Is my thermal imager defective?

[mikegb]

Hi everyone. I've recently bought a thermal imager( a UNI-T UTi260B ), from AliExpress. When I opened it, I saw that the infrared lens, as well as the screen had a few scratches, so I'm suspicious that I may not be new as advertised.

 I'm testing how it works and I've found something odd, I was watching my soldering iron(set to 380 C, in picture IMG_0027.jpg ) and I can see the IF light reflected on my table, but the odd thing is that I can see other kind of reflection with the shape of the iron's tip, but inverted( which to me sound like a lens problem).

I also pointed to my ceramic stove and the same thing happens, you can see in the picture(IMG_0019.jpg) the reflection on the wall( which is made of reflective ceramic tiles) as expected, but also the other reflection(on the right) which I think is a camera's problem, in picture IMG_0020.jpg I took a similar picture but with my hand covering the spot where the odd reflection appears, and as it can be seen the reflection still appears even though my hand( which is quite close to the lens) is covering it, so that gives me more confidence there's something defective with the camera.

Even more weird is that this inverted reflection stuff only appears with very hot things( like >100 C or so), but not with cooler things like with my hand( as you can see in picture IMG_0034.jpg).

In picture IMG_0025.jpg I took a similar picture to IMG_0027.jpg, but with a piece of cloth covering the table to avoid IR reflection, and the weird inverted thing still appears.

In picture IMG_0030.jpg I took an image of the same soldering iron, but I just moved the camera a little bit so that the iron is more centered and the odd reflection superimposes over the soldering iron, if i slightly move the camera( like changing the angle a little bit) the inverted thing moves over the display, so all of this seems like a camera problem.

I should clarify that all the pictures are taken with High gain mode, in Settings->Temperature scale, but the exact same problems happen if I set the camera to Low gain mode.

 I've seen videos on youtube about IF thermal imagers, and this problem I have never appears, I even watched a video with a person pointing to a soldering iron with the same model of camera as mine and that inverted reflection doesn't appear, so I'm even more convinced my camera is faulty. I'm not an expert in thermography, so I ask you those who know more than me what you think about this, I haven't confirmed the reception on the AliExpress webpage, so maybe I should ask for a refund. Thank you for your time and have a nice day!.

[Bill W]

It certainly looks like an optical internal reflection, so a design 'characteristic'.  Anyone had one of these apart to know if the rear lens element is concave or not ?

It may though be a coating batch problem in that if a bit more than usual is reflected back from the detector front into the optics there is more to come back again.

I am not surprised your hand does not create this effect.  In energy terms the soldering iron is not just 30x hotter (25-35 vs 25 - 325) but more like 1500 times (IIRC).  The optics do need to be quite good to avoid a 0.1% return - or designed out by a convex rear element or angled windows

Bill

[IR_Geek]

Agree, that's an internal lens reflection issue.  I could come from bad (or non-existent) coatings or just a bad optical design.

[mikegb]

Thanks, that explains it. should I then ask AliExpress for a refund or a partial refund?.

[c1400700226]

Check the teardown I posted. The quality control is very poor. Maybe the lens inside yours is particularly dirty, so it causes a lot of reflection.

Added: I just tested mine. Seems it's normal to have some reflections. For a general photographic camera, it's necessary to have very high quality coating on both the lens and the sensor to reduce those ghosting.