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How Close Do You Need to Be?

One of the most frequently asked questions in thermography is, “How close do I need to be to my target?” The answer depends upon target size and the type of data that are desired.

Appropriate distance is largely dependent upon three factors: target size, IR equipment optics, and detector resolution.

With qualitative thermal imaging, the maximum viewing distance is achieved where the object and any possible anomalies can be clearly resolved. If a target cannot be clearly distinguished, it will be necessary to move closer or to use a telephoto optic.

When using an imaging radiometer, obtaining accurate temperatures will require substantially shorter distances than those required for thermal imaging. Obtaining accurate quantitative data requires that the radiometer’s spot measurement size is smaller than the area being measured. If it is determined that the radiometer’s spot size is larger than the area being measured, it will be necessary to move closer or use a telephoto optic calibrated for the imager.

Because there is no method for correcting for errors caused by imaging at excessive distances from a target, it is imperative to always ensure appropriate distance prior to recording images.

Spot measurement size and its impact on accurate temperature measurement is one of the many topics covered in the Level II Infraspection Institute Certified Infrared Thermographer^® training course. For information on thermographer training or to obtain a copy of the Standard for Measuring Distance/Target Size Values for Infrared Imaging Radiometers, visit us online at www.infraspection.com or call us at 609-239-4788.