Basics of IR thermography

All matter above absolute zero continuously emits energy in the form of infrared (IR) radiation. This emitted energy is detectable and quantified as an object's temperature through the technique of infrared thermography. The human eye can see in a very narrow range of the electromagnetic spectrum, in wavelengths from 0.

03/15/2003


Key Concepts

 

  • Incandescent objects emit visible and invisible radiation.

  • Infrared instruments can be spot, line scanners, or imagers.

  • IR transmission losses can be caused by anything in the sight path.

    Sections:
    Infrared thermometer elements
    Temperature instruments
    Causes of temperature error

    Sidebars:
    Advantages of thermal imaging
    Elements of sensor selection


    All matter above absolute zero continuously emits energy in the form of infrared (IR) radiation. This emitted energy is detectable and quantified as an object's temperature through the technique of infrared thermography. The human eye can see in a very narrow range of the electromagnetic spectrum, in wavelengths from 0.4 to 0.7 microns. The infrared portion of the spectrum ranges from 1 to 100 microns (Fig. 1).


    A sufficiently hot object will emit light or visible radiation, a phenomenon called incandescence. A light bulb filament, smoldering ember, and a billet of red-hot steel are examples of this phenomenon. The hotter the object, the brighter and whiter its color. It is possible to estimate the temperature of an object this way. Experienced steelworkers do this regularly.

    Not as widely recognized is the fact that incandescent objects emit a tremendous amount of invisible infrared radiation. For example, the radiance of a steel billet at 1500 F is 100,000 times greater in the infrared spectrum than in the visible spectrum.

    Infrared energy is part of the electromagnetic spectrum and behaves similarly to visible light. It travels through space at the speed of light and can be reflected, refracted, absorbed, and emitted. The wavelength of IR energy is about an order of magnitude longer than visible light. Other common forms of electromagnetic radiation include radio, ultraviolet, and x-rays.

    Infrared thermometer elements

    A simple analysis of the eye, one form of a radiation thermometer, reveals the basic components used in an infrared thermometer. The eye contains a lens, which focuses the photon flux from the emitter onto the retina or radiation detector of the human system.

    The retina is stimulated by the incident radiation and produces a signal that is transmitted to the brain. The brain serves as an indicator or recorder, which measures the radiance of the emitter and, if properly calibrated by experience, relates this radiance to temperature.

    The same basic elements comprise an industrial infrared thermometer. These include collecting optics, a radiation detector, and some form of indicator. It is the capabilities of available detectors that result in the effectiveness of present day infrared thermometers.

    Temperature instruments

    Spot instruments are the oldest form of IR thermometer. They are noncontact thermometers that measures a spot on a target (Fig. 2). These instruments are used for controlling ovens, furnaces, and heaters. They can measure as low as -50 F and as high as 6500 F and measure spots as small as 0.017 in. in diameter at speeds as fast as 25 milliseconds.


    Units are available in single and dual wavelength models. Lenses can be fixed or focusable, or fiber optics may be used to transmit energy from the target to the sensor. Spot instruments are useful for moving targets because they do not interfere with the process. A disadvantage is they only measure one spot on the target.

    Line scanners were developed to measure the temperature of hot webs of moving material, such as steel and glass. A single detector with a 45-deg angle mirror creates a 90-deg scan angle across the target as it is moving to develop a two-dimensional (2D) thermal image.

    With software, the target can be divided into spots for online and offline analysis and data storage. These instruments measure temperatures from 50 to 4000 F at speeds of 100 Hz.

    Thermal imagers are the ideal IR thermometer for static targets. They provide a thermal image of the entire product (Fig. 3). They can measure from %%MDASSML%%40 to 4500 F and can see small targets. They are available in almost every wavelength that is provided by spot instruments.


    The detector for this instrument is a focal plane array. Each pixel acts as an individual infrared thermometer. The detector is radiometric. This means that the signal from each pixel is changed to a temperature indicator.

    Causes of temperature error

    Several factors must be considered when installing and operating an infrared thermometer. Errors caused by these factors can be sizeable but are avoidable, so the instrument will provide the correct subject temperature.

    Emissivity. All infrared thermometers are calibrated to a device called a black body. A black body is considered the perfect emitter. This means it is the only target that emits maximum infrared energy at any temperature and wavelength.

    Every other target emits less than the maximum and has an emissivity error. For example, oxidized steel has an emissivity of 0.8 at almost all wavelengths, which means it emits 80% of its infrared energy compared to a black body. As a result, when an instrument that operates at 1 micron sees this target at 1600 F it would indicate a temperature of 1563 F, an error of 37 deg F.

    To correct for this error almost all infrared thermometers have an emittance control. When it is set at 0.8, a 20% gain is added to the infrared signal to allow the thermometer to indicate the true temperature. Values for emissivity vary with the subject and the instrument wavelength but can be obtained from published tables.

    Reflections. Many applications for infrared thermometers require measuring the temperature of a subject inside an oven. The problem is, the oven is hotter than the subject, and every target has a factor called reflectivity.

    For example, steel with an emissivity of 0.8 is a 20% reflector. When looking at steel inside a reheat furnace, the instrument measures the radiant energy from the steel and 20% of the reflected energy from the furnace walls. The final temperature reading would be as much as 200 deg F too high.

    The basic solution to eliminating reflections is to measure the temperature of the subject as it exits the oven where the background is cooler than the target and reflection is not a problem (Fig. 4).


    Transmission. For transparent materials, such as glass, plastic, and some semiconductor products, there is a factor to consider that is called transmission. If the instrument is looking at a target that is being heated from the other side, the instrument could measure the heater rather than the transparent material.

    For example, glass is opaque to any thermometer that operates at wavelengths of 5 microns or longer. When viewing a light bulb with a 7.9-micron thermometer, the instrument will only measure the glass temperature and not see the 3000-F filament inside the bulb.

    Focus. All infrared thermometers have a specific target they can measure. For the thermometer to indicate the correct temperature, the hot target should be at least two times larger than the design spot size of the instrument. A common question is "What effect does distance have on temperature?" The answer is none, except the spot size gets larger and a larger target is required.

    Interference. When looking at a hot target the instrument needs a clear line of sight (Fig. 5). Dust, smoke, and a dirty lens can all cause the instrument to indicate an incorrect temperature. For dusty applications, the dust is usually moving, and if the eye were as fast as an infrared thermometer the real target would be seen intermittently. If the lens gets dirty, use a clean, dry air purge.


    Most infrared thermometers have a function called a peak picker that picks hot spots and prevents the instrument from indicating cold spots when dust is in the way.

    If flames are clean gas flames, the instrument looks through them. If flames are dirty, such as oil or coal flames, the instrument cannot see through them.

    Calibration. All infrared thermometers should be calibrated on a regular basis. Most ISO procedures suggest once a year as a normal time period. The only way to properly calibrate the instrument is to use a certified black body source as the standard.

    Plant Engineering magazine extends its appreciation to Ircon, Inc. ( ircon.com ) for its assistance in the preparation of this article and in providing the cover photo. We also wish to thank FLIR Systems Inc. ( flirthermography.com ), and Raytek Corp ( raytek.com ) for their assistance in the preparation of this article.

    More Info: For more information on IR thermography visit the following web sites: infraredtraining.com , snellinfrared.com , and prothermographer.com . Additional technical information is available at the web sites of companies who contributed to this article.



    Advantages of thermal imaging

    The instrument can provide a full 2D image of a static target.

    Each pixel on the detector provides an exact temperature at that point.

    The instrument can be used as a vision system for detecting parts.

    Software can provide as many as 50 areas of interest.

    The instrument can take as many as 60 images per second.

    Elements of sensor selection

    What is the temperature range of the target?

    What is the size of the target?

    How close to the target can the instrument be installed?

    Does the target fill the field of view?

    What is the target material?

    How fast is the target or process moving?

    Will discrete objects or a continuous process be measured?

    What is the ambient operating temperature?

    Are the ambient conditions contaminated by dust, smoke, steam?

    Will there be a connection to control equipment?

    Do records for audit or quality programs need to be kept?



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