Message Board Thread - "??? IFOV ???"

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??? IFOV ??? Thermoimagen Roberto Cruz 3/17/2004
i found this text:

The detail discernible in an image is dependent on the spatial resolution of the sensor and refers to the size of the smallest possible feature that can be detected. Spatial resolution of passive sensors depends primarily on their Instantaneous Field of View (IFOV). The IFOV is the angular cone of visibility of the sensor (A) and determines the area on the Earth's surface which is "seen" from a given altitude at one particular moment in time (B). The size of the area viewed is determined by multiplying the IFOV by the distance from the ground to the sensor (C). This area on the ground is called the resolution cell and determines a sensor's maximum spatial resolution. For a homogeneous feature to be detected, its size generally has to be equal to or larger than the resolution cell. If the feature is smaller than this, it may not be detectable as the average brightness of all features in that resolution cell will be recorded. However, smaller features may sometimes be detectable if their reflectance dominates within a articular resolution cell allowing sub-pixel or resolution cell detection.

So, when we want to detect hot spots of 1 square inch at 60 feets away, for example bad conections at high voltage towers, what IFOV needs my camera have?.

please see this picture:
http://www.geog.nottingham.ac.uk/~dee/gif/ifov.gif
 
Re:??? IFOV ??? Gary Orlove 3/18/2004
For your example the IFOV needed to just subtend your 1 inch target at 60 feet away is easily calculated this way (remember we must convert feet to inches):

1/(60 x 12) = 0.00139 radians

Now this is the IFOV needed to place one pixel of your camera’s detector on the target. Most references suggest that in order to detect a hot spot, the hot spot should subtend about 2 x 2 pixels on your detector, so the IFOV must be halved:

0.00139 radians/2 = 0.00069 radians or more conveniently 0.69 milliradians (mrad)

Now this is to be able to detect the hot spot, not measure it. In order to measure, the target needs to illuminate more detector elements on your infrared camera. Most modern IR cameras with focal plane arrays can do a decent job of measurement if the target subtends 4 x 4 detector elements. So now calculating the IFOV you need to measure this target:

1/(60x12x4) = 0.35 mrad

Older cameras using a mechanical scanning mechanism typically require the target to illuminate more IFOV’s in order to provide reasonable measurements.

Gary Orlove
Infrared Training Center
 
Re:??? IFOV ??? Thermoimagen Roberto Cruz 3/18/2004
thanks Gary,
sure your answer will help us (lot) of thermographer s to take care about this important factor and limitations of our imaging equipments.
 


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