Thermal Camera - A413

 

Thermal Camera
Document Type	Article
Document Identifier	413
Tags	Deposition

Introduction

A thermal camera is a device that can be used for temperature monitoring. It relies on the infrared radiation emitted by a body to measure its temperature without contact. The emissivity of the measured material must be known to obtain accurate measurements. The influence of the environment must also be minimized to avoid impacting the temperature reading.

Scope

This article provides an overview of the working principle, advantages, and limitations of a thermal camera.

Significance

Thermal cameras are widely used to monitor parts temperature during manufacturing. They are an alternative and complementary measurement to thermocouples, by offering a full-field, non-contact temperature measurement of a visible surface. However, some important parameters, such as the material’s emissivity, must be well-known to obtain reliable results. More information on alternative temperature sensors can be found here.

Overview

A thermal camera works in a similar way to a regular camera, except that it reacts to the infrared wavelengths of the light spectrum. These wavelengths are longer than the visible light spectrum (typically between 1 µm and 1 mm), making them invisible to the human eye. Every object emits infrared radiation at an amplitude that depends on its temperature and its surface properties. The thermal camera can measure the amplitude of radiation emitted from a given point in space and calculate its temperature. Long-wave infrared wavelengths are typically used for thermal imaging.

Figure 1 – Light spectrum, including an enlarged view of the section corresponding to the visible light spectrum, located between wavelengths or approximately 400 and 800 nm. Infrared wavelengths are located on the right of the visible spectrum, between approximately 1 µm and 1 mm.

Working Principle

All objects emit radiation when they have a temperature higher than absolute zero, due to the molecular agitation. The intensity of the emitted electromagnetic radiation depends on the material of the object and its temperature, as stated in the Stefan-Boltzmann law. This radiation is detected by the thermal camera, and its intensity is converted to a temperature value.

Important Parameters

The most important parameter to know to obtain reliable temperature measurements with a thermal camera is the emissivity of the observed material. This property defines the amount of radiation emitted at a given temperature; it varies between 0 and 1 and has no unit. An emissivity of 1 corresponds to a perfect black body.

The emissivity depends not only on the material itself, but also on its surface roughness. In fibre-reinforced polymer composites, the polymer is directly visible to the thermal camera, and its emissivity is around 0.9 to 0.95. However, every material should be characterized beforehand to validate the value of emissivity. It must also be noted that emissivity varies with temperature, especially when the polymer is melting, which can also impact the temperature measurement.

Another important parameter is to limit reflection on the material by removing external heat sources in close proximity. Infrared radiation from a secondary body can be reflected on the surface of the sample and be detected by the thermal camera, affecting the reading. The angle of observation is also critical to limit reflection. The camera should be ideally placed in a position to observe the part in a normal direction to its surface. Also, thermal camera measurements are more accurate with opaque material, as transmitted radiation coming from heat sources located behind the samples can also be detected. Heat sources in the background or behind the sample should therefore be avoided.

Figure 2 – Schematics of the different types of radiation measured by a thermal camera. Only the emitted radiation must be considered to measure the temperature of the object, so other sources should be minimized or removed.

Data analysis

The data acquired by the thermal camera is analyzed in dedicated software. Different results can be obtained:

• Thermograms: Spatial temperature distribution on a surface at a given time. They are useful for detecting the heat concentration points or the general heating behaviour of a part.
• Time-temperature curves: the user defines one or multiple regions of interest (ROIs) on the thermal image, and the software produces a time-temperature curve for that location. ROIs can be points, lines, or surfaces, with the temperature on the curve representing either the average, the maximum, or the minimum value observed in the corresponding ROI.

Equipment

Various thermal cameras are available on the market. For temperature monitoring in composite materials processing, temperature typically varies between room temperature and a maximum of 400-450 °C. The lens of a thermal camera can typically be changed to offer a wider or narrower field of view, depending on the part size.

Entry-level long-wave infrared thermal cameras are the most common. These cameras target a spectral range between 8 and 14 µm, and their resolution is typically relatively low (320 x 240 to 640 x4 80), with a frame rate below 60 Hz. They are affordable and easy to integrate solutions for general process monitoring and temperature monitoring, in both industry and research/academia. Some examples are the FLIR A400 and A700 series, the Optris PI-series, or the DIAS Infrared Pyroview entry models.

For applications requiring a more detailed temperature acquisition, high-performance infrared cameras are also available on the market, such as the FLIR X8580 series, the Telops FAST-IR series, or the InfraTec ImageIR high-end cameras. These are typically larger and more expensive, but they offer higher resolution (up to 5 megapixels) and higher frame rate (up to 1 kHz). Their applications are fast heating processes and high-resolution thermal imaging to identify local temperature variations.

Thermal cameras can also be coupled with external heat sources to offer NDT thermography systems. These non-destructive methods allow for to detection of delamination, barely visible impact damage (BVID), porosity mapping, and quality inspection of welded joints. Some examples of thermography systems are Dantec Dynamics ThermoScope, MoviTHERM NDT solutions, Automation Technologies, Telops TESTD series, and Infratec active thermography setups.

Finally, a few compact thermal cameras are available on the market, such as the FLIR Boson series and the Teledyne Calibir. These light and small cameras can be easily integrated into an automated system or mounted on a robotic arm. They can be used for inline inspection during the AFP/ATL process, for example, or any other automated process. These cameras offer limited resolution and frame rate due to their small size.

Comparison with Thermocouples

Thermocouples offer point measurements, whereas the thermal camera allows to get a complete full-field measurement. In addition, thermocouples are invasive, while thermal cameras are a non-contact method to measure temperature. However, thermocouples are typically more accurate, and their use does not require knowledge of the material’s property (such as the emissivity). They can also be embedded in the part to obtain inner temperature evolution, which is impossible with a thermal camera that can only record the temperature on the outer surface. In conclusion, thermocouples and thermal cameras should be used jointly as they offer complementary results.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

The failure mode characterized by a partial or full separation of adjacent laminae. Delamination is unique to laminated structures, and can be induced by impact load, fatigue load, and quasistatic load. (Also known as interlaminar separation)