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Blog #045 – Your THERMOGRAPHY COMFORT LEVEL – Instantaneous field of view (IFOV), spot size

Although IFOV or spot size is not usually a huge issue in building diagnostics a thermographer certainly needs to understand what it is.  I’ll explain why a little later.  The definition for IFOV as found in the glossary of the ASNT Nondestructive testing Handbook 3rd Edition Volume 3 Infrared and Thermal Testing is:  “Angular subtense (expressed in angular degrees or radians per side if rectangular and angular degrees or radians if round) over which an instrument will integrate all incoming radiant energy;  the projection of the detector at the target plane.  In a radiation thermometer this defines the target spot size; …”   Now we are really confused, right?   My definition is: “the target area seen by a single detector expressed in degrees or milradians”.  What does all of this mean and why do we need to understand it?  We assume the temperature reading on our thermal imaging camera is accurate (within the calibration specifications from the factory) if the target surface has a very high emissivity, right?  (Remember:  The higher the emissivity of the target surface, the more accurate the temperature read out on your camera.  The lower the emissivity of the target surface, the higher the reflectivity and thus the more inaccurate the temperature reading.  What does that temperature reading on your IR camera represent?  It is the average temperature of an area as seen by one detector.  Therefore, one of the major things to remember is to check your distance from the target and the size of the target.  Why?  To illustrate: picture in your mind that I’m holding a slide projector in my hand.  As I walk closer to the wall the screen size on the wall shrinks.  As I walk away from the wall the screen size increases.  We move the projector to the right distance in order to fit the projection within the screen, right?  Think of the “IFOV” or “spot size” on your infrared camera as the size of the projection (although it is not projecting anything).  If you stand back too far your detector will give you the average temperature of your target plus the surrounding area.  If your target is 168°F and your surrounding area is -22°F and your spot size (IFOV) is greater than your target size then your reading will be corrupted.  You will get an average temperature of the 168°F area and the -22°F area.  My Fluke Ti32 thermal imaging camera has a spot size ratio (standard lens) of approximately 784:1.  That means that if I stand 65.33 feet from the target it would be important that my target size is 1”x1” or larger.  On the other hand, if I’m using my Fluke TiR1 camera (standard lens) the spot size ratio is approximately 392:1.  So, at 65.33 feet my target needs to be 2”x2” or larger.  Notice the image below.  This was taken on Dec 23 at 6:45am.  I’m using my Fluke Ti32 infrared camera with a 320×240 resolution, a 50mK detector system and the standard lens.  The target is the moon, which is approximately 238,857 miles from earth.  Using the Ti32 the spot size (IFOV) is approximately 304 miles x 304 miles and the temperature reading on that area of the moon in this image is 168°F. (If I were using my Fluke TiR1 the spot size would be 612 x 612 miles, twice that of the Ti32.)


This image was taken with a Fluke Ti32 on 12/23/10 at 6:45am.  The registered temperature is 168°F.  Since the moon is  approximately 238,857 miles away, the spot size is approx 304 miles x 304 miles.

This image was taken with a Fluke Ti32 on 12/23/10 at 6:45am. The registered temperature is 168°F. Since the moon is approximately 238,857 miles away, the spot size is approx 304 miles x 304 miles.




I started this blog by saying that IFOV or spot size is not a huge issue when using thermal imaging cameras for building diagnostics.  Why?  We are usually standing very close to the target and the size of the anomaly is usually large.  Think of missing insulation issues or moisture issues or air leaks.  All of these anomalies will be larger than a couple of inches, wouldn’t you say?  A normal distance from the target surface in a home is 5-10 feet.   When you stand 10 feet from the target using a Fluke TiR or TiR1 (160×120 resolution and standard lens) the spot size is 0.3” x 0.3”.  If you are using the Fluke TiR32 or Ti32 (320×240 resolution and standard lens) the spot size is 0.15” x 0.15”.  Like I said, IFOV or spot size doesn’t present much of an issue in building diagnostics.

How’s your comfort level in thermography?
(The above comments represent my opinion).

Rod Hoff / Restoration Consultants Inc
Thermography Instructor / IR camera sales
3284 Ramos Circle, Sacramento CA 95827
toll free 888-617-3266 ext 301
fax 916-736-1134

Provider of Fluke TiS, TiR, TiR1, Ti, Ti25, TiR27, Ti27, TiR29, Ti29, TiR32, Ti32, TiR3, and TiR4 infrared cameras.


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About infrarod

Rod Hoff is a Thermographer and instructor with Restoration Consultants, Inc. He teaches a two-day IR class in moisture and building envelope investigations. A graduate from Florida State University, with a degree in education, he received his formal training in Thermography from Snell Infrared and Restoration Consultants, Inc.
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