Note that the ZnSe lenses come in Planar-Convex and Bi-Convex. With Planar Convex you may get a better image quality but you would need to experiment with which surface (Flat or Convex) faces the item under observation. With my commercial Meniscus close-up lenses the concave side faces the object under observation but users of Planar Convex ZnSe lenses report that they get best results with the convex side facing the target.
The ZnSe supplemental lens is just like placing reading spectacles on your camera. It provides close focus but you lose distance imaging and there is a definite small depth of field over which good focus is available. The camera may need to be held on a tripod for best focus stability.
There are different qualities of ZnSe lens. Some are designed for use on cameras and in labs. They cost a lot of money. The cheaper ZnSe and GaAs lenses we buy from Asia are intended for use in CO2 laser systems and are of variable quality. There is a standard lens type that is the cheapest and an ‘HQ’ lens that is said to be better optical quality. The HQ lens is a bit more expensive but still not a bad price. Expect to pay $30 to $50 for one. Neither type of CO2 laser lens is actually designed for use on a thermal camera and the manufacturers advise that the lenses are for non critical optical applications where some distortion is acceptable. It is a happy coincidence that the CO2 operating wavelength falls within the LWIR band and so the CO2 laser lenses are AR coated to work at LWIR. Whilst they work well as a close-up lens in our application, do not expect the very highest optical performance from them. Often centre focus is good, whilst edge focus is degraded somewhat. For their cost, these lenses cannot be beaten however
If you want better close-up optics for your camera, there are commercial supplemental lenses for FLIR and FLUKE cameras but these high quality lenses cost more than your camera
I have managed to buy some used close-up lenses at reasonable prices on eBay however.
Also.... remember that adding a supplemental lens to your camera does effect the measurement accuracy. The readings will be slightly low with a ZnSe lens fitted. You can test this by imaging a surface of known temperature with and without the ZnSe lens fitted. Temperature measurements should be carried out with the target area in focus.
Fraser