I agree with Spirit 532.
The seller of the supplemental lenses for the Seek camera is supplying a market segment that does not have the knowledge or skills to make their own. The seller is using a standard cheap ZnSe CO2 engraver lens that us very common on eBay. Such a ZnSe lens ranges in price from $10 to around $50 depending upon size and quality. Laboratory grade ZnSe lenses are priced in the hundreds of Dollars due to their guaranteed optical performance. Companies like Edmund Optics sell such lab grade lenses.
The ZnSe lenses that I, and many others on this forum, use are intended for use in optically non demanding roles such as CO2 lasers and low resolution imaging. As such they are not guaranteed to be very low distortion. That said, they are good enough for close-up Imaging with our relatively low resolution thermal cameras. The CO2 laser lenses come in a range of diameters and focus distances. I personally tend to use 100mm focus distance with the FLIR E4 and it’s relatively wide FOV. Other focus distances include 50mm, 66mm and 75mm. These lenses are affordable so different focus distance supplementary lenses can be kept ready for differing needs.
The diameter of the supplemental ZnSe lens should be marker than the camera primary lens objective to avoid edge distortion and Vignetting. The centre region of these ZnSe lenses generally provides better performance than the edge region. You can use a larger diameter lens than is actually needed but not a smaller diameter. The supplemental lens needs to be mounted reasonably close to the cameras primary lens. Consider the primary lenses FOV. You want the Supplemental lens to completely cover the FOV to avoid weird edge effects.
Mounting a Supplemental lens is often a challenge on cameras that are not designed to gave such fitted. The only requirement is to hold the supplental lens reasonably securely in front of the primary lens objective. This can be achieved with all manner of contraptions including cardboard and elastic bands
Take a look at mobile phone supplemental lens sets. They are very common and cheap. I bought two types that comprise a range of very nicely made aluminium lens barrels containing different diameter lenses that provide wide to telephoto imaging for the cell phone. The glass lenses are easily removed and a ZnSe one fitted in its place. The mounting to the phone is either a spring loaded clip or a combination of an adhesive metal ring on the phone around the lens and magnetic ring on the lenses. The magnets hold the lens firmly on the camera. These inexpensive multi lens mobile phone supplemental lens kits are brilliant for experimentation as you get a mounting system plus 5 or 6 different sizes of lens barrel to experiment with. Mine had lens locking rings so we’re easily dismantled and repurposed.
Regarding any ‘characterization’ Of the ZnSe lens with regard to the effect on measurement accuracy. Such is not that difficult to carry out yourself. Whether the seller of the Seek lenses actually did this or not isolen to discussion as they could just have used the generic transmission curve for such a lens and applied that to theirs as a ‘calibration’ curve.
Thankfully ZnSe is a high transmission material at LWIR so the losses through a single lens element are quite low. A crude lens characterization test just involves looking at a decent thermally emissive target, at different temperatures, with and without the lens in place. A very crude test you can easily carry out is what I call the coffee cup test. A metal beaker would be better though. A Piece of common PVC electrical tape is a good decent emission target for the test and it is heat resistant. Fill the mug or container with water at about 40C and wait fir the mug to take up the heat before starting the test. Aim the thermal camera at the insulation tape and take a temperature measurement. Now add the close-up lens, and move to the focus distance of the supplemental lens until focus is achieved. Measure the temperature of the tape. Repeat this test for a number of different temperatures by using hitter and hotter water until you reach boiling water temperature. That is as high as you can go with this simple test. At the end of the test you will have two sets of temperature readings, with and without the ZnSe lens fitted. These figures will hopefully give you an adequate idea of how much the ZnSe changes the measurements. You may even be able to calculate a general transmission figure for the supplemental lens that may be applied to any reading taken using it. My Inframetrics lenses have just a single stated transmission figure of around 96% that I can use to offset any readings taken whilst using them
Best Wishes
Fraser
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