Thermography. Making the invisible visible with thermal imaging cameras.

Table of contents.

1. Preventing system failures using thermography
2. Thermography in electronics development
3. How does a thermal imaging camera work?
4. Reading thermal images correctly
5. What the resolution says about the quality
6. Thermography ABC
7. Selection criteria for a thermal imaging camera

Preventing system failures using thermography.

Defective components in electrical and mechanical systems represent a high safety risk and can lead to cost-intensive downtimes. For reliable electrical infrastructures in industrial and utility companies, thermal imaging technology offers a particularly convenient and safe way of regularly checking system components and ensuring grid safety and efficiency.

Portable thermal imaging cameras allow you to efficiently localize temperature anomalies and visualize electrical asymmetries and energy losses. The cameras provide you with detailed images and precise measured values in real time. They can be used without interfering with the running operation and at a safe distance from the system. In critical electrical installations, round-the-clock monitoring with permanently installed (stationary) thermal imaging cameras can also be useful in order to avoid missing any energy anomalies and to detect potential causes of failure in good time.

The use of thermal imaging cameras as part of regular inspection and maintenance measures enables companies to maximize operational energy efficiency and avoid unscheduled repairs and downtimes.

Fig: Application examples for thermal imaging technology (from left to right): loose fuse connection in an electrical system, overheating motor bearing, heat development in a transformer substation

Thermography in electronics development.

During the development of an electronic circuit and the associated circuit boards, a lot of time is invested in the design and arrangement of the components. A great deal of effort often goes into controlling the thermal loads, e.g. with thermocouples for temperature measurement, in order to ensure the safety and function of the circuit. In this context, heat development and heat dissipation via heat sinks play an important role. Possible questions that development engineers might ask themselves about the design include:

• Is the distance between the conductors correct?
• Are the components correctly dimensioned?
• Are the heat sinks calculated correctly?
• Where are hot spots on the circuit board?
• What are the temperature curves?
• How do different powers affect the components?

Measurements using a thermal imaging camera can easily answer such questions and avoid costly redesign measures. This allows the development team – or quality assurance – to ensure that a design also meets the thermal requirements as early as the prototype stage.

How does a thermal imaging camera work?

Defective electrical and mechanical components in a system usually become hot and can lead to costly breakdowns. Thermal imaging cameras are extremely effective diagnostic tools for many applications. In addition, industry's efforts to make production processes more and more efficient and to save energy resources are expanding the range of applications and measurement tasks for thermal imaging cameras (infrared cameras).

Infrared light and thermography.

The wavelengths of thermal energy (infrared light) are much longer than those of the visible electromagnetic light spectrum. Thermal radiation is imperceptible to the human eye, comparable to radio waves (high-frequency waves). With thermal imaging technology, we can expand the spectrum visible to us by visualizing the thermal energy emitted by a target object.

Every object that has a temperature above absolute zero (-273 °C) emits electromagnetic heat radiation, including cold ice cubes, for example. And since visible light works independently of thermal radiation, thermal energy can be detected in both light and dark environments. The higher the temperature of an object, the more intense the infrared light emitted. Thermal imaging cameras produce images in which infrared radiation (heat radiation) is visible. They therefore enable non-contact and very precise temperature measurements.

Use of thermal imaging technology in practice.

A thermal imaging camera (IR camera) is a non-invasive measuring device that can detect thermal energy (infrared radiation) and convert it into electrical signals. These signals are shown on a display and used for temperature calculations. With the ability to precisely quantify heat in this way, performance monitoring can also be carried out.

The measurement is contact-free, i.e. without interfering with the running operation. It is therefore a non-destructive testing method that is suitable for almost all areas of industry, research or the electrical trade and can deliver fast results.

Useful areas of application for thermal imaging cameras are in hardware development, electronics, construction, quality assurance and production. Measurements with a thermal imaging camera can also provide information about potential sources of interference during service or maintenance.

Application examples for thermal imaging technology:

• Inspection and maintenance of industrial plants
• Condition monitoring of pipelines
• Inspection of HVAC systems, ovens and boilers
• Energy audit, building inspection and building diagnostics
• Testing of electronic components
• Biosciences

Reading thermal images correctly.

When starting with thermography, you should be aware that a surface that appears blue on the screen of the thermal imaging camera is not necessarily "cold". The decisive factor is the intensity of the heat radiation and the temperature difference between the respective surfaces (Figure in German).

What the resolution says about the quality.

The IR detector of the thermal imaging camera has a significant influence on the measurement result. The quality of a detector is assessed on the basis of its geometric and thermal resolution. The geometric resolution determines whether anomalies in the thermal image are sufficiently recognizable. The thermal resolution (Noise Equivalent Temperature Difference NETD), on the other hand, describes the smallest temperature difference that a thermal imaging camera can measure. This is particularly relevant for objects with very small temperature differences; the smaller the value of the thermal resolution, the more precise the measurement result.

Pixel count as a quality criterion.

To generate a thermogram, the IR detector of a thermal imaging camera captures several image dots (pixels). These are arranged in a sensor matrix. A matrix of 120 x 90 pixels comprises a total of 10,800 measurement points and therefore 10,800 individual measured values. A detector with 240 x 180 pixels generates four times as many measured values (= 43,200 pixels). This means that even small objects that are surrounded by a larger area can be easily recognized, e.g. a thermal bridge on a building façade.

Thermal imaging cameras in the entry-level price segment are available with a resolution from 80 x 60 pixels. For very small components, such as a hot wire in a switch cabinet, this may not be enough. Objects at a greater distance, e.g. in building thermography, also require a higher resolution.

Detector resolutions with corresponding number of measured values:

Thermography ABC.

Noise Equivalent Temperature Difference (NETD)

The NETD is an indicator for the smallest possible temperature difference that a thermal imaging camera can measure (resolve). The lower the value of the thermal resolution (given in millikelvin, mK), the more precise the measured values are.

Reflectance ρ

The reflectance is the ratio between reflected and incident infrared radiation or the ability of a material to reflect infrared radiation. This depends, among other things, on the surface quality of the material. A smooth or polished surface is usually more reflective than a rough or matt surface of the same material.

Thermography

Thermography is an imaging measurement method for displaying the surface temperature of objects. The intensity of the infrared radiation emanating from a point serves as a measure of temperature. A thermal imaging camera converts invisible infrared radiation into electrical signals and visualizes them as a thermogram on a display.

Transmittance τ

The transmittance describes the permeability of an object or material to infrared radiation. When IR radiation strikes an object, it is partially reflected at the boundary surfaces, depending on the material properties (thickness, type, etc.), and partially or completely absorbed as it passes through. Most materials are not permeable to long-wave infrared radiation.

Selection criteria for a thermal imaging camera.

The market offers a practically infinite variety of thermal imaging cameras for users in the mechanical and electrical sectors. One of the basic considerations when choosing your camera is what type of system you want to check.

Maintenance professionals are interested in discovering problems as quickly as possible or enduring long inspection periods. Particularly ergonomic and user-friendly cameras that offer sufficient flexibility for a wide range of applications are suitable for this purpose.

In areas with motor control units that are difficult to access or on high-voltage systems, on the other hand, special lenses (wide-angle or telephoto lenses) can be useful for measurements from a greater distance. And if your tasks include measuring very high temperatures, e.g. on rotary kilns, your camera should be able to be calibrated accordingly. Which camera is right for you?

Important selection criteria for your thermal imaging camera:

• Field of application and use 
• Measuring distance
• Number of temperature measurement points (resolution)
  Selection options generally from 80 x 60 pixels to 1,024 × 768 pixels, with software support even up to 2,560 x 2,048 IR pixels; the resolution affects the measuring distance as well as the image quality and measurement accuracy; for inspections of objects at a great distance or of very small components, you should aim for the highest possible resolution quality
• Temperature range
  Selection options generally from -40°C to +2000°C
• Accuracy
  usually from ±1°C or ±1% to ±2°C or ±2% of the scale value
• Thermal sensitivity
  usually from < 0.1 to < 0.02°C (20 mK)
• Lens aperture angle
  Generally available between 6° and 90° (telephoto lenses and wide-angle lenses); available as interchangeable lenses or dual lenses (with two views)
• Measurement and analysis tools
  for detailed diagnostics on site or at a later date, e.g. multiple measurement points, Delta T (temperature difference), isotherm, power and current analysis functions
• Other features
  e.g. wireless communication for data transfer, automatic report generation, touchscreen control, comment function, automatic screen alignment, compass function, GPS