Hello.
With this core came from the seller also a nice Germanium Lens, but without any specifications.
In the goal to replace the genuine lens from the Indigo Core (8mm/1.2) i wanted to know more about this lens.
Physically, the lens is housed in a kind of ceramic (i suppose) sleeve or jacket.
The lens is apparently, when observing my own reflexion, a plano-convexe element (i suppose single, i was measuring rear distance from the edge of the tread and front distance from the front edge of the jacket/sleeve, and i obtained just 2 or 3 mm of thickness on the periphery of the lens itself, roughly).
- Convexe side in forward direction, and plane side in backward direction.
- The clear diameter of the lens is around 40mm.
- Length of the brown ceramic sleeve/jacket (without the thread), around 83 mm.
What i was missing is the focal length of this lens.
Today being a sunny day, i decided to simply aim the Sun, which i believe to be a good source of LWIR, and i fixed the lens on a tripod with behind a brown card-box screen which had apparently some decent emissivity. And i used my Guizmo as a monitoring tool for finding, by moving the screen, the focal plan of my setup wherein the sun was appearing the most brillant and with the smallest spot or projected image.
My wording is a bit vague, because, with the resolution of my thermal camera, i had to repeat several times the experience, moving back and front the screen, the image being not super defined as you can imagine, and i had to average my brightness illumination, from that.
But finally, when considering the lens as roughly equivalent to a thin lens, i obtained a focal length around 40mm.
Therefore, i suspect my unknown lens to be a 40mm/ "around f/1". Which would be a nice replacement lens for my DIY camera.
Later on, i will precise my measurement by comparing the FOV given by the genuine 8mm/1.2 lens with the FOV provided by this other lens.
Here some photos of the "experimental set-up". a crude, rudimentary one, but at least giving some indications for me....
It is always fun and rewarding to be able to use a non finished project as an already more or less functional tool for investigating further aspects for itself....
Cheers.
Stéphane