Iwi004,
Sadly ND filters for thermal cameras are not common. The reason for this is that the maximum temperature capability of a microbolometer thermal camera is set using the pixel bias voltages. Cameras that need greater temperature range just have two or more bias voltages that create additional ranges. As the range is increased, the sensitivity normally decreases. For very high temperatures, cameras do need a filter or attenuators to be attached in the optical path. Such cameras are usually of the Industrial variety.
Now back to your situation........
Your E4 should be capable of measuring most electronic components within its maximum temperature capability. Temperatures much higher than 260C would normally indicate a serious situation that could lead to self desoldering of the component ! Are you able to expand a little on your situation ?
The Emissivity of electronic components can be an issue so you need to ensure your camera is set up correctly respect. You can carry out a temperature accuracy check on a certain component by using a surface contact thermocouple and then comparing the reading with that produced by the thermal camera. A difference in the readings could be due to an incorrect Emissivity setting in the camera for the particular surface it is viewing.
There are simpme 'tricks' that enable a thermal camera to view targets that exceed the cameras maximum temperature, but they are crude and uncalibrated. I used such a method to view a gas burner head.
Some experimentation is required but here is an insight into what can be done simple.....
A thermal attenuators is just a material that does not provide 100% transmission of the thermal scene to the camera sensor. By this definition, all thermal camera lenses are attenuators ! I.e. They all introduce losses into the optical path. If we take that idea a step further, we can deliberately insert a material into the optical path that we know to have poor transmission in the LongWave thermal spectrum. The material does need to be of reasonable 'optical' quality though.
Polyolefin film, as used in consumer product shrink wrapping, is a decent material for the transmission of thermal energy. It is used as a simple thermal camera lens protector. It is available in various thicknesses and in the case of a lens protector, I recommend 12micron or 15micron for good transmission. As the thickness increases, the transmission decreases due to losses within the material. So we need to consider both the material and its thickness when making an attenuator.
I have successfully used a standard photographic Tiffen Haze filter on my E4 to view the gas appliance head and flame. The filter is made from optical glass that is not too thick and is NOT multi coated. Glass has very port transmission figures in the LW thermal spectrum but ir is not 0% transmission. It therefore acts as a large attenuation block in the optical path. It also acts as a lens protector if viewing boiling fluids such as oil.
As to gaining ant sort of decent temperature measurement.....that could be challenging. There are two approaches that could be used. Both have their error factors though !
1. Set up a test with a test target operating at the predicted temperature that will be observed when using the attenuator. Measure the target accurately with a thermocouple ot PT100 contact sensor. Observe the target using the thermal camera with the attenuator in place. Make a note of the temperature indicated on the camera. This is the temperature you should expect to see on target device if it has the same Emissivity as the test piece that was just used. The reverse process could be used. The target temperature could be measured with the camera and then the camera aimed at the test piece that has a contact temperature sensor fitted. The temperature of the test piece could then be adjust until the camera reads the same temperature as the target device. The true temperature may then be read off of the test piece.
2. A standard comparative calibration test could be carried out in order to produce a simple temperature conversion chart. A variable temperature test source is needed for this process. I would suggest that a variable temperature controlled soldering iron may be useful as a test source. Its bit should be Dull and not shiny. A really old and pitted bit is best ! This keeps the Emissivity reasonable. Unless the calibration of the so,dering iron display is trusted and reasonably ac irate for the free air temperature of the bit, a separate Thermocouple or PT100 temperature monitor should be attached to the bit.
TIP: if you do not own an old corroded soldering iron tip, you can buy Matt black high temperature exhaust system paint from a motor factors. Spray the shiny tip with this to form a decent Emissivity surface good to at least 600C. Some HT paints can offer higher temperatures but the soldering iron will 'top out' before 600C anyway ! You can get similar paint for cast iron stoves and BBQ's as well.
The test set up will involve pointing the thermal camera at the soldering iron bit without the attenuator fitted, followed by the use of the attenuator.
a) set the soldering iron to its lowest temperature setting and allow to stabilise. Check the temperature with a thermocouple or PT100 meter. Adjust the temperature control if required to provide a round number such as 200C. Measure the tip temperature using the thermal camer without an attenuator fitted. If the reading is not 200C, adjust the cameras Emissivity setting until it does provide a reading of 200C. We are trying to get the Emissivity setting of the camera to match that of the dull soldering iron tip. Avoid pointing the cameras measurement area at shiny parts of the soldering tip !
b) Create a table on paper with the following columns....
Target Temp actual
Target Temp TIC no attenuator
Target Temp TIC with attenuator
You need to decide on the resolution of your calibration table. I would suggest 10C steps but it is your choice. The temperatures are listed down the page, like so....
200C
210C
220C
230C
240C
250C
Etc.....
For each temperature you will enter the required heading data. First the targets temperature as measured by its calibrated display or a suitable contact temperature sensor. Then check the target temperature using the camera without the attenuator, followed by with the attenuator in front of the lens. Change the temperature of the target to the next incremental value and allow to stabilise. Repeat the readings at that temperature. Continue this process until the camera reaches its maximum capability without the attenuator. It will normal, indicate an over range situation. Beyond that, calibration of the camera is unknown. At this point, you can stop doing the camera without attenuator readings and just leave the attenuator in situ.
c) After all the required temperature steps have been documented you will have a reference chart that will enable the readings of the thermal camera to be checked against it and interpreted. Why did we test the camerawith no attenuator in place ? These cameras become less accurate at the extremes of the temperature ranges. It also gives you an indication of the attenuators effect on the readings as a confidence check that the camera is not producing a very non uniform measurement response with the attenuator in situ.
Now the bad news.... neither method detailed above is with out its limitations and inaccuracies. Emissivity of materials can change with temperature and the passband of the attenuator is unlikely to be flat. Hence why we cannot just determine an attenuation offset rather than individual comparisons to a known target at a known temperature. There are several other issues that can introduce errors in the readings, such as surface reflectivity and background ambient temperature, but I suspect Young can still achieve the desired accuracy by just using the above methods and setting the ambient or background temperature in the camera.
This is a time consuming process but the good news is that method b) would only need to be carried out once and then you have your temperature translation table for raw particular attenuator.
3. You can experiment with various smooth surface materials to assess their thermal spectrum attenuation capabilities. Very thin glass is a possibility. Glass cover slips for microscope slides could be tried. Various plastics are decent attenuators of thermal energy. They need not be transparent at visible light wavelengths ! ABS modelling plasticard could be tried as it comes in various thicknesses. Just consider anything relatively thin and these it by viewing a soldering iron or gas burner head through it. If you are lucky enough to have a plain wafer (unpopulated) of silicon that also works as a good attenuator at LW. Avoid metals, they will not work.
Finally, a word of caution. When observing a target that is emitting large amounts of thermal energy, it can start to heat the attenuator material and this will cause measurement accuracy issues. In industry, an air purge is used to maintain the temperature of the front window. This is why that is done. Suvh air purge systems are needed when observing furnaces and very hot targets in the steel industry.
Hope this helps
Fraser
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