Products > Thermal Imaging

Narrow lepton Field of View

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Bill W:
From another thread..


--- Quote from: Vipitis on October 19, 2017, 10:11:50 pm ---
My problem is that I am limited to high fov and low resolution with my phone. I can increase the resolution partially by super resolution. But lowering the fov will need a lens. And the offer I got from TPLogic is kinda expensive and might not work. So diy and a ZnSe solution might help to lower fov and get detail at range - for better stiching. A tripod with a panorama mount that fits my phone could help.... That might be printable.


--- End quote ---


--- Quote from: Ultrapurple on October 20, 2017, 07:56:58 pm ---I suggest you consider starting a new topic (something like 'Narrowing Lepron FOV'), but I am also happy to continue the discussion here if you prefer.

I know very little about the Lepton. I assume it's not possible to remove the lens assembly. That really only leaves you the option of making a telescope arrangement - which can work well. Try using a reflector telescope and putting a germanium or ZnSe lens in place of the eyepiece. You may be able to use it to form the light so it can be seen by the presumably fixed-focus Lepton. I use a simple astronomical telescope like this, except I do not need the extra lens for my setup. I suggest using a hot target such as a soldering iron a few tens of metres from the telescope. Set it up with its original eyepiece so you can be certain it's pointing accurately. Then start playing with lenses and the position of the Lepton to see if you can get anything resembling an image. I expect you so get poor sensitivity and high magnification - but once you have demonstrated that it works you can start optimising different bits.


--- End quote ---

I have taken a lepton apart and am indeed going to be trying out a number of other lenses on it.  Summer has got in the way, but yes the lens can come off although I might also have to sacrifice the shutter at least for now.
Next step is making up a good mounting to line up the available lenses and see what works, and also making the battery connection and the output interface to the phone more robust with the case out of the way.

The biggest issue is that the lepton is 12┬Ám pixels which will be way beyond the design intent of the available lenses.  I expect some bad surprises but perhaps some good ones too.

The first good surprise was that just refocussing the lens supplied helped quite a bit.  I suspect the Chinese factory have been screwing them in fully regardless.


Bill

Vipitis:
Hey Bill,

Thanks for starting this topic up again.

The Lepton I have is integrated in my phone and I won't be taking it apart.

I reached out to CAT, their service partner and Bullit but I got no information whatsoever on the lens used to cover the Lepton. I suspect it's similar to what is used in the flir one. And that would be silicon.

I was about to do some test to confirm the fov of the sensor and do measurements with callipers on my own.

From what I got from TPL, I can say that their adapter has an opening for the Lepton lens, but not for my phone and therefore would result in large vignette and loss of pixels.

I plan to buy lenses and print the mounting solution on my own, similar to what Boris achieved.

I was going to make a thread to compile all attempts, like some links to Boris and his YouTube, the Fresnel attempts by Ultrapurple and others as a knowledge base.

I got something to take care off ATM and will do some later this day.

Attachment: little friend in my basement trying to hide - I will "take care" of him/her now!

Fraser:
Please do not kill the mouse !

I always live capture them and release a good distance away from my house.
I kill only when there is no other option. It is good Karma :) Please do have mercy on the little guy and live capture it.

Fraser

Vipitis:
So let's start this.

I will collect a few links to different experiments, people and examples of building custom lenses for thermal imagers. The topic might be lepton, but the examples I will present include Seek, Flir One, Therm App and other cameras. In the end it should work for any camera out there anyways.

Uho Boris
Boris has built quite a few modifications for Seek and Flir One. His YouTube channel has a few videos on his creation. He is active on this forum under the name Uho
Relevant projects include:

x6 Telephoto lens seen in
This is an auxilary telescope made for the Flir One v2 smartphone dongle. It uses 2 lenses(as far as I understand it) and is mounted on a selfmade case with a thread.

T10 upgrade as seen in this post
This is an additional lens to the T10 imager by Torres Pines Logic. It is made of at least 2 germanium lenses and 3D printed parts.


***

I will do the next references another time.

Fraser:
Some information on decreasing a thermal cameras Field of View.....

Refracting telescopes are used as an auxiliary lens to decrease the field of view on some thermal cameras.

Refracting telescopes commonly come in two types, Keplerian and Galilean.

The Keplerian is commonly used in astronomical applications but may be used in front of a thermal cameras objective provided the correct materials are used for the lens elements. Only two lens elements are needs, both Bi-Convex. The disadvantage of the Keplerian telescope is that it is inverting so the image appears on the camera upside down and reversed left to right.

The Galilean is also used in astonomical applications and it has the advantage of being non-inverting. The telescope comprises two lens elements, but one is bi-convex whereas the other is bi-concave. The Galilean suffers from relatively poor field of view.

Keplerian telescopes are relatively easy to construct providing suitable lenses are available. Such is not normally a problem with visible light lenses, but lenses suitable for thermal imaging wavelengths are harder to source at reasonable cost.

How to select a lens set for a thermal telescope......


In the Keplerian telescope there is a front lens of long focal length and a rear lens of short focal length. The front lens should be as large diameter as practical in order to collect plenty of energy. The image energy is inverted and passes to the rear lens where it is converted to parallel energy beams to illuminate the thermal cameras objective lens. The magnification factor (and subsequent reduction in camera FOV) is calculated by dividing the front lens Focal Length by the rear lens Focal Length.

For example:

Front lens FL = 150mm
Rear lens FL = 50mm

150/50 = x3 magnification and a reduction in FOV by a factor of 3. A 36 degree FOV Camera objective would be reduced to 12 degrees.

This may sound great but it is rarely so simple. The selection of the two lenses is essential to decent performance. A large front lens is desirable, as already stated, otherwise the telescope will have high losses. The lens length must also be considered. The refractive index of the lens element material will directly effect the length of the lens for a given magnification. Germanium has a very high refractive index of ~4, whereas Zinc Selenide is closer to visible light lens refractive index of ~2.4.  A x3 telescope that uses Germanium lenses can be shorter than one that uses Zinc Selenide lenses of the same dimensions.

The Galilean telescope uses a Bi-Convex front lens and a Bi-Concave rear lens. Finding a suitable Bi-Convex lens for thermal imaging wavelengths can be challenging, but finding a suitable Bi-Concave lens can be even more so !

The magnification calculation is detailed here:

http://www.citycollegiate.com/geometricaloptics2.htm

I have detailed telescopes that are placed in front of the thermal cameras original objective lens. This basically acts like an optical translator for the objective to produce a magnified image of smaller Field of View.

There is also the option to remove the thermal cameras original lens block and replace it with a lens block that provides a greater distance imaging capability. Such lenses can often be found on the secondary market and are often a precision assembly incorporating quality Germanium, ZnSe or Chalcogenide IR Glass lenses. The Focal Length of the lens needs to taken into consideration and some form of mount devised to hold the lens block at the correct back focus distance from the microbolometer.

Now a word about lens blocks. I use the term lens block to describe a complete lens assembly incorporating all required lens elements. This avoids confusion when talking about "lenses" and "lens elements" which can be quite different !

A lens block is designed to match the sensor that it illuminates. This needs to be considered when sourcing a lens block on the secondary market. If a lens block is designed to illuminate a thermal sensor of area "X", if it is placed in front of a sensor of area 0.25 X, the field of view specification for the lens block will be reduced by a factor of 4. Many thermal camera lens blocks sourced from older thermal cameras are a bit of a mixed bag. Whilst they will likely contain decent quality Germanium lens but will have been designed to illuminate a sensor with large pixels compared to more modern microbolometers. This directly effects the size of the sensor array and so, how the lens block illuminates it. Over illumination of the sensor array is common in such mixes of older lens block with modern microbolometer. This fact needs to be considered when selecting a certain FOV lens block for a modern sensor array. The up side of this over illumination situation is that the lens block provides a telephoto characteristic :) The Lepton microbolometer area is tiny when compared to the relatively large BST imaging sensors from around the year 2000. The BST sensor pixel array has a corner to corner dimension of around 15mm (best guess). Bill W will know a more accurate figure.

Another important factor to consider when planning to use a lens block designed for older thermal cameras, such as the BST based units, is pixel size and lens resolution. 50um pixels were common and are huge when compared to modern 17um and 12um pixels. The older lens block resolution was chosen to match the large pixels sizes of the time. Placing an older lens block, designed for 50um pixels, in front of a 12um pixel Lepton microbolometer is a bit of an unknown. Depending upon the lens element quality, the resolution may, or may not be adequate for decent imaging with such a small pixel size.

Older lens blocks will likely be physically larger than more modern lens blocks that are designed for the smaller sensor arrays. It is easy to end up with a huge lens with a small thermal camera like the Lepton hanging off of its rear ! Such could be an issue in Drone applications where weight can be an issue. Germanium lens elements are relatively heavy.

Finally, if considering the purchase of a lens block on the secondary market, ensure that it is designed for use in the Long Wave thermal spectrum as a "Short Wave only" lens will not work with a microbolometer sensor. Some dual band lenses are available that work in both LW and SW. They will be marked as such.

I hope this helps those unfamiliar with lenses, lens blocks and auxiliary telescopes.

Fraser

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