Dahua Reference Blackbody - under the covers :)

[Fraser]

The high emissivity coating is fragile and all Blackbody manufacturers warn not to touch it or attempt cleaning with anything more ‘aggressive’ than a photographic lens blower brush. This is good advice as the surface is very Matt and easily takes up oils from fingers or scratches from ‘tools’. Many relatively gentle solvents will damage the coating so do not even think of using a solvent on it.

As to the damage you have on your emission plate……. Well the good news is that what you see in the visible light domain is often not what is seen in the thermal energy domain. That is to say, a rub on the coating may look nasty, but it does not change the emissivity enough to make it visible at LWIR wavelengths  A scratch that gets through to the aluminium or copper plate is a different matter. That will be seen as bright metal or oxidised metal in the VL spectrum and sadly such damage will effect emissivity at that point. Such serious damage will be visible at LWIR but it may be possible to ignore it or touch-in the damage with some suitable very Matt paint. I use Krylon very Matt black camouflage paint but the solvent in it can attack the original high emissivity coating. Matt black enamel paint may also work well enough to cover a bright metal scratch and recover acceptable performance. The Krylon very Matt paint was tested by NASA and found to have excellent and predictable emissivity over a wide temperature range.

https://www.krylon.com/products/camouflage-paint/

In your case, I believe the ‘damage’ is limited to scuffing of the surface with possible light penetration to the aluminium plate. It is likely best left alone. You cannot restore that surface to factory state by any easy method and you will likely end up with a lower performance coating if you respray it. The original high emissivity paint coatings are both hard to source and expensive.

Fraser

[Bill W]

A scratch that gets through to the aluminium or copper plate is a different matter. That will be seen as bright metal or oxidised metal in the VL spectrum and sadly such damage will effect emissivity at that point. Such serious damage will be visible at LWIR but it may be possible to ignore it or touch-in the damage with some suitable very Matt paint. I use Krylon very Matt black camouflage paint but the solvent in it can attack the original high emissivity coating. Matt black enamel paint may also work well enough to cover a bright metal scratch and recover acceptable performance. The Krylon very Matt paint was tested by NASA and found to have excellent and predictable emissivity over a wide temperature range.
https://www.krylon.com/products/camouflage-paint/

Fraser

There are two other saving factors here.
Depending on the lens being used, the emission surface will be a (long/ very very long) way out of focus for a calibration routine, as the camera is failry close to the source.
Some calibration routines take a large number of frames over several seconds, so a 'shaky hand' can be used to blur these effects too.

Bill

[f3d3r8]

Thanks Folks!

Sounds like good news: I don't have another project to do... for now

I had a few other questions to finish out my thought process:

* Could you recommend any good papers on the more advanced high emissivity paints? Are they using Vanta Black or some variant of it?
* Would a DI water mist and a light brush do damage? (I'm not going to risk it now, just if it gets progressively worse...)
* If I end up having to resurface the plate, would you just spray over the factory coating?

Fun adventures in thermal imaging - this is a dangerous subject for my wallet 

[Fraser]

This thread should help you……..

https://www.eevblog.com/forum/thermal-imaging/black-body-thermal-references-and-fraser_s-new-black-body-project-)/

You will find discussion on coatings there.

As to whether to strip the original coating to bare metal before repainting….. you will likely have to do that to avoid a reaction between the solvent in the new paint and the original coating.

Fraser

[Fraser]

The NASA paint testing page appears to have moved/disappeared.

The Wayback machine comes to the rescue though 

https://web.archive.org/web/20190412075821/https://masterweb.jpl.nasa.gov/reference/paints.htm

Fraser