Thermography and Radiometric Properties

Explore the principles of thermography and the radiometric properties essential for accurate temperature measurements with infrared cameras. Learn about the factors that influence field of view, emissivity, and how environmental conditions affect the precision of your infrared readings.

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Infrared Camera IRSX Series

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Field of View and Temperature Measurement

The Figure below, shows the horizontal (HFOV) and vertical (VFOV) field of view. Depending on the distance of the camera to the observed scene, the maximum scene width increases. As the distance increases, the resolution on the object to be measured changes.

Since real optics produce an unavoidable blur, the object to be measured should be at least 3 pixels in size to obtain an accurate measurement result.

Thermographic principles for radiometric temperature determination

This chapter is intended to provide a brief overview of the radiometric temperature measurement with infrared cameras. Prerequisite for a temperature measurement with the IRSX is that the camera has a calibration. This can be recognized by the GenICam Nodes Radiometric Control in the camera properties.

The camera series IRSX is exclusively equipped with thermal sensors. An essential feature of these sensors is the direct proportional relationship to the radiant energy incident from an object on the detector. The radiant energy depends nonlinearly on the object temperature and is influenced by environmental conditions as well as by object parameters.

In order to be able to measure the temperature of an object via its infrared radiation, it is essential to know some parameters of the measurement situation. The most important parameters are illustrated in the sketch below.

Emissivity

The emissivity depends on several object properties such as material, surface condition, viewing angle and also on the object temperature itself. Thus, this parameter is one of the most fundamental in thermographic temperature measurement. In practice values between 0.1 (e.g., polished metal surfaces) to 0.98 (e.g., human skin) are common.

The functional relationship between the object temperature and the signal S_Det of the camera, taking into account the viewing scene, is described by the following formula:

In order to calculate the object temperature T_Obj, the camera signal S_Det has to be corrected with respect to the environmental influences.

Determination of the Object Emissivity

The emissivity of an object can be provisionally determined, for example, by the methods described below. The object must have a temperature that differs significantly from the ambient temperature. For your orientation, a table with typical values for the emissivity of different materials can be found in the appendix (see section Emissivity Table).

Measurement of the Object Temperature with a Thermocouple

A simple method for the provisional determination of the emissivity is obtained by measuring the object temperature at a point using a thermocouple. Now align the infrared camera to the same point on the object and adjust the emissivity until the temperature displayed by the camera matches the value measured with the thermocouple. The set value is then the emissivity of the object. With this procedure, however, it should be ensured that the temperature of the measurement object differs significantly from the temperature of the environment.

Using a Reference Material

In this method, a reference material with known emissivity is applied to the measurement object. For example this can be paint or a piece of tape. By adjusting the emissivity of the reference material on the camera and measuring the temperature of the ink / adhesive tape, the object temperature is first determined. The emissivity of the measurement object is obtained by looking at the object itself with the camera and adjusting the emissivity until you get the same temperature reading as on the reference material. Again with this method, the object temperature should be significantly different from the ambient temperature.

Temperature Calculation from camera signal (Flux Linear)

The non-linear relationship between camera signal and object temperature is described by the following formula:

The coefficients R, B, F are based on the physical Planck function. The coefficient O describes the signal offset (property of the detector). The determination of these coefficients is part of the factory calibration. The coefficients are found in the GenICam Nodes under the Radiometric Control group in the camera properties.

Tip

The exchange of the lens has an influence on these coefficients. If the lens must be replaced, a recalibration is recommended.

Most IRSX models can store two calibration sets. This allows a replacement of the lens without recalibration. The calibration sets belonging to the lenses can be selected in the GenICam Nodes under the camera property Lens Selector in the group Lens Control. When delivered without a replacement lens, the relevant calibration set is stored under Lens1 as standard. For each calibration set, you can switch between the high and low temperature measurement range using the Temperature Range Selector property.

In the temperature calculation, the external environmental influences such as ambient temperature, must be taken into account. The object temperature can be calculated according to the following formula:

In order to calculate a temperature value from the camera signal, the R, B, F, O parameters are be required. These can be read out of the camera. In addition, the environmental parameters must be known.

Table : Parameters

For the transmission of the atmosphere, 𝜏_Atm = 1 can be assumed for short distances. If no protective window is installed, 𝜏_Lens = 1 can be used. To calculate the temperature from the signal value, the following parameters must first be calculated.

Calculation of the emissivities:

Calculation of the radiation of the environment (I_amb), of the atmosphere (I_Atm) and of the protective window (S_Atm):

Calculation of the radiation components (K₁, K₂):

Calculation of the object signal (S_Obj):

With the signal S_Obj and the parameters R, B and F, the object temperature can be calculated in Kelvin.

Temperature Calculation with Camera Temperature Linearization (TLinear)

The IRSX series allows to calculate the output signal directly proportional to the object temperature. There are two levels of accuracy, which differ in their dynamics. The function can be selected in the GenICam Nodes under Radiometric Pixel Format in the group Radiometric Control.

The Figure below shows the selection of temperature linearization with the factor 0.04:

Assuming the signal value in a pixel is 7600, the temperature is calculated as follows:

Temperature in Kelvin

Temperature in Celsius

For measuring temperatures with this method, ambient properties such as emissivity, transmission and temperatures must be set in the camera. You will find these settings in the GenICam Nodes under Radiometric Control.

Emissivity Table

Material	Surface	Temperature [°C]	Emissivity
Aluminum	polished	20	0.04
	oxidized, strongly	20	0.83-0.94
Copper	polished	100	0.05
	oxidized, strongly	20	0.78
Iron	cast, oxidized	100	0.64
	sheet, heavy rusted	20	0.69-0.96
Nickel	polished	20	0.05
Stainless steel (18-8)	polished	20	0.16
	oxidized	60	0.85
Steel	polished	100	0.07
	oxidized	200	0.79
Brick	red	20	0.93
Carbon		20	0.93
Concrete	dry	35	0.95
Glass		35	0.97
Oil, lubricating		17	0.87
	0,03mm film	20	0.27
	0,13mm film	20	0.72
	Thick coating	20	0.82
Oil paint	Mix from 16 colors	20	0.94
Paper	white	20	0.07-0.90
Plaster		20	0.86-0.90
Rubber	black	20	0.95
Human skin		32	0.98
Soil	dry	20	0.92
	saturated with water	20	0.95
Water	distilled	20	0.96
	frost crystals	-10	0.98
	snow	-10	0.85

Non-destructive Testing (NDT)

Active thermography, as a method of non-destructive testing, makes it possible to quickly and reliably detect hidden defects such as cracks or delaminations. For this purpose, the component is stimulated and heated by an external energy source. An infrared camera then records the temperature distribution on the surface, which is influenced by irregularities within the material. This results in high-contrast images that enable precise quality control – contact-free, efficient, and without causing any damage to the component.