lens coating - Page 1
Forum section: electronics / general

author: Etakarinae
author_rank: Newbie
timestamp: June 09, 2023, 07:46:45 am
content: Hello,I'm a newbie in thermal imagingI have bought old lenses from the 90sI would like to know if you can you MWIR lenses on LWIR camerasSome have coating damaged can they still be used ?How about a nitogen cooled camera, can they be powered w/o damage if the nitrogen tank is empty for testing ?thanks

author: Fraser
author_rank: Super Contributor
timestamp: June 09, 2023, 09:54:11 am
content: Welcome1. The AR coating on a thermal camera lens has a “pass band” characteristic so can act as a very broad filter to incoming thermal energy. This means that a lens AR coating for MWIR will be almost opaque to LWIR energy and little energy will pass to the LWIR sensor array.(often a microbolometer) The performance of the coating will dictate how much thermal energy leaks through the AR coating. It is worth noting that MWIR lenses are also quite ‘slow’ compared to that of a LWIR lens as the sensor system in a cooled MWIR camera is far more sensitive than a typical LWIR camera. Higher f numbers are to be expected in MWIR lenses. Lenses AR coated fir LWIR will present the same pass and filter characteristics so little thermal energy will get through to the sensor system. There are dual band lenses for use on both MWIR and LWIR cameras and these are like a ‘universal’ thermal lens with a specialist ‘dual- and’ AR coating. Such lenses will be marked up as broadband, dual band or the two wavelength ranges marked on their casings. They are rare.2. Liquid nitrogen cooled thermal cameras contain a Dewar for the liquid nitrogen. The thermal detector array or sensor is mounted in the wall of the Dewar with its rear in thermal contact with the Liquid Nitrogen. Running a liquid nitrogen cooled camera with an empty dwar does no harm at all as the cooling is NOT used to prevent overheating of the detector array. The detector will be in noise saturation so do not expect any thermal imaging capability in a ‘dry’ test.Hope this helpsFraser

author: Etakarinae
author_rank: Newbie
timestamp: June 09, 2023, 11:49:28 am
content: Thanks a lot FraserHow about the damaged coating : will it harm the images ?Can I uses these old objectives with "newer" cameras, like this one ISG Infrasys Core Kern 320 x 240 30 Hz (see attached pics) ?My goals are to experiment/learn the thermal world and then capture images about distant landscapes and wildlife (hogs, ...)

author: Fraser
author_rank: Super Contributor
timestamp: June 09, 2023, 12:48:51 pm
content: There are different types of damage that can occur to thermal imaging lenses.1. Scratches on the AR coating - not normally that serious as thermal cameras are low resolution and the scratches will have little effect on the image produced. This is very different to scratches on conventional visible light camera lenses where image artefacts result.2. Removal of the AR coating in small areas due to over cleaning or incorrect cleaning technique - this is a bigger issue than scratches as the AR coating enhances the lenses transmission, especially if the lenses are made of germanium (~40% uncoated vs ~80% coated). Small areas of AR coating loss are unlikely to be very visible in the thermal image but larger area of loss may cause noticeable effects in performance.3. Peeling or flaking away of the AR coating - this is serious as it indicates age or environmental factors are causing loss of the AR coating and this may lead to almost total loss of the AR coating over time. Attempts to clean the lens tend to make matters worse as much of the AR coating may come away from the lens substrate. The cause of this delamination is sometimes purely age related on early lenses, whilst in other cases it may be due to storage in damp or humid conditions. If there is delamination visible on the Objective lens, it is also not uncommon to find delaminating of the AR coatings on internal lens elements. The combination of AR coating loss throughout the lens assembly can seriously degrade its performance. That said, the lens may still operate to an acceptable level for low resolution, non-radiometric thermal imaging. It can look worse than it is when it comes to using the lens for imaging. It just needs to be tested with a core to see how it performs.4. Corrosion of the Germanium metal lenses causing lifting of the AR coating - corrosion of the Germanium lens material surprises people as they are not used to a lens made of metal. The corrosion is often the result of regular exposure to water or incorrect storage when wet, or in damp conditions. This is a common scenario for fire fighting cameras and manufacturers do their best to protect the lens by either placing it behind a Germanium window or using a special, very robust, AR coating on the front lens element. Hard Carbon coating is a diamond hard like coating that can protect against aggressive working environments. That said, any breach of the coating at the edges of the lens or on the lens surface can lead to moisture getting under the hard carbon AR coating and lifting it away from the lens substrate in flakes. Once corrosion of the Germanium lens has occurred, it cannot be polished by owners and the damage is permanent to both the lens material and its AR coating. The damage will degrade lens performance but the lens may still remain useable for low resolution, non radiometric, thermal imaging.5. Deliberate abuse causing AR coating loss - Any attempt to remove the AR coating from a thermal imaging lens will end in its destruction. Some users have attempted to remove the MWIR AR coating from lens elements with abrasives in order to attempt use at LWIR. There is no sense to this action and it just destroys the lens surface and its imaging performance. Avoid any lens that looks dull grey with a non reflective surface. It will be good only for scrap germanium. The same applies to other thermal camera lens materials such as ZNS, ZnSe, and Chalcogenide IR Glass. It is also worth noting that some MWIR lenses actually use silicon or sapphire lens elements that are low transmission at LWIR, so removing the AR coating achieves nothing.Hope this helpsFraser

author: Etakarinae
author_rank: Newbie
timestamp: June 09, 2023, 12:58:07 pm
content: Thanks again, I have learned a lot

author: Hydron
author_rank: Super Contributor
timestamp: June 09, 2023, 09:15:55 pm
content: Quote from: Fraser on June 09, 2023, 12:48:51 pmYou'd be surprised what you can get away with even at visible wavelengths and resolutions:https://www.lensrentals.com/blog/2008/10/front-element-scratches/

author: Fraser
author_rank: Super Contributor
timestamp: June 09, 2023, 10:06:00 pm
content: Hydron,Lens damage becomes most obvious in conditions where the lens is heavily illuminated such as looking towards a light source. All manner of unpleasant optical effects can then result. Even dust on the lens can be a problem in bright light source scenarios. The same can be true when a damaged thermal imaging lens views a high thermal energy scene.Fraser

author: Bill W
author_rank: Super Contributor
timestamp: June 10, 2023, 12:49:41 pm
content: Quote from: Fraser on June 09, 2023, 12:48:51 pmI would add that the coatings are potentially quite nasty materials. Definitely a wash hands after / gloves approach.Some very old lenses may even have radioactive coatings (Thorium), while as alpha decay it is not severe through air, and stopped by paper, it would be very bad if swallowed etc.