I do not see that, in the FT pictures the underside looks flat to me.
One could experiment with two of these directly on top of each other. You will loose some light from the extra reflections off the extra surfaces, but it will more or less act like one thicker lens :-)
Yeah, this is how I understand it, assuming everything else is the same,
100mm lens with low convex profile - long focal length, project smaller spot (lesser light captured)
100mm lens with higher convex profile - shorter focal length, project larger spot (more light captured)
But they both have the same light intensity in the spot.
You may use dual-lens system to ‘shorten’ the the focal length of the 100mm lens - resulting a larger projected spot with still the same intensity.
(I might not use the correct technical terms here.)
It adds a couple extra reflective surfaces but back to back, the focal point should be closer and capture more light. Bridging that thin gap with optically clear silicone could eliminate or at least drastically reduce those reflections.
Interesting. Says 80mm in the link, but I'm not surprised it is wrong. Are they actually 24mm thick as advertized? If so, they it would seem the lens in the OP would have an even longer FL since it is only 19mm thick. How are the lenses working for you?
Had to get out to try it out. This lens has to be built into a zoomie because fully focused you need binoculars to see out far enough to to use the hot spot. The hot spot is too small within the range of normal eye sight. Check you my 20 minute to build light:
LOL. Yeah, totally surprised me too. I expected junk for $9. I'm thinking the issue is that it might focus the image of the die with some artifacts. Could just be my set up though with the white PVC. Well, I have to crash for work tomorrow. Night folks.