sensor window

[johnelot]

here is a couple of pics of an older thermal core - i just wondered if anyone knew what the gold flecks are in the material , it looks like it might be better if they were not there ?

[Fraser]

It is always a challenge to interpret such pictures as the above illumination reveals every tiny spec of dust and light scratch and makes them look awful. Like flash photography.

That microbolometer window does look a little ‘abused’ though. It looks to be Germanium but those scratches concern me and are not ‘normal’. Very light fine scratches are not normally a problem

Ok, regarding contamination on, or in, the window material at time of manufacture...... it is possible for a window to have tiny imperfections or contamination during production.

The problem is that the camera is then calibrated and the effects of any contamination are captured and countered during the calibration process. If you subsequently remove such contamination the calibration that captured the defects becomes invalid and weird things happen, such as a bright spot on the produced image due to removal of attenuation from the window. Care is needed when changing anything involved with the microbolometer.

Apart from dust that attached itself since production, you should not concern yourself with the window appearance.

Fraser

[johnelot]

it does seem to have a few small scratches from somewhere , but as for the contamination in the window - intresting that they just calibrate it out - but on something expensive like this surely they could use a nicer cleaner window , unless the white/gold color  contaminant is just as transparent to ir as the rest of it i guess. i have a flir core i might try to get a pic of it too to compare

[Ultrapurple]

So here's my question: how big does a speck have to be before it is noticeable on a thermal image?

Assuming the window is about a mm or so above the FPA and the rays from the lens are converging, any speck is going to appear on the FPA slightly smaller than it really is. The actual geometry depends on the lens characteristics and front of window to FPA distance but let's say for the sake of argument that a 10um speck casts a 9um shadow on the FPA.

Now we have to consider the microbolometer pixel pitch. Let's start with 17um. Assuming a 100% fill factor (which is essentially unachievable but makes the assumptions easier), one would need a carefully positioned speck to shade just one pixel. Based on the earlier assumptions, a speck of about 18 or 19um would be big enough to obscure a 17um pitch pixel. But in the 'best' position, equally shading four adjacent pixels, the effect on each pixel would be to shade just one quarter of each. Would that be enough to show up? Or are we now in  the position that we need a (square) speck 35um on a side before there are discernible effects?

If the rays converge quickly from the lens and/or the front of window to FPA is larger than my guesses, a much bigger speck is needed to achieve the same masking effect. On an older 50um sensor you'd need an easily visible speck around 0.1mm square to have a major  effect.

The sharpness of the lens will have a big impact too. LWIR lenses are generally made from a number of compromises in a row and are nowhere near as sharp as a DSLR lens (they don't have to be) but this might increase the speck tolerance by say 50%. So now we're looking at perhaps a 0.15 x 0.15mm speck - that's well into blob territory!

I am quite happy for anyone to correct my assumptions and arithmetic but this little thought experiment does begin to indicate why really small, shallow scratches are nothing to worry about.

[Fraser]

Do not forget self radiation from the microbolometer die hitting the contamination on the highly transmissive window. The die runs at around 32C when operated in open loop thermal equilibrium mode (No TEC)

Fraser

[Hydron]

I know that window manufacturing defects are definitely a problem with (at least) the ULIS 17u sensors. They are pre-screened before sale but some will still show image defects with smaller aperture lenses (the f/1 aperture lenses will render defects more out of focus than smaller aperture lenses and thus have more of a chance of hiding the problem). This is similar to dust on a SLR/MILC sensor - it can be invisible when using a lens at f/1.4 but stop down to something like f/11 (or use for macro photography) and it can ruin pictures easily.

Edit: btw the defects appear as circular non-uniformaties and can't be easily masked with FFC.

For an example of why small amounts of dust or other contamination is only really a problem near the focal plane, see this post on the lensrentals blog (which also contains a lot of other interesting optics posts): https://www.lensrentals.com/blog/2008/10/front-element-scratches/