Do you have a calibrated source for calibration, i.e. a blackbody source?

Assuming you are able to adjust the basic user controlled parameters of ambient temperature, emissivity, distance image plane (plane) to oblect plane (target), and humidity, questions arise concerning the optics.

Is the material of the telephoto lens the same as the lens calibrated for the camera? If there exists a difference, then some adjustment is made in the transmissivity response. This would be calculating the difference after recording or observing the data via thermograms. Under ambient temperature conditions (conditions tolerant for the camera and for us) the transmission bands of infrared optics are relatively similar and does not grossly alter temperature ranges. Yet if the lens material is different from the calibrated lens on the camera. there will be some error, +/-, due to the transmitted flux. Some IR cameras provide the capability to adjust the transmissivity.

When testing the camera, such as calibrating and parameter adjustment, use an object/target with a known high emissivity, set the cameras emissivity to 1.0, ambient temperature, distance, and humidity. Ensure that sources of radiant heat on the object surface are reduced or negligible. Take temperature readings at 'normal' angle of incidence. Without altering the camera's parameters or position, change to the telephoto lens and take temperature readings. Always maintain a clear focus of the oblect (resolution). Compare your data. If the object/target source's emissivity is known with relatively high certainty, then repeat the process of data collection by adjusting the camera's emissivity and compare the results.

This is a rather rough in-field test. The accuracy of the data can be improved by using a black-body source.

Using a black-body source with the ability to test the cameras at various temperature points is optimum, yet if your IRT work is constrained to ambient and near ambient temperature ranges the in-field method can provide reasonable temperature difference readings. The degre of accuracy for this process will need to be calculated from the data.

Hi All! Thanks for the answers. I will try to post a better answer soon, but we are currently in the field working in Galapagos, so, alas, no access to a blackbody source! I will try some of what you advised. Still looking up the lens material but I think it is coated germanium. Anyhow know the IR transmission of that material? It was foolish of me to expect a non FLIR telephoto to be compatible without first checking the calibration! Basically, my cooler temperatures are being overestimated and my higher temperatures underestimated. It doesn't help that we are also working outdoors so the lens will heat up to an unknown temperature.

I will try Don's advice above and see if it yields a partial solution. Meanwhile I've taken the telephoto off and will work with a properly calibrated camera.

The gernanium lens should not adversely effect the temperature readings since the transmission band in the IR spectrum is 1.8 to 23 microns (approximately). I assume your Flir camera is a long wave IR, 7.5 to 14 microns, 8 to 13 microns. The germanium telephoto lens was most likely manufactured for use with short or long wavelength IR and therefore had been coated to raise itd transmittance to approximately 95 to 97 per cent of the wavelength band.

If I understood more about the specifications of your camera and the objects/targets for analysis, I may be able to troubleshoot the difficulty you have found in the field. If you are 'scanning' in an outdoor open environment you will also want to taken into account the cooling effect of wind. Measuring temperature with an IR camera when the wind equals or exceeds 9 metres per second is not recommended. A correction factor for wind speeds of 1 to 8 metres per second range from 1.00 to 2.54.

Working outside under direct sunlight over extended periods can heat up the lens assembly and provide false readings due to the sun's overwhelming radiant energy. You can ameliorate these conditions to some extend by shielding the lens assembly with a cardboard hood or a reflective material. The on board parameter settings...ambient T, atmospheric T, reflective T, humidity, distance, et cetera will help reduce reading errors. The IR camera has an in-built temperature detector (at least most recent models) that compensates for detector and system heat build up. The specs will indicate the useful limit for working temperature and humidity.

Are you thermally analyzing water contamination, plant life and animal surface temperatures? If you cannot answer for research reasons, I understand.

And by the way nothing you have said or done can be taken as foolish.

Let me know how I might assist from a distance.

Incidently, I utilize a Flir T440 and the ISG 6800, with high temperature range. They are LWIR (7.5-13.5 micron band). I have used several SWIR models for furnace interiors. LWIR are not adequate for 'thru'smoke, gas applications, rather they read the surface of the smoke, gas.

Hi Don!

Wow, thanks for all the help. Let me try to answer a few questions. I currently have a FLIR SC660. Dont have the manual on me so I forget the exact wavelengths, but I assumed between 5-20um? I could be wrong. Anyhow, foolishly, I attached a telephoto designed for a completely different camera by a different company. a 7-14 um telephoto attachment with no electronics or temperature measurements of the lens temperature. Initially during tests at room temperature everything seemed fine. I never noticed any under or overestimates of known thermal sources. However, we are putting the cameras to their limits in the field doing wildlife thermography. It's fine if you just want to produce pretty pictures, but we need to be quantitative since we are using surface temperatures to estimate the degree to which certain parts of the body exhibit blood flow.

Needless to say, in the hot tropical sun, the lens will be heating up and I can't really control this in the field.

Alas, I have tried some of the suggestions above (using a 36oC point source with and without the telephoto) and then trying to use ExaminIR's 'external optics compensation adjustment, but in order to obtain a value that makes sense, I would have to put in a transmission value of 0.59 for the lens (assuming lens temp is 26oC which is room temperature). I just dont trust this since I know the lens is germanium with coating to allow it to be ~97% transmission. Anyhow, I have gone out to collect new data. It's just a shame since the telephoto allowed me to obtain closer images on wildlife than I would otherwise have done.

Maybe once I'm back home in my lab I'll try this out under more controlled conditions.

I'll be back at Brock University in mid may. I'll let you know if I have some more answers.

Cheers

Glenn

Sounds like a good plan. ExaminIR may allow some 'massaging' of the data you have collected with the telephoto lens. I believe Flir would be interested in looking at the data for R&D purposes and may provide insight for you recovering useful thermographic data.

FYI, and not intending to instruct you, your camera:

Flir SC660

640x480 pixels (307, 200) resolution

7.5 - 13 micron spectral range

0.65 mrad - 24deg lens

0.33 mrad - 12deg lens

1.30 mrad - 45deg lens

It appears the disparate temperature readings you are experiencing may be the result of the variable for spatial resolution and image performance, IFOV. For a given viewing distance, the IFOV covered by an individual pixel would be given by the equation IFOV = FOV/(total pixels). The telephoto lens will affect the resolving function of the IR detector.

The spatial resolution of an imaging system is its ability to distinguish separate objects or parts of an object within its field-of-view. The smallest image element is determined by the IFOV. There may be different vertical (VIFOV) and horizontal (HIFOV) instantaneous fields-of-view. For an object at some range, R, from the detector, the IFOV will encompass some length. For example, if the IFOV = 1 mrad, then at 1000 m, the instantaneous field-of-view will cover 1 m in length. The spatial extent of the IFOV at object range R is estimated (for small angles) by height: Dh R x VIFOV width: Dw R x HIFOV. For infrared imaging systems, which detect the thermal radiation from objects, an important measure of performance is its ability to detect small changes in temperature. The smallest temperature difference a system can detect (and therefore can display differently) is called the thermal resolution. Changes which are too small to be distinguished from the background noise in the system will not be detected.

wow. 6 years ago I posed this question. I now have learned so much and I do realise just how difficult this would be to do without knowledge of the transmission loss of the external lens + the temperature of the lens itself. I should go back to these data some day to see if they might be useful to demonstrate why not to do this! I never sat down to analyse the images, since I could tell that the surface temperature estimates were very wrong.