Message Board Thread - "IR from a Helicopter"

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IR from a Helicopter dcnewt2 3/25/2004
I have been asked to do IR scans on distribution lines in remote areas from a helicopter. Does anyone have any experience or suggestions on doing this. The helicopter has regs that it can only get so close to the lines, so the prop wash on the target should not be a problem. I also have a seven degree lens so I should be inside my spot size ratio. My concern is with the prop wash between me and my target-Any suggestions?
Thanks,
Dave Newton
 
Re:IR from a Helicopter Thermoimagen Roberto Cruz 3/25/2004
hi,

7 Degree lens?.. i assume you have Agema 470 unit?..what is your Spot Size?.. is the distance according with the IFOV of your camera? (7 Deg give you 1.4 mRad) ..this is important if you want to get accuracy temperatures. besides what type of camera is SW or LW (agema 470 is SW means that maybe will have sun reflections problems when make inspections at sunlight.

well, i would like to have more info on what equipment you are going to use, maybe i am wrong.

regards
 
Re:IR from a Helicopter Doctir bob 3/30/2004
The IFOV is the resolution spot size which is about 50% MTF. For measurement, you need close to 100% MTF. The MFOV, measurement field of view, is typically 3 to 4 times the IFOV.

The 7 degree lens will be a challenge in terms of target acquisition. You will be bouncing around trying to see the power lines with a telescope lens. Without stablized IR camera mounting, this will be difficult.

I used a 3X telescope in the '80s for this, hand held, and it was difficult. Emissivity, wind and other environmental effects can cause additional problems. But we successfully found several bad splices in spite of all the challenges. As far as getting accurate temperature measurements, I didn't even try. We categorized the problems based on apparent temperature, understanding we did not know the emissivity well and did not have sufficient MFOV. But everything was done about the same distance under the same load. So, we ranked the problems from the highest apparent temperature rise to the lowest.
 
Re:IR from a Helicopter Greg @ AITscan 4/1/2004
Here's more than you probably wanted to hear, but I just cut and pasted it from a paper I wrote once...

See aitscan.com for more on aerial IR.

Detecting electrical faults on high voltage electrical transmission lines is fairly easy and can be accomplished rapidly from a light aircraft. However, even from short distances, accurate temperatures of electrical faults are impossible to measure [quantify]. There are several problems associated with temperature measurement from the air which include spot size to target distance ratios, reflection of the objects surveyed, having a sufficient load on the line at the time of the survey among others. The spot size to target distance ratio is the number one problem with temperature measurement. Specification writers have not yet realized the seriousness of this problem and continue to ask for quantitative data on fault areas. The fact is that infrared cameras that are in general commercial use today cannot measure accurate temperatures on small objects from distances of 50 feet...much less from reliably safe flying distances. A one-inch (relative size of a transmission line splice) target cannot be measured from that distance, plain and simple, although it can be detected. These spot sizes are unmanageable and inaccurate on any target that does not have a large homogeneous heat signature. The GRE is critical to the measure of spatial resolution in aerial infrared thermography. Nyquist's frequency theorem states that an object less than two times the size of a sensor's GRE cannot be resolved for measurement, so a 3x3 pixel or GRE spot is needed for reliably obtaining measurements.
This shortcoming may be addressed by using more powerful lens to reduce the GRE for a given distance, but then the sensor's FOV is then reduced, limiting the area covered over a given period of time. So, if one is using a small format IR camera (256x256 pixels) in a helicopter only 50 feet away from a 1 inch “hot spot”, it is impossible to obtain accurate temperatures using a standard lens. The smallest “hot spot” that could be accurately measured with one of these imagers is over 2", even at that extreme short distance. Also, from the air, using a more powerful lens does not work well because vibration is more evident in the form of image 'shaking'. Image 'smearing' may also occur due to an increase in the apparent speed of the sensor's view across the ground. In the air, there are few substitutes for a large pixel array, but even using large format detectors, one cannot and should not profess to measure temperatures on very small objects. These anomalies can be seen, and by comparing them to similarly loaded phases or equipment, potential problem areas can be identified, saved and marked on a map. For ‘good’ measurements, a ground verification team should be used to inspect suspect hot spots from the ground (cloudy nights are best) and verify the findings of the aerial IR survey. They will be closer to the target and with a powerful lens on a stable surface, much more accurate.
High Voltage Electric Utility Distribution Lines
Because they are smaller, lower to the ground and often run through populated areas, high voltage electrical distribution lines are much more difficult to see against all the thermal clutter on the ground such as trees, street lights, people, animals, etc., than transmission lines. Therefore they are best left to ground-based infrared thermographers.

Greg S.
 
Re:IR from a Helicopter Derrick 4/19/2004
Please give me a call. I work for a large utility and we have been performing IR from a Helicopter since the early 70's , myself since 1991.
I may be able to help you with your questions.

705-795-3626
 


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