Why optical zoom and auto fucus are so rare for thermal cameras? - Page 1
Forum section: electronics / general

author: yucel
author_rank: Newbie
timestamp: April 27, 2019, 02:37:28 pm
content: Those functions are so common on a normal camera, but they seems to be very difficult to get on thermal cameras, why?

author: Vipitis
author_rank: Frequent Contributor
timestamp: April 27, 2019, 02:55:15 pm
content: most camera use a fixed focal length (prime) lens. These are much cheaper to build and also simpler in design. There are some zoom lenses for uncooled cameras and you can find a good overview on the OphIR website.Zoom lenses are more common in cooled cameras, for example airborne systems. They give the flexibility needed by the customer for example law enforcement or military. Cooled cameras (mostly MWIR) are more sensitive and therefore work with less light. Zoom lenses can be much more compact when you don't need a focal ration (f-number) of 1.0.Autofocus can be found in thermal cameras. High end handheld FLIR T6xx comes to mind. However, it is contrast based focussing as I am not aware of any phase detection focus system that work in LWIR.A lot of lower end cameras FLIR One Dongle, Cx, Exx, some monoculars and a lot of survalliance cameras are fixed focus. Therefore additional "macro" lenses are used by enthusiasts to focus close up on electronics for example.In professional cameras you might find auxiliary lenses to put put in front that either change the fov ("zoom") or allow close focus ("microscope").Also note that you don't always find continuous zoom. Some lens contraptions have multiple elements that rotate in and out to give multiple, but fixed options. One example I can give is thishttps://youtu.be/o3RVKp7i_hAwhich could be found on eBay not long ago.

author: Fraser
author_rank: Super Contributor
timestamp: April 27, 2019, 03:19:48 pm
content: Still available for ~$3600Cooled and USA only though.https://www.ebay.com/itm/AXSYS-LONG-RANGE-TRI-FIELD-OF-VIEW-COOLED-THERMAL-CAMERA-200MM-LENS-40-000-/273527169222Fraser

author: Fraser
author_rank: Super Contributor
timestamp: April 27, 2019, 03:43:03 pm
content: As has been well covered by Vipitis, traditionally a high cost item on the BoM was the Germanium lens assembly. The lens elements were so expensive to produce that the number used in a thermal camera lens block was kept to a minimum. Zoom lenses were considered an expensive luxury and found mainly in military systems where cost was less of an issue. More Germanium lens elements tended to mean higher cost and greater weight.As already stated, and seen, it was often a multi magnification lens block that was found in military cameras. There was not the need for a continuous zoom as commonly found in a consumer SLR VL camera. There was no ‘pretty picture’ requirement, only a hunt, identify and, if necessary, kill requirement. Two or three magnification levels provided all that was needed for such and simplified the optical design. True continuous zoom lenses can be optically complex. Telephoto lens attachments are simpler.With the advent of cheaper and more easily produced Chalcogenide IR Moulded Glass lenses there would appear to be an opportunity to build affordable continuous zoom lenses for thermal cameras. The industry appears to prefer interchangeable prime focus lenses and supplementary lenses that mount in front of the primary. Interchangeable primary lenses offer the advantage of being faster than a zoom or a combination of primary and supplementary lens.Multi magnification lenses would appear to remain a mainly military use technology. Modern thermal cameras use advanced interpolation to mimic a true continuous zoom function using electronic zoom principles. The IFOV remains the same however so it is a compromise solution as you would expect.With regard to autofocus. My PM695 cameras have this built into them. Whilst the hardware is not a great additional expense, it was usually provided only on remote manual focus cameras that already contained the required motorised lens linear drives. Such cameras were already expensive because of motorised lens drives. The cheaper alternative to auto focus was fixed focus lenses. They offered the benefits of lower complexity, lower cost, ease of use and great depth of field. They could not match a manual focus lens in terms of image sharpness at several distances but then F1.0 manual focus lenses have quite a shallow depth of field necessitating regular manual focus adjustments. Users often do not like to have to keep revisiting the focus on their cameras. Auto focus was intended to provide the user with the benefits of sharp focus at all distances whilst also handling the tedious need for regular adjustment of focus due to shallow DoF. It does work but the motorised lens adds noise, weight, bulk and, importantly, power consumption to the camera design. The technology has improved over the years though, thanks to improvements in auto focus VL camera lens mechanisms.Auto focus remains a luxury fitment on a thermal camera and so is found only on the more expensive models. While users continue to accept the limitations of fixed focus lenses and inconvenience of adjusting manual focus lenses, the manufacturers have little incentive to change this situation.Fraser

author: Vipitis
author_rank: Frequent Contributor
timestamp: April 27, 2019, 05:39:35 pm
content: hm, I actually had a different eBay listing in mind. this time it is a scrapyard find, different seller but same YouTube channel and same room:https://www.ebay.com/itm/Giant-weapon-site-zoom-thermal-imaging-IR-FLIR-germanium-non-cooled-Lens/222756509586https://youtu.be/FsvPOMIiTfgthe lens seem to have no export restrictions, but the camera built in there has. I might ask the seller just out of curiosity.

author: Fraser
author_rank: Super Contributor
timestamp: April 27, 2019, 06:20:05 pm
content: Ah yes, the Frankenstein camera !Fraser

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2019, 06:58:04 pm
content: IR lenses are expensive and still not very good (some absorption and background) compared to lenses in the optical range. A zoom needs several lenses - so not really practical in the IR range.For low noise one also wants high aperture, as it's not just lost intensity, but also extra emissions unless the lens / aperture is cooled. So the only real option is a set of interchangeable fixed lenses.The resolution is often limited by the sensor. So one may not really need auto-focus, though a rough focus adjustment (e.g. far / close / macro) could be a good idea.AFAIK some thermal cameras, especially the better ones are export restricted in the US. The really good one may need even more paperwork.

author: Spirit532
author_rank: Frequent Contributor
timestamp: April 27, 2019, 11:13:00 pm
content: I should also note that "athermalization" is a process unique to infrared imaging. The spectrum of wavelengths imaged is so incredibly broad(spanning several thousand nanometers), that it's very difficult to optically correct for chromatic aberration, as the refractive index of the elements, regardless of material, varies quite wildly throughout the range. This would be drastically amplified by adding a simple varifocal assembly into the mix, making the images very very blurry, hence why the zoom lenses are complex, expensive, and very heavy.

author: Fraser
author_rank: Super Contributor
timestamp: April 27, 2019, 11:57:32 pm
content: Athermalisation is a different issue pertaining to ambient temperature change induced defocusing of optics and the design features used to compensate for such.https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/10/Povey-1986.pdfI know what you mean about the broad LWIR spectrum and it’s potential to cause complications in some simple lens block designs. It is an important consideration for the lens block designer though thankfully the relatively low resolution of a thermal camera can be far more forgiving than in VL camera applications that are effected by such issues.Fraser

author: Vipitis
author_rank: Frequent Contributor
timestamp: April 28, 2019, 01:08:17 am
content: the broad spectrum gets countered by the high refractive index of Ge compared to quartz glass. We haven't mentioned all the coatings you find on the lenses yet.

author: eKretz
author_rank: Frequent Contributor
timestamp: April 28, 2019, 02:11:01 am
content: Right, some IR lenses even have multiple coatings - to help with different wavelengths.

author: Bill W
author_rank: Super Contributor
timestamp: April 28, 2019, 10:54:17 am
content: Thermal behaviour is a pain, although on fire cameras it can be called 'auto focus' as when it gets hot the focus comes closer - like you'd want inside a very hot place.Some lens designs mixed in a ZnS element, also helping on chromatic performance, others with several elements could use natural expansion of materials to help refocus (passive athermalistation) or you could measure temperature and use a motor / encoder to refocus actively without being a true autofocus.More recently the move towards 'chalcogenide materials' has helped, these are lower index (n) and also much lower thermal constant (dn/dT). So while pixel pitch and focal length have come down the use of a mechanically fixed lens is still generally OK.Bill