
• Avoid direct temperature comparisons between surfaces with very different emissivity values.
• Be cautious when inspecting polished metals, bare busbars, stainless steel, aluminum, and copper.
• Create high-emissivity reference points before equipment is placed into service.
• Apply reference materials while equipment is open and de-energized.
• Use electrical tape, high-temperature paint, high-emissivity labels, or thermal paint where appropriate.
• Account for infrared window transmission when inspecting through IR windows.
• Document emissivity settings.
• Take measurements with proper calibration and documentation, including setting emissivity and reflected temperature parameters in the camera, as part of the inspection program.
• Use repeatable inspection routes so readings can be compared over time.
• Confirm suspicious readings from reflective surfaces before diagnosing a fault.

These practices help reduce false readings and improve the quality of thermal inspection data by ensuring accurate temperature measurement accuracy.

Common Materials and Their Emissivity Values

Emissivity values range from 0 (a perfect mirror) to 1 (a blackbody), with real materials typically falling between 0.01 and 0.99. Different materials and surface conditions have different material emissivity at a given temperature, and these values can change depending on the specific surface condition and temperature. Highly polished metallic surfaces, such as copper or aluminum, usually have an emissivity below 0.10, while roughened or oxidized metallic surfaces can have values of 0.6 or greater. For metallic materials, surface roughness significantly impacts emissivity; for example, wrought iron with a rough surface has an emissivity of 0.94, while polished wrought iron has an emissivity of only 0.28. The following values are typical reference ranges for material emissivity at a given temperature, not fixed values for every inspection. Thermographers should verify actual values using approved methods, manufacturer guidance, or known reference materials, as different materials and their surface conditions can greatly affect the accuracy of thermal imaging.

The key point is that shiny bare metals are usually low-emissivity surfaces. Matte, painted, oxidized, taped, labeled, or coated surfaces usually provide better thermal measurement targets. Always consider the emissivity values range, the type of material, and the surface condition when performing thermal imaging to ensure accurate temperature readings.

How Emissivity Supports Predictive Maintenance Programs

Predictive maintenance depends on trustworthy data. If thermal inspection readings are inaccurate, the maintenance program is built on weak evidence.

When emissivity is calculated correctly and proper inspection procedures are followed, it is possible to trend the data collected accurately. This helps maintenance teams catch problems earlier by ensuring temperature measurement accuracy and reliable detection of anomalies.

In electrical systems, that may mean identifying a loose termination before it damages the connection point, spotting load-related heating before it causes downtime, or verifying whether a suspected issue is real or simply a reflection from a nearby heat source.

Infrared Inspection Challenges in Industrial Environments

Industrial environments create difficult conditions for infrared thermography inspections. Challenging factors such as ambient temperature, clear sky conditions, and the presence of electrically energized or difficult-to-access physical objects can complicate accurate temperature measurement. Electrical cabinets, switchgear, panels, transformers, motors, control systems, and busbars may include many different surfaces in one inspection area. Some are painted, some are oxidized, some are bare metal, and some are reflective. Each physical object’s surface may behave differently in a thermal image due to variations in emissivity.

Ambient conditions add more complexity. Nearby hot equipment, sunlight, lighting, heaters, cooling systems, or reflective surfaces can affect what the camera sees. Dust, moisture, viewing angle, distance, airflow, and enclosure design can also influence readings.

Infrared windows add another factor that must be managed. When inspecting through an IR window, the thermographer must account for the window’s transmission rate as well as the emissivity of the target surface. If either setting is wrong, the calculated temperature may be inaccurate.

Best Practices for Accurate Thermal Imaging Measurements

Accurate thermal imaging starts before the inspection begins. Use these best practices to improve measurement quality and ensure accurate temperature measurement:

• Set emissivity based on the target surface, not a default value.
• Use high-emissivity targets on reflective parts.
• Keep inspection angles as close to perpendicular as practical.
• Avoid measuring shiny metal surfaces without compensation.
• Watch for reflected heat sources behind or beside the thermographer.
• Record load conditions during electrical inspections.
• Compare similar components under similar operating conditions.
• Account for IR window transmission when using infrared inspection windows.
• Use the same target points during each inspection cycle.
• Review thermal images alongside visual images whenever possible.
• Treat unexpected readings on reflective surfaces with caution until confirmed.

Proper thermography training is essential for learning how to take accurate temperature measurements. These courses teach the correct techniques for taking measurements, including how to set emissivity and compensate for reflected temperature, which are critical for accurate temperature measurement in thermal imaging.

Good thermography is not just a matter of pointing the camera at equipment. It requires control over the variables that influence the reading.

Improving Infrared Inspection Accuracy with Proper Emissivity Understanding

Emissivity affects how thermal cameras calculate temperature and how thermographers compare components, directly impacting temperature measurement accuracy and the ability to determine the true temperature of assets.

Low-emissivity surfaces can reflect the surrounding environment and make faults appear cooler than they really are. Incorrect camera settings can distort both temperature readings and ΔT comparisons, leading to inaccurate results.

The solution is disciplined inspection practice. Understand the surface. Set the camera correctly. Use high-emissivity reference materials where needed. Standardize targets while equipment is open and de-energized. Document the method so future inspections can be repeated with confidence.

When emissivity is managed properly, infrared inspections deliver accurate results, improved temperature measurement accuracy, and reliable determination of the true temperature of components. This makes inspections more accurate, more repeatable, and more useful for condition-based maintenance. That gives maintenance teams better information, clearer priorities, and a stronger basis for preventing equipment failure.

Frequently Asked Questions

What is emissivity in thermal imaging?

Emissivity is a measure of how efficiently a surface emits infrared radiation compared with an ideal blackbody. In thermal imaging, emissivity affects how accurately a thermal camera can calculate surface temperature. A high-emissivity material gives a more reliable reading, while a low-emissivity material can reflect surrounding heat and distort the measurement.

Why is emissivity important for thermal imaging accuracy?

Emissivity is important because thermal cameras do not measure temperature directly. They detect infrared radiation and convert that signal into a temperature value. If the emissivity setting is wrong, the camera may overstate or understate the real temperature, especially on shiny, reflective, or bare metal surfaces.

What should I set my emissivity to on a thermal camera?

The correct emissivity setting depends on the material being inspected. Many painted, rubber, plastic, oxidized, or non-metallic surfaces are often set around 0.90 to 0.95. Bare metals are usually much lower and require extra care. For accurate inspections, thermographers should use the known emissivity value of the material or apply a high-emissivity reference material.

What causes inaccurate thermal readings during inspections?

Inaccurate thermal readings can be caused by incorrect emissivity settings, reflective surfaces, distance from the target, viewing angle, atmospheric conditions, background temperature, dirty lenses, poor focus, or measuring the wrong spot. Low-emissivity surfaces are especially difficult because they reflect infrared energy from nearby objects, including the thermographer.

What are low emissivity surfaces in thermal imaging?

Low-emissivity surfaces are materials that do not emit infrared energy efficiently. Instead, they reflect a large amount of infrared radiation from their surroundings. Common examples include polished aluminum, copper, stainless steel, brass, and other shiny metals. These surfaces can make a thermal camera display reflected heat rather than the true surface temperature.

How do shiny metal surfaces affect infrared inspections?

Shiny metal surfaces can act like infrared mirrors. During an infrared inspection, they may reflect heat from people, equipment, sunlight, walls, or other nearby objects. This can make a component appear hotter or colder than it really is, which can lead to false alarms or missed faults.

Does emissivity change with temperature?

Yes, emissivity can change with temperature, surface condition, oxidation, contamination, coating, roughness, and viewing angle. A clean polished metal surface may have very low emissivity, while the same surface may become easier to measure after it oxidizes, becomes dirty, or is coated with paint or another high-emissivity material.

How can emissivity be improved for thermal inspections?

Emissivity can be improved by applying a high-emissivity material to the inspection target. Common methods include using thermal paint, electrical tape, labels, markers, or other approved reference materials. The goal is to create a known, stable surface that emits infrared radiation consistently so the thermal camera can provide a more accurate reading.

What materials have low emissivity values?

Low-emissivity materials often include polished aluminum, polished copper, stainless steel, brass, bronze, silver, gold, and other clean or shiny metals. The exact value depends on the surface finish. A polished metal surface may have very low emissivity, while a rough, oxidized, painted, or coated version of the same metal may have much higher emissivity.

How do thermographers measure emissivity accurately?

Thermographers measure emissivity by comparing the target surface against a known reference. One common method is to apply a high-emissivity material, such as thermal tape or thermal paint, allow it to reach the same temperature as the target, then adjust the camera settings until the untreated surface matches the reference temperature. Accurate measurement also requires proper focus, angle, distance, reflected temperature compensation, and stable inspection conditions.