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.htmI 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