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Point Radiometers & Spot Measurement Size
Tip written by: Infraspection Institute

With awareness of infrared technology at an all time high, point radiometers have become a common tool in many areas. Frequently, knowledge of proper operation lags behind instrument popularity. Understanding how spot measurement size affects accuracy is imperative to collecting meaningful data.

All radiometers are limited by a characteristic known as spot measurement size or spot size, for short. Spot size is determined by a radiometer’s detector and optics. Typically, spot size increases as distance to the target is increased. For accurate temperature measurement, spot size must always be smaller than the target being measured. When using a point radiometer, be sure to keep the following in mind:

• Point radiometers are usually supplied with a Distance to Spot Ratio value. To determine spot size, divide distance to target by ratio value.
• Point radiometers have minimum focus distances. At lesser distances, spot size will not decrease.
• Single, laser-generated aiming dots do not represent spot size
• Multiple, laser-generated aiming circles/dots often understate spot size
• Beware of stated spot size ratio values. Spot size ratios are frequently quoted at 90% radiance (accuracy) or less

When using a point radiometer, be sure to understand the limits of your instrument and the challenges presented by your target. Always use correct emissivity values and stay within the limits of your instrument.

Spot measurement size and its effect on non-contact temperature measurement is just one of the many topics covered in all Infraspection Institute Level II Certified Infrared Thermographer^® training courses. For more information on open enrollment classes or our Distance Learning opportunities, call 609-239-4788 or visit us online at www.infraspection.com.