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I have designed an imaging lens assembly incorporating two plastic lenses. The first is Zeonex E48R and the other is PMMA. The manufacturer has supplied the lenses well within tolerance, including the surface irregularity P-V. I have remodelled the system incorporating all supplied component tolerances and surface irregularities and i am never getting above a 50% MTF contrast of 50lp/mm compared to the design value of 110lp/mm. The worst case modelled MTF over 1000 Monte Carlo runs is 80lp/mm. Note that i have also included the lens barrel tolerances in the model.

Any suggestions as to what could be causing the further consistent loss of MTF, particularly wit plastic lenses? The lenses come from several tooling cavities, but this has no impact on the results. Illumination is at 850nm and the transmission of the filters used has been confirmed as correct. 

I would suspect the refractive index of the lenses, the homogeneity of the index, and stress birefringence in the material.

maybe also the thermal expansion coefficient? It is order of magnitude greater than glass and I saw myself how even 10 or five degrees can destroy the MTF

Thanks for the answers. Very useful input for me to investigate. Would you suspect that the PMMA is a bigger culprit with regards to the stress birefringence than the Zeonex E48R?

With regards to the thermal explansion, i will investigate this further, but the lens manufacturer surface metrology on sample parts from both cavities fall within the modelled tolerance. We are currently using an external metrology house to confirm the lens manufactureres claims

Hey Craig, try putting each lens between crossed polarizers and look for any fringing.

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