Understanding Infrared

What is infrared thermography? It is the process of using an instrument and a method to detect infrared energies emitted from an object, convert these emitted energies to temperature values, and display an image of the object, showing its temperature distribution. The instrument is an infrared thermograph and the method is called infrared thermography. However, the current trend is to incorporate both instrument and method in the terms infrared thermography or thermal imaging.

Infrared energy is an electromagnetic wave, part of the electromagnetic spectrum of radiation that transfers energy through space. It was discovered by the British astronomer, Sir William Herschel, around 1800. When dispersing sunlight using a prism, he accidentally found that there was an invisible energy just below the red colour wavelength, and he noted that the wavelength increases with temperature.

The infrared wavelength ranges from 0.7 µm to 1,000 µm (1 mm), and its frequency is 300 GHz or greater.

Infrared is invisible, since its wavelength is longer than visible light. It has nothing to do with the brightness or darkness of visible light.

Infrared is emitted naturally from any object that has a temperature of absolute zero (Zero Kelvin) or higher. Therefore, it can be applied to any field. It has a characteristic of heating an object. Therefore, it is sometimes called the ‘heat wave’.

Because it is a kind of light (being an electromagnetic wave), infrared is transmitable through a vacuum. There is a correlation between infrared energy and the temperature of an object. Therefore, it can be used to measure the temperature of an object, and is especially useful in industry to locate hot spots in machinery and equipment, which usually indicate hidden problems.

The characteristics of infrared thermography instrumentation are as follows:

1.
Captures surface temperature distribution and displays it as visible information.

2.
Temperature is measured from a distance without contacting the measured object.

3.

Temperature is measured in real time.

The merits of infrared thermography instrumentation include:

a)
Relative comparison of surface temperature distribution is made over a wide area.

b)
Allows temperature measurements of hazardous objects from a safe distance.

c)
Measures temperature of small objects without interference.

d)
Temperature of food, medicine or chemicals can be measured in a sanitary fashion.

e)
Temperature of an object with dynamic temperature changes can be measured.

Frequently asked questions (FAQs) about infrared thermography instrumentation include the following:

Q. Isn’t the infrared camera
emitting something?

A. No, it is not emitting anything. It merely detects infrared energy emitted from an object in a passive manner. It should be noted that the energy detected may be affected by reflected energy from the surroundings.

Q. Isn’t the temperature
measured by the wavelength?

A. No, temperature distribution is measured by the amount of infrared energy.

Q. Can you see through objects
using infrared instruments?

A. No. Infrared energy is emitted from the surface of an object; you cannot see through an object. However, it may be possible to estimate the rear-side temperature if the temperature distribution appearing on the front surface is due to thermal conductivity from a temperature difference.

Heat transfer occurs in three ways (Fig. 1). Emission is where the heat is transferred directly from the surface of an object as an infrared energy. Convection transfers heat to a gas or liquid as it passes an object. Conduction transfers heat mainly through a solid object. An object that absorbs infrared well also emits infrared well.

Taking a measurement by infrared thermography instrumentation (also called an infrared camera or thermal imaging device) involves the following sequence: The instrument detects infrared emitting from an object; it transforms the amount of energy measured into temperature; and it displays the object as an infrared image (Fig. 2). The relative temperatures of the object are indicated by different colours on the image, from blue for the coolest temperatures and rising through green, yellow, red and finally white as the hottest temperature (Fig. 3).

Infrared thermography instruments display the temperature distribution image data using a matrix of pixels (each pixel being a miniature infrared energy detector). In Fig. 3, there are 320 horizontal x 240 vertical pixels (detectors).

Thermal image data from each of these pixels/detectors may be stored in the thermography instrument or transferred to a PC for further analysis. The displayed thermal image is coloured pixel by pixel to show the temperature distribution values converted from the detected infrared thermal energy. MRO

This guide to understanding infrared thermography was compiled from data provided by Soltec Corporation, a subsidiary of NEC Avio Infrared Technologies Co. Ltd. For more information, visit www.solteccorp.com.

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