Author Topic: Cleaning lens of TI?  (Read 6706 times)

0 Members and 1 Guest are viewing this topic.

Offline frenky

  • Supporter
  • ****
  • Posts: 971
  • Country: si
    • Frenki.net
Cleaning lens of TI?
« on: May 27, 2016, 11:48:11 am »
This one is probably for Fraser but I wouldn't mind if anybody else answers it.  ;)

How to clean the lens? What kind of cloth and cleaning solution? Would Isopropyl alcohol (isopropanol) and ear cotton swabs be ok?
I guess it also depends on the lens material. Does anybody know what material SeekThermal uses?

Seek:


« Last Edit: May 27, 2016, 11:51:00 am by frenky »
 

Offline encryptededdy

  • Frequent Contributor
  • **
  • Posts: 358
  • Country: nz
Re: Cleaning lens of TI?
« Reply #1 on: May 27, 2016, 12:16:12 pm »
The seek thermal uses a Chalcogenide Glass lens.
 

Offline Fraser

  • Super Contributor
  • ***
  • Posts: 9441
  • Country: gb
Re: Cleaning lens of TI?
« Reply #2 on: May 27, 2016, 01:03:21 pm »
Thermal camera lenses are relatively fragile due to the Anti Reflective coatings that can be damaged if too much pressure is applied.

The SEEK does indeed use A Chalcogenide lens but it's cleaning is the same as for Germanium. ZnSe cleaning is different as ZnSe is relatively soft and it is quite a reactive material.

In general, thermal camera lenses do not need much cleaning. Unlike visible light camera lenses, dust is not a great problem at thermal wavelengths. If dust is to be removed I would first use an optical grade air duster or air brush to blow loose dust away. A microfibre spectacles cleaning cloth may then be used to gently wipe off stubborn dust or finger grease

IPA is OK to use on the Chalcogenide and Germanium lenses but never rub the lens, just drag the dampened cloth around on the surface (drag cleaning) and very gently wipe the surface with a dry microfibre cloth afterwards.

For the SEEK lens, just a gentle wipe with the dry microfibre cloth is usually enough.

Why so much care when cleaning thermal camera lenses ? Well some of mine originally cost over $15K so you did not want to damage it ! In general, treat the thermal cameras as you would an expensive SLR lens and you should be OK.

DO NOT, I repeat DO NOT use aggressive chemicals or rough materials on a coated thermal lens. You will literally rub the AR coating off of the substrate, ruining the lenses performance. You sometimes see thermal cameras for sale with a weird discoloured area on the lens....... With Germanium lenses, this can actually be normal colouring, but a close look can reveal a lens with destroyed AR coating...... Very bad news indeed. If you do, by accident see slight damage to the AR coating, DO NOT try to 'polish it out' as that just does more damage. Leave damaged AR coatings alone and you may get away with it.

The relatively low resolutions of thermal cameras and their operating wavelengths are really quite forgiving of dust and dirt on the objectives front face. Professional surveillance thermal cameras often only have a rain and dirt wiper for the visible light camera and do not wipe the thermal camera window as that actually causes more issues and damage.

Fraser
 
The following users thanked this post: DaneLaw

Offline frenky

  • Supporter
  • ****
  • Posts: 971
  • Country: si
    • Frenki.net
Re: Cleaning lens of TI?
« Reply #3 on: May 27, 2016, 01:17:31 pm »
Tnx, I'll be carefull. :-+

I have a big round spot on all images... I just hope it's on the lens and not the sensor.
 

Offline Fraser

  • Super Contributor
  • ***
  • Posts: 9441
  • Country: gb
Re: Cleaning lens of TI?
« Reply #4 on: May 27, 2016, 02:51:31 pm »
Hmmmmm, sadly a big round spot on the image that is not an obvious lump of dirt on the objective lens often means something has got onto the microbolomter window. If you are very careful you may be able to shift any contamination on the microbolometer window with low pressure air. High pressure air is risky when die bond wires are not protected. You can blow air through the FFC flag hole.

Fraser
 

Offline frenky

  • Supporter
  • ****
  • Posts: 971
  • Country: si
    • Frenki.net
Re: Cleaning lens of TI?
« Reply #5 on: May 27, 2016, 08:56:44 pm »
The spot looks like this 35px diameter:
(Left is uncooled and right is cooled sensor)


I'm really afraid that by blowing air on the sensor something like this would happen:

Source: https://plus.google.com/photos/115054970849159689228/album/6166783672525608961
« Last Edit: May 27, 2016, 09:00:34 pm by frenky »
 

Offline Fraser

  • Super Contributor
  • ***
  • Posts: 9441
  • Country: gb
Re: Cleaning lens of TI?
« Reply #6 on: May 27, 2016, 09:14:48 pm »
Regarding blowing on the sensor, you are OK. The microbolometers that appear in your picture live inside a vacuum with a thermally transparent window protecting them. A low pressure air stream will not break the microbolometer window and the VoX pixels will not be exposed to any stress.

Fraser
 

Offline frenky

  • Supporter
  • ****
  • Posts: 971
  • Country: si
    • Frenki.net
Re: Cleaning lens of TI?
« Reply #7 on: May 27, 2016, 09:19:29 pm »
Tnx, I'll try it with a blower that I used in my old dslr. I can easily regulate the airflow by hand:

 

Offline Bill W

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: gb
    • Fire TICS
Re: Cleaning lens of TI?
« Reply #8 on: May 27, 2016, 09:32:38 pm »
There are several different anti-reflection coatings in use on thermal lenses.  Most common is  'high efficiency' which are the most delicate.These can be damaged by water, this is the cause of the effects that Fraser described.  Small scratches in the coating allow water in and the coating peels off.  This is usually only used on internal surfaces, although a few lens suppliers do a 'durable' version that can survive external use.
The other main type is 'hard carbon' which is black.  This can survive harsh abrasion and salt etc, so is used on external surfaces, although it is lower transmission, about 80%, compared to 95% for a good high transmission coating.  This then allows a choice of replaceable HE coating, or putting HC on the front lens element.
It is also possible to make an antireflective surface using micromachining instead of coating, and these CANNOT be cleaned at all.

Bill
www.fire-tics.co.uk

Offline Bill W

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: gb
    • Fire TICS
Re: Cleaning lens of TI?
« Reply #9 on: May 27, 2016, 09:35:12 pm »
A blob on the image can also be due to a dirt spot that has now fallen off, that would create a more sensitive spot as opposed to a less sensitive spot

Bill

www.fire-tics.co.uk

Offline Fraser

  • Super Contributor
  • ***
  • Posts: 9441
  • Country: gb
Re: Cleaning lens of TI?
« Reply #10 on: May 27, 2016, 10:01:18 pm »
I hope this is just a blob of dirt on the lens.

The less pleasant possibility is damage caused by your low temperature experiments. The die is directly bonded to the cameras pcb. if you drop the cameras temperature below the freezing point of water any trapped moisture under, or inside, the die will expand as it freezes.this would be very bad news and, I would say, an unlikely event.

It looks to my eyes like a spot of dirt on the microbolomter window. Even a small spec close to the VoX pixels can appear very large on the image. Normally I would expect a hot spot though as the dirt is heated by the microbolometer self radiation and is reflected back onto its sensor pixels.

As Bill states, there is also the possibility of dirt that was on the microbolomter window at manufacture coming away and skewing the NUC table for the affected area.

Fraser
 

Offline frenky

  • Supporter
  • ****
  • Posts: 971
  • Country: si
    • Frenki.net
Re: Cleaning lens of TI?
« Reply #11 on: May 27, 2016, 10:49:36 pm »
I cleaned lens with isopropanol but the spot is still there. The interesting fact is that spot wanishes if I use external reference instead of internal shutter. Perhaps the dirt is on the inner side of the lens? That would be really hard to clean.
 

Offline jeffreythe00

  • Contributor
  • Posts: 9
Re: Cleaning lens of TI?
« Reply #12 on: June 11, 2016, 12:28:40 am »
Note: I'm not affiliated with first contact in any way. I am just very happy with the product. I know this post is a bit old but I don't see too much information about this kind of thing.

I use First Contact cleaning solution for both my Seek compact and Therm-app imagers. The cleaning solution was designed to be used on telescope mirrors and laser optics and is safe for most coatings. I've use it to clean diode facets, diode windows, various optics with dielectric coatings ect with no issues. I've only had it damage one of my optics and this was a spare front surface mirror (Which it DOES say specificlly not to use this solution on FS mirrors).

The people are very friendly and will gladly answer any questions you have I'm sure. The only downside is the solution is VERY expensive ($220 + shipping for a cleaning kit). Though, since this is a forum. Perhaps you thermal imager guys would want to do a group buy on it? The amount of solution you get from the kit will last you forever.
« Last Edit: June 11, 2016, 12:30:16 am by jeffreythe00 »
 

Offline bangbang99

  • Contributor
  • Posts: 35
  • Country: ua
Re: Cleaning lens of TI?
« Reply #13 on: June 12, 2016, 06:03:45 pm »
From "AGA Thermovision 680 Operating Manual", Operator Maintenance section (Germanium lenses):
 

Offline Fraser

  • Super Contributor
  • ***
  • Posts: 9441
  • Country: gb
Re: Cleaning lens of TI?
« Reply #14 on: June 12, 2016, 06:40:24 pm »
banbang99,

Thanks for the manual excerpt.

Whilst what it says is correct. It comes from a period when the lenses could easily cost $40K each and so EXTREME care was taken with them. It is absolutely correct that careless cleaning can damage the lens coating, as I stated above. But in more modern cameras some basic rules apply. Do not use harsh or abrasive cloths and use IPA only when absolutely necessary.

I once had the misfortune to meet a chap who cleaned his lenses with kitchen towel and soapy water. The kitchen towel is abrasive and he not only rubbed too hard, but also cleaned the lenses too often.They are a it of a mess by the time he brought them to me for cleaning ! Scratches everywhere and corrosion around the edges. I have heard various horror stories involving unsuitable cleaning fluids but thankfully I only had the one 'genius' to deal with. He was a little upset when I advised him that he had damaged around £100K worth of Germanium lenses.

I take the view, clean only when absolutely necessary, and clean very gently with a super soft clean cloth and IPA when marks are stubborn. If a mark does not come off with IPA, it is either permanent, a natural shade in the lens surface, or it needs expert attention to assess it under a microscope. I had one chap ask me to 'polish out' a mark on his cameras lens. Under the microscope I could see that it was not a mark but rather a crater caused by something hitting the lens. That was the bad news. The good news that I delivered to the owner was that the crater was so small as to be of no consequence to the image produced. I warned him against ever asking someone to 'polish' a thermal camera lens as that would be a disaster. I ended up owning that camera several years later and the lens still has the crater in it of course !

By the way. I do not know how available ether is these days but you might get some questioning if asking for it. Such is used to kidnap people !

Fraser
 

Offline Fraser

  • Super Contributor
  • ***
  • Posts: 9441
  • Country: gb
Re: Cleaning lens of TI?
« Reply #15 on: June 12, 2016, 07:10:59 pm »
I just dug out the thermal camera lens that has a crater in its objective.

Photographing a Germanium lens is challenging as it acts like a mirror at visible light wavelengths. I positioned my camera so that the crater is actually at the centre of my cameras lens reflection so that you can actually see it ! It is really small in reality but any attempt to 'remove' it by the owner would have destroyed the lens so be warned. Sometimes the best action is no action.

This lens is 30 years old and as you can see, it has dust on it but I would blow it clean as there are no finger marks on it. The lens is still in great operational condition because it has been well cared for and not over cleaned.

I have bought and used thermal camera lenses that are covered in tiny cleaning scratches. I am pleased to advise that these micro fine scratches do not significantly effect the thermal image produced by a 320 x 240 pixel camera. Rubbing away the AR coating would be a very different matter though.

Fraser
« Last Edit: June 12, 2016, 07:20:05 pm by Fraser »
 

Offline bangbang99

  • Contributor
  • Posts: 35
  • Country: ua
Re: Cleaning lens of TI?
« Reply #16 on: June 12, 2016, 08:05:42 pm »
Fraser,

I am not a huge expert in optics, just got info from good book about optical technology. So, petroleum ether consists of light gasoline fractions and  is widely used in optical production, and spatially for cleaning a surface before coating. I think they meant petroleum ether. Nothing else.
 

Offline bangbang99

  • Contributor
  • Posts: 35
  • Country: ua
Re: Cleaning lens of TI?
« Reply #17 on: June 12, 2016, 08:31:30 pm »
Result of irresponsible treatment. Got the lens in this condition. I do not even worry about it until it is obvious that this does not affect the image. Don`t know jet  :-\
 

Online helius

  • Super Contributor
  • ***
  • Posts: 2995
  • Country: us
Re: Cleaning lens of TI?
« Reply #18 on: June 12, 2016, 09:00:43 pm »
petroleum ether is chiefly pentane (formally it is a fraction of aliphatic alkanes C5 to C6). it's available more or less everywhere.
 

Offline Bruno28

  • Regular Contributor
  • *
  • Posts: 105
  • Country: au
Re: Cleaning lens of TI?
« Reply #19 on: January 25, 2017, 01:45:19 am »
What's the best way to clean a therm-app TH lens with the 6.8mm? Just use the dust blower thats I have for my dslr and then a microfiber towel like the ones from Giottos cleaning dslr lens kits?
 

Online helius

  • Super Contributor
  • ***
  • Posts: 2995
  • Country: us
Re: Cleaning lens of TI?
« Reply #20 on: January 26, 2017, 09:51:22 am »
There is a very gentle lens cleaning method that uses finely powdered graphite. The idea is that contaminants tend to cling to carbon, and it can then be brushed away. This may or may not be suitable for thermal cameras; germanium is more reactive than glass and chemical compatibility must be considered.
 

Offline Bill W

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: gb
    • Fire TICS
Re: Cleaning lens of TI?
« Reply #21 on: January 27, 2017, 12:15:40 am »
Do remember that you are not dealing with Germanium on the lens faces, you are dealing with a multi layer thin film coating.  If you get down to Germanium it is scrap as without the coating the transmission is very low.

Hard Carbon coatings (eg FLIR E4 and Tau derivatives, Avon T & P) are pretty bomb proof as they pass rubbing with sand filled cloths.

Many fire cameras (e2v / Avon Argus) have a 'durable' external coating, not quite hard carbon but reasonably robust.

Internal surfaces and sensors will have high efficiency coatings which are the most fragile and do peel if wet.

Bill


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf