This lens good anything (thermal)?

[Vipitis]

Glad you were able to take it apart. Your diagram shows something like an aspherical element. Which is difficult to manufacture and design but works against distortion. 4 elements rise the value of the unit and lot; and they are also 35mm diameter so work for larger sensors.

About handling them, you can use an lens assembly tool, those with a little vacuum cup or some soft pliers - or just graphics the elements on the side. I dropped my elements onto the floor and all and they survived it so far.

The edge is uncoated and exposes the germanium. It's not part of the optical function for the element and usually covered in the assembly.

If your elements look like a mirror in sunlight and give a green or red cast in artificial light; that's a good sign for a LW coating.

Try to grab a few of those units if you can!

[Fraser]

Eeeek .... dropping Germanium lenses 

They are brittle and of a monocrystaline structure so can shatter. Chalcogenide IR Glass lenses are not as fragile in this respect. When you see a Germanium lens crystal being grown, cut, profiled on a diamond lathe and polished..... you tend to treat them like a very fine champagne glass 

The AR coatings are reasonably robust if not mistreated but I have seen a lens where a user rubbed the objective to clean it after every use... he asked me how to remove the weird grey blotch that was growing out from the middle of his very expensive lens. That 'blotch' was the raw Germanium that had been exposed by excess friction on the AR coating. The AR coating was gone from the middle of the lens and I estimate around 70% area coating loss had occurred. The lens was effectively scrap in terms of its calibration profile. The owner was understandably  quite sad about that.

AR coatings on LW cameras can take on many appearances in different light. They sometimes look Blue, other times almost black and can appear like petrol on top of water with a lovely blue/green spectrum rainbow effect. Some have asked me to polish a lens that looked like the petrol on water effect, thinking it was some nasty surface contamination. I have been pleased to allay their fears and did not 'polish' the lenses. It is totally normal to see a sort of graduated rainbow effect on some Germanium lenses.

Thankfully the OP in this thread has already tested the lens for a LW AR coating. It does pass LW but could actually be MW and LW coated as the M7500 is a multi band series of cameras. Inframetrics produced multi band coated lenses for their MW and LW scanning cameras. If at all possible, always check a lens for LW AR coatings as a MW AR coating renders the lens pretty much useless for LW working. If you can see through a lens with a LW camera, it is coated for LW. If it appears like a mirror to the camera, it is coated for something other than the LW band. Simple but not always possible when buying off eBay ! Be careful how much you risk paying on an unknown lens !

Fraser

[Fraser]

Just some pictures of the rainbow effect on thermal camera lenses.

[ivaylo]

Speaking of different color coatings... I did get a few more of these lenses and when I went home I noticed some of them have a different tint. One is red-ish, the other is blue-ish (I am red-greed color blind, keep in mind). Anything I've posted so far (and the three lenses I shipped to people here so far) is about the lens with the red-ish coating. I did run a quick experiment and to my untrained eye it looks like the Seek Thermal sees the same through both. Same lens structure inside. The color difference is consistent between both front and back elements. Pictures attached...

[Hydron]

I have seen quite a variation in coating colours too, even between different samples of what is nominally the same lens. The only one that seems consistent is the dark grey "Diamond like Carbon" that is often used for extra ruggedness on the front surface of an assembled lens.

[Fraser]

Indeed, the lens colour of Fire fighting cameras is often the grey colour from the hard carbon coating.

I do not know why the colour of the AR coatings varies. I assumed it was different batches of lenses and that as the coating is applied, minor differences in the process effects the colour, bit not the AR performance. As stated, some lenses have several bands of differing colours on them. This is, of course, in the visible light domain and, as we know, that means nothing in the Longwave energy domain.

As a side note, Hard Carbon coatings are known as "diamond like" coatings due to their resistance to scratching. That are not indestructible though ! They should be treated with care like any optical coating.
Sadly lenses that are subjected to long term damp contions can suffer degradation of their AR coatings. Hard Carbon coatings are no different. The damage is a form of corrosion and is seen as spider lined under the coating, white corrosion breaking through the coating, lifting of the coating or mottling of the coating surface. In the case of lifting, the coating can flake off in whole areas, it can rub off easily if cleaned or it becomes loose crazing all over the lens that can easily rub off if touched.

Loss of the AR coating is bad news for Germanium Lenses as it directly effects the transmission of the lens. The lens will still function, but with reduced performance.

Keep your lenses dry !

Fraser

[Hydron]

Mine arrived safe and sound (thanks OP!) and a _very_ quick check suggested that the lens is designed to work placed directly infront of an FPA at a back focal distance in the range of single digit to low 10s of mm. Will have a bit more info if I'm not too busy tonight (the quick check was done by simply holding the lens infront of a bare FPA so is highly unscientific - treat my initial take with a grain of salt!). Of course we don't know what magnification this is actually supposed to work optimally at, but I guess we just use it in the way it works best for our needs.

[ivaylo]

Glad to hear you got it! If anyone still wants one, please PM. I got a few more when I got them for these guys. After which I may try and move them on eBay or something.

[Vipitis]

I hold back on getting more lens elements arm, sorry. Can anyone with the elements give an estimate of what kind of focal length they are? My interest would be to repurpose the elements and use the rest of my collection to build a doublet refractor.

[Fraser]

Thanks to the very kind efforts of Ivaylo, I now have two of these interesting lenses 

I will have a look at their characteristics as soon as possible but a few higher priority tasks must be completed first. The lenses certainly look to be very nicely made. A quality product, but sadly also quite a specialist one due to their close focus design. I have a use for such though so they are perfect for my needs.

Thank you very much Ivaylo. I may just have to buy another !

Fraser

[Fraser]

I have just written to a company with direct connections with Mikron products requesting any information that they might hold on these lenses. A long shot but it is always worth asking.

Fraser

[Hydron]

I will have a look at their characteristics as soon as possible but a few higher priority tasks must be completed first. The lenses certainly look to be very nicely made. A quality product, but sadly also quite a specialist one due to their close focus design. I have a use for such though so they are perfect for my needs.

I'm in agreement with this and in a similar position regarding time unfortunately. The limited playing I have done with mine suggests that they work well at magnification ratios of ~0.5 with a working distance of ~4cm from the front element. With 12mm between the FPA and rear of the lens (shortest I can easily do without making a new mount - it's 44mm in diameter!) I get ~0.3x magnification and 88mm working distance. 1:1 (i.e. true macro) performance would require the target to be touching the lens (0 working distance) so is not practical. Performance looks to be better than what can be obtained with a non-close-up lens at these magnifications. Definitely good fun for the price regardless of the eventual practicality!

[INFINIUM]

Somebody uses the  ANGENIEUX  ir  Lents ? , I want to transplant this  F50/F1,5 From one AEG PTSi old camera to one from ALDI, use for infrared analisis in old paintings or ? chips repairs  , bearings , etc.
https://www.hotukdeals.com/deals/night-vision-device-9999-aldi-768263

[Fraser]

That Aldi unit is a Generation 1 image intensifier that operates in the visible and Near IR wavelengths.

Your thermal camera operated in the Mediumwave Thermal band. The AEG camera is a thermal imager and is very different to an image intensifier. Your lens will not work on the Aldi device and you cannot carry out thermal imaging with the Aldi device unless the target is over around 400C when it starts to radiate NIR wavelengths.

Fraser

[Fraser]

My second pair of the close focus lenses have arrived so it is high time for me to start experimenting with them

Looking at the excellent sketch of the internal lens element arrangement I am minded to remove the rear lens and see if this assembly will work as a supplimental lens with a roughly parallel beam output. Sadly life is rarely that simple though. I will report the result soonest.

Now that I own four of these lens assemblies I have the opportunity to experiment moving lenses from one into another to change the characteristics. Buying such large diameter Germanium lenses and a suitable lens tube from Thorlabs would be very expensive indeed !

Fraser

[Vipitis]

Do you see a possibility in stacking a few of these units for even closer focus?

I was wondering if a laser (with a wavelength in the LWIR band) and indicator paper(the one that is thermochromatic and loses the traces when it's cooled again) would help with trail and error of lens arrangements as it makes visualization much easier.

Excited for what you come up with, but I am not yet sold on getting one of these units for the pure purpose of stockpiling elements.

[Fraser]

You and I think alike on the thermochromatic film. I bought the most sensitive that I could find and have yet to play with it.

Regarding closer focus. I think a small explanation of the beam path might help. I am in bed so cannot draw a pretty picture at the moment. Referring to the OP's lens cross section sketch and considering a beam entering at the left and travelling to the right......

The focus point of the first lens element is the source of the 'beam'. Let's just say it is at 20mm from the front of the lens. It passes brought the first lens and exits it in a parallel beam that enters the second lens. Upon leaving the second lens the beam is refracted to the extent that it travels to the opposite side of the Third lens face. This is the point of image inversion. The Third lens receives the beam and refracts it to a roughly parallel beam again. The beam enters the fourth lens and is refracted to create a focal point at the rear of the lens where the sensor array will be located. Looking at the lens system very roughly in pictorial form, you end up with an expanding cone of beams coming from a focal point entering lens one the left, an image inversion at the mid point, and a contracting cone of beams converging to a focal point on the right after leaving lens four. The focus point of the lens would normally be set by the distance between lens two and lens three. This could possibly be adjusted for closer focus in front of the lens. Back focus distance must also be adjusted correctly.

I am looking at removing lens four to see just how parallel, or not, the beams are that leave lens three. I would effectively be removing the beam convergence stage to make a three element supplimentary lens. Sods law will likely ruin this plan and require some additional beam collimation though.

You would not achieve much by lens stacking in my opinion. Better to tweak the focus point of the original design.

Fraser

[Fraser]

Vipitis,

Regarding stockpiling lens elements..... fair comment. For the cost though, these lenses are superb. I buy cheap ZnSe elements to act as supplimentary close-up lenses. The elements within the Mikron lens would be suited to such a task as well and they are far higher quality meniscus Germanium types. So for those wanting to add a really good quality Close-up lens element to their camera, these lens assemblies may be a good source of such.

The Mikron price on these close focus Mikron lenses was apparently around $5K ! Sadly I cannot source any technical specs though as Lumisense flat out ignored my RFI.

Fraser

Update to post : Rough beam path Sketch added