thermal sensitivity vs thermal accuracy

[cevans (Chuck Evans, CMI)]

I think it’s time to banish this clown

[Rye_Zers]

I know the FLIR i7 is a severe threat to home inspection business because home owners can buy their own thermal imager and check where the aircon is leaking. But don’t worry… because companies can be inspected only using your expertise so why not try companies inspection instead of just home. This way. the i7 is not a threat to your business.

I ordered the i7 the other day because the next cheapest 160x120 is the e40 and it costs $4500 in the US. At least the i7 at $1995 is affordable enouigh.

[jgromkoski (John Gromkoski, License No. 16000020183)]

@cevans

I think it’s time to banish this clown

I couldn’t agree more.

[dandersen (David Andersen, TN HI# 40)]

So take your $1995 camera and use it like all the homeowners that have purchased them and found out that it is not a “point and shoot” camera.

Go to the ITC (Infrared Training Center) web site and look at all the nice vidieos that people spent many hours producing to answer questions just like yours.

There is no one here that is qualified and has the time to teach you.

[Rye_Zers]

@cevans

Thermal sensitivity and thermal accuracy are different specifications. There is another factor that you need to be aware of which is spot size. An infrared imager (imaging radiometer) cannot accurately measure temperatures at the single sensor (pixel level)

Spot size defines the minimum size object that can be accurately measured with a given lens at a particular distance with a particular imager. Anything smaller than the spot size cannot be accurately measured, although it may be readily seen. The spot size will cover some number of sensors and is an important consideration when performing quantitative anaysis.

Thermal sensitivity specifies the minimum change in signal due to variation in incident radiation that the sensor can detect (i.e., how small a temperature variation can the imager see). The sensor can detect temperature variations in things that are too small for it to measure.

Each sensor in the array will have a different response curve to infrared radiation. Each cell in the array will vary in signal response from other cells around it for the same incident radiation. This is corrected through the camera firmware (set during the factory calibration process) and is periodically normalized through the NUC (non-uniformity correction) process, which corrects for drift. Most current imagers NUC automatically.

Basically, your imager will periodically flip a uniform temperature screen up in front of the sensor and readjust for the signal level of each pixel in the array so that it renders the same temperature across the whole array. Think of it like the mirror in an SLR camera. Every time your imager freezes and makes the familiar clicking noise, which is the screen flipping up in front of the sensor, it is going through the NUC process.

The NUC process is what eliminates the “smearing” that you refer to.

This is also the reason that your uncooled imager should be given time to warm-up and reach equilibrium in the environment in which you are using it. You may also notice improved image clarity an accuracy immediately after the NUC event.

This is but one of many reasons that thermography is not simply point and shoot. If you are doing energy audits you should be focused on the thermal patterns (qualitative analysis)

Yes you can see significant differences between imagers with different spacial resolutions and thermal sensitivities.

Very excellent description. But for those that says speciificaly that 0.05 c at 30 degree. Does this means at 50 Celius or 80 Celsius. The 0.05 C sensitivity no longer works? I thought from the begnning hte 0.05C sensitivity is across all temperature range. If not true. Then at 100 Celius, what is the thermal sensitivity… would be become like 3 Celsius or something?

[Rye_Zers]

Let me just share what I found out about the theoretical limit or maximum megapixel for thermal camera before I leave this forum and join an actual IR class and give you guys peace

The formula for airy disc diameter is 2.44 * wavelength * focal-ratio.
For visible light wavelength of 550 nanometer (green light) and focal-ratio of 1.5 (most thermal imagers have focal ratio less than 2 to make the spot smaller as larger focal ratio like f/10 can make it larger), airy disc diameter is 2.44 * 550 nanometer * 1.5 =
2 micron. This is for visible light camera.

For thermal imagers. Wavelength is say 10 micron (middle of 7-13 micron) and focal ratio of 1.5 gives 2.44 * 10 micron * 1.5 = 36.6 micron

That is. Thermal camera spot size is 18 times larger than that of visible light therefore there is an upper limit to the megapixels.

From 36.6 micron… signal sample theorem gives about 18 micron minimum size pixel. Anything smaller and it’s a waste of megapixel (it’s not like in telescope where you can use barlows to increase pixel scale or make smaller pixel to image better double stars).

Now if you will check out the specs of FLIR cameras. They use about 17 micron for the SC600 for 640x480 and the camera is much larger than the E60 or Fluke Ti32 which only use 320x240.

Bottom line is. Because of the 36 micron spot diameter of the airy discs of thermal cameras. 320x240 or slighter larger camera for 640x480 is the ultimate megapixel as far as hand held portable thermal imager is concerned. They have now the 1 MP Atom thermal camera but it is quite large and not unlike the E60 or SC6000. Of course one can make 2Mp or even 10 MP thermal imagers but the camera would be as big as a desktop computer or even room size. This is because you have to create very large lens as the chip of 2 MP thermal camera that uses minimum 17 micron is larger.

Conclusion. 320x240 and 640x480 thermal camera in portable handheld format would be here to say for the next hundred years. And therefore it is not a bad idea to invest in a 640x480 as it will be useful for generations to come where even your grandchildren can still make good use of it.