More than just point & shoot

Infrared (IR) thermometers and imaging cameras can locate thermal hot spots on ICs, assembled circuit boards, and mechanical devices without physical contact. Like any measuring instrument, IR thermometers (also called "pyrometers") and cameras perform only as well as their calibration. Blackbody temperature sources let you calibrate IR temperature instruments and minimize errors.

An imaging-camera calibration lab uses numerous black-body calibrators. Courtesy of FLIR Systems.

A blackbody calibration source often contains a thermal cavity with an aperture opening that provides the IR thermometer or camera with access to the cavity (Figure 1). A heating or cooling unit, a temperature probe (usually a thermocouple), and a temperature controller monitor and control the cavity's temperature. Calibrator temperatures can reach 2000°C.

Not all blackbody temperature calibrators contain cavities. Some use a thermal plate (Figure 2). Blackbody plate calibrators let you calibrate low-end IR thermometers and perform checks of IR cameras prior to calibration.

IR imaging camera manufacturers often start with a plate calibrator. A blackbody plate calibrator provides a consistent temperature across the place from which a technician verifies that all of a camera's sensors produce the same color on a computer screen. With the plate, a technician can adjust the camera to nullify offsets that produce errors.

Once a camera produces a consistent color, it's ready for a full calibration. At FLIR Systems (Billerica, MA; www.flirthermography.com), the calibration lab consists of several blackbody calibrators arranged in a quarter circle. The camera sits on a small platform at one end of the arm; the other end connects to a vertical rotating pole at the center of the circle. Two wheels that support the table end of the arm let technicians rotate the camera to point at any blackbody calibrator.

The calibrator temperatures range from –196°C to 2000°C, says service manager Pat McDonough. A monitor displays the image that the camera captures. A technician takes readings at each temperature and enters them into the camera. A curve-fit algorithm generates a calibration look-up table of digital values versus temperature for each of the camera's calibration ranges.

Ircon (Niles, IL; www.ircon.com) takes a different approach. Instead of using numerous blackbody calibrators, Ircon uses one. Technicians calibrate IR imaging cameras at five temperatures, says VP Vern Lappe. This process takes longer than using multiple calibrators, but it costs less to maintain.

Ircon's calibration procedure includes temperature cycling of the camera. To perform a calibration, a technician places the camera in a thermal chamber that cycles from 0°C to 45°C for each of the blackbody calibrator's temperature settings. The camera's thermal chamber contains an opening for the infrared sensors.

Regardless of whether you use one or many blackbody calibrators, you need to properly place the IR instruments relative to each calibrator. To perform an accurate calibration, you must place the instrument so its measuring area (field of view) is smaller than the area of the target (the calibrator's plate size or aperture size). Ideally, the target area should be twice that of the field of view, because an instrument takes the average temperature within its field of view and the temperature differs from the center to the edges.

Figure 3. Calibration requires placing the thermometer or camera at the proper distance from the calibrator.

Ircon's Lappe offers more tips for calibrating IR thermometers.

• Always place the thermometer on a solid stand and aim it at the cavity.
• Check the thermometer's calibration as soon as it comes into the lab. If it is out of calibration and the factory uses ISO procedures, notify the quality staff about the instrument's condition.
• Clean the instrument's lens according to the manufacturer's instructions. Some thermometers have three lenses to clean. Remember, IR thermometers and imaging cameras make optical measurements, so a clean lens improves accuracy.
• After cleaning, aim the instrument at the blackbody calibrator and check the temperature. Check the lower, middle, and upper ranges of the instrument.
• During the calibration, make sure the instrument is completely warmed and the blackbody calibrator's temperature is stable.
• Check the thermocouples used in blackbody sources at least once a year because thermal cycling of a blackbody source can, over time, damage its thermocouple. At temperatures such as 2000°F, the cavity glows (Figure 4). Thermal cycling causes expansion and contraction of the thermocouple's materials, which can degrade the sensor.

Because a calibration is only as good as the calibrator, a blackbody calibrator itself needs calibration. High-accuracy IR thermometers, with calibrations traceable to NIST or another national lab, serve as transfer standards and usually carry uncertainties four times better than the blackbody sources they calibrate.

You can also calibrate a blackbody calibrator with a spectroradiometer, says Robert Madding, director of training at FLIR Systems' Infrared Training Center (www.infraredtraining.com). A spectroradiometer measures the IR power spectrum of its target, producing a plot of IR power as a function of wavelength. "Spectroradiometric calibration is the only way to truly calibrate the irradiance from a blackbody source," claims Madding.

Blackbody calibrators provide manufacturers and third-party calibration labs with accurate temperature sources for IR thermometers and imaging cameras. Most users send imaging cameras to the manufacturer for calibration. Proper care and good calibration practices will produce instruments that consistently meet manufacturers' accuracy specifications.

"IR Thermometer Calibration," Omega Engineering, Stamford, CT. www.omega.com/literature/transactions/volume1/calibrate1.html.

Madding, Robert P., "Spectroradiometric Calibration of Blackbody Sources," Proceedings of Thermosense (XXIII), Vol. 4360. International Society for Optical Engineering (SPIE), Bellevue, WA, April 2001. p. 363. www.photonics.org/web/abstracts/4300/4360.html.

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