When it comes to infrared, “if you can’t see it, you can’t measure it”
This is a key point because whether we’re engineers checking bearings or electrical inspectors checking for bad connections, an inspection that is blind to a fault is worse than no inspection, as it can lead to a false sense of security.
The Field of View (FOV) is simply a measure of how much your camera can see. It is similar to the width of a car windscreen. If it’s a wide windscreen, it has a wide FOV.
Instantaneous FOV (iFOV) asks us, what is the smallest object we can see through our windscreen?
In this picture, we see a typical car windscreen with a wide FOV (in this case it is about 125 degrees). Through his screen, the driver can just make out the sign warning of wandering cattle, some meters ahead. Anything smaller than this he can’t see.
The sign is the driver’s iFOV. The smallest object he can define.
When measuring temperatures with Infrared (whether a thermometer or a camera), what we are measuring is NOT defined by the helpful laser spot marker equipped on most devices.
The laser merely marks the very (sometimes very, very) inaccurate center of a measurement spot, whose size varies when we get farther from our target.
With an IR thermometer, it’s fairly simple. We have a simple distance-to-spot ratio as shown in the diagram below. The further away we get from the target the bigger the measurement area becomes.
If the measurement spot is bigger than the target, the temperature displayed will have massive errors! In fact as the measurement spot gets larger than the target, the errors grow exponentially!
So it is mission critical that the measurement spot MUST be smaller than the target. An IR thermometer is a thermal camera with a 1-pixel by 1-pixel resolution. A thermal camera has many pixels, sometimes several thousand!
Because you can’t be sure that the fault image will fall squarely on a single pixel receptor, one needs more than 1 pixel within the measurement spot. There is a generally accepted idea in the IR Inspection world that:
To see a fault, one needs a measurement spot containing 3-pixels
To measure a fault, one needs a measurement spot containing 5-pixels
So the mathematics gets a little more complex, but don’t worry! Homershams have constructed a spreadsheet to do these calculations for you.
Thermal Imaging cameras, range from $2,500 to well over $80,000, so clearly there are some big differences between models. How can the potential user select the appropriate model?
The single most important factor in selecting a thermal imaging camera is TARGET SIZE. “How big is the object I’m wanting to measure and how far away from it am I?”
In one fell swoop, the answer to this question reduces ones short list from tens of models to a handful.
The danger of not understanding target size vs. model selection is that one may choose a camera that either misreports what the temperature is or doesn’t even detect a fault! In this case it is worse than having no Infrared inspection at all, as one might be lulled into believing all is well, where in actuality, faults exist.
Now this danger exists both for close-up work (e.g. switchboard inspection) as well as distant work (e.g. Power Line Maintenance).
For example a reader of this article might be surprised to read that perhaps NZ most popular camera, is NOT good enough for many switchboard inspection jobs.
The good news is that checking the target size requirements is “science not sales!” One can reasonably simply calculate the required specification for any given target size/distance using trigonometry, but as mentioned above Homershams have helpfully done this on a spreadsheet to make it easier.
(It is generally accepted by the industry that to detect a fault one needs a spot size, containing 3-pixels, that is smaller than the target and that 5-pixels are required to measure the actual temperature.)
Here is a theoretical example: As mentioned a very popular, high-end brand, IR camera sold in NZ in good numbers, has a minimum 5-pixel spot size of 11 mm at its closest focus distance. 11 mm is just too big if you consider electrical cables of 3 mm diameter. One might easily completely miss a fault and only become aware of it, when the Fire Brigade or the customers insurance company contact you!
Here is a real example:
One of Homershams sales team was delighted to discover a line fault in Kaianga Road, Christchurch during 2012.
When walking out of a customer’s premises, they looked up at a power pole with their IR camera.
Here is the image that was captured.
Typical camera resolutions are as follows
120 X 120 pixels
160 X 120 pixels
320 X 240 pixels
384 X 288 pixels
640 X 480 pixels
1024 X 768 pixels
2048 X 1536 pixels
320 X 240 is a very common size for Preventative Maintenance and Electrical Inspection for the reasons mentioned in the article above.
But moving to 384 X 288 pixels gives a significant increase in quality, greater than the simple 20% increase in pixel count, due to the excellent ULIS sensor.
(Some of Homershams Optris cameras use the same sensor)