Temperature Range extender for thermal cameras - Cheap option from Fraser

[Fraser]

I have previously been asked if there is a cheap way to extend the thermal range of a thermal camera so that it can image very hot targets such as gas flames. The FLIR E4 does not have the range to cope with such a high temperature source so goes into over range mode.

Some cameras are limited to quite low maximum temperatures, such as the F1G2 and 'B' versions of FLIR cameras.

If you have a need to image a very high temperature target and only have a camera capable of say 100C or 250C, I have a crude solution that may assist you. The bad news is that calibration is totally lost, but the good news is that imaging is possible without over-ranging the camera.

What is my solution? ...... a simple filter that is placed in front of the cameras Objective.
Is the filter anything special for thermal imager use ? ...... no, its available on the high street 

What is it ? ..... It is nothing more sophisticated than a Glass UV filter for a visble light camera.

No Way! I hear you say.... Glass blocks Long Wave thermal energy....... well yes and no. In the normal state of affairs, glass does indeed block LW thermal energy. However when looking at a very high energy thermal source, some energy does make it through to the camera lens. The filter behaves like an attenuator.

Take a look at the attached pictures taken with my E4, with and without a Tiffen Haze 1 filter fitted. Judge for yourself. As you will see, the camera reading on a hot spot drops from Over-Range, to around 30C to 40C, well within the cameras capabilities 

You can experiment with different materials to see which work best in your application. I needed to image (but not measure) a gas jet so needed a lot of attenuation for my E4. My experiments proved that I could use a standard UV filter as an attenuator for my application. Note that adding another UV filter DOES NOT double the attenuation provided, it only increases the attenuation slightly as your are dealing with the passband of the filter glass and not the thickness per-se.

Go on, experiment and see what materials suit your requirements for high temperature imaging.

Fraser

[Fraser]

For those wondering what advantages the attenuator shows apart from measured temperature in the above images. The filter enabled me to see the effect of the gas jets playing on the surface of the hobs burner central metal cover piece. The moving hot and cooler areas due to air draughts could not be seen without the filter present. Adding attenuation enabled the camera to reveal subtle details and to decrease the temperature span that it automatically selects.

Fraser

[Fraser]

I almost forgot, as the filter is optically clear, the MSX camera still functions as normal and MSX may be used 

Fraser

[Fraser]

I also tried a silicon wafer as an attenuator but that did not work at all well. It heavily attenuated the thermal energy but the target was unrecognisable as your can see in the attached image of the same hob gas burner.

The fact that the wafer is populated and not bare may not have helped matters 

Fraser

[tomas123]

I love this forum.
Thank You 

Unfortunately it doesn't work with my B+W UV Haze MRC 010 filter.
This UV filter is a good thermal mirror and opaque for thermal radiation.

[Fraser]

@tomas123,

I will test some other filters tomorrow. I have some single coating Skylight 1A filters that are a possibility. I chose the Tiffen Haze-1 because I already knew it had thermal transmission capabilities from tests I carried out some years ago. At the time I did not need an attenuator though so just filed the knowledge away.

I do not know the glass and coating spec used for the Haze-1 but doubt it is anything very exotic. Single coating appears important from your findings though.

Happy experimenting

Fraser

[Fraser]

I will test some other glass and plastic products to see how they work as cheap thermal wavelength attenuators. Microscope slides and their associated thin glass cover slips come to mind as possible candidates

Some interesting filter passband details here from SCHOTT. We are making use of the camera filters low EM energy transmittance at um wavelengths.

http://www.schott.com/advanced_optics/english/download/schott-optical-filter-glass-description-2015-eng.pdf

Fraser

[2lps]

There is some attenuation using the thin plastic screen protector of a new mobile phone (the one put in the factory, and you remove). MSX doesn't work with it, as it needs to be very close to the object, in order to see clearly through it.

left - the protector is in front of the camera. In the first one, it is very close.
right - without the protector for comparison.

[Fraser]

You can also buy the armour glass type thin glass phone screen protectors. Not tried one of those yet but have a few somewhere.

I bought some more camera filters to test but have not tried them yet. From memory they are another Haze-1, two Skylight 1A's and a linear poloriser.

I have piles of unusual 20mm diameter Russian filters to test as well,

Materials that appear opaque at visible wavelengths can be transparent at thermal wavelengths so it is worth trying various plastics as well.

The advantage of clear attenuators is the continued presence of MSX data of course.

Fraser

[Bill W]

Black bin bags are about 50% transmission

Regards

Bill

www.fire-tics.co.uk