FOV reduction with Fresnel
I have a simple optical system that works based on energy collection. (i.e. I have a photo diode that counts photons). I don't do any imaging. For my application, I need to have a specific FOV and I have been trying to build this FOV using off the shelf Fresnel lenses. (my FOV is 200x200mm high at 300mm away from the lens system, I am trying to reflect this to a 2.5x2.5 detector)
I basically, take an image of my FOV (i.e. assume my FOV is an image) at target distance, I ray trace through three fresnel lenses (F=15, F=15, F=10.2 all same diameter), I position the lenses such that each one is 1 mm away from each other on the same axis. 791
Each one is 2mm thick. The final image of FOV falls on to the detector.
When I measure each lens individually, they all adhere to their spec (ie. focal length), however when I construct my system (really putting one after the other with 1mm gap), I think there is a problem:
1. The energy collection is not what it is supposed to be (the diameter of the first lens is 10mm, so I expect roughly 10x amplification but I only see around 5)
2. I take a bright light (white LED) and shine to lens system (at this moment system is in my hand and I am reflecting to a surface) hoping to see a bright image of the source. I do not see the source brightly, if anything it is very weak.
3. I take out the third lens and use the first two and repeat the bright light experiment and same result, image is brighter compared to three lens but not as bright as a single lens experiment. I assume the loss is about 5% on each surface so this loss on brightness is puzzling.
Now, does it make sense to do what I am trying to do with Fresnel lenses? Assuming my trace is correct (I am pretty sure it is), where else I should be looking?
What other methods you recommend, I really need a very low cost optical front end to reduce the FOV to detector. 791