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FFT MTF: Unexpected Values

  • 5 January 2021
  • 4 replies
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A possible impact on MTF of a complex beamsplitter coating has been investigated on a simple system comprising three lenses and two prisms on top of each other. Coordinate breaks and MIRROR surfaces were used to 'construct' the prisms in 2-D. The said coating has been applied to one of the MIRROR surfaces.


The system is polychromatic, on-axis (one field only) and 'polarisation' has been checked in the FFT MTF Setup Window.


Now a surprise: with the coating applied or not the values of MTF differ by up to approx. 3 precentage points in the medium frequency range in 'all fields' mode as compared to the 'field 1' mode. This is not dramatic but very probably shouldn't happen...


Any explanation?


Thank you, Dusan

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Best answer by Dusan 11 January 2021, 15:32

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Mark.Nicholson
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Hey Dusan,


Without seeing the file it's hard to say, but I'm the opposite polarity to you: I'm not surprised by this and would find the opposite result surprising. Unless your coating has flat transmission and flat phase (i.e., doesn't actually do anything) I'd expect the coating would affect the MTF. Anything that affects the amplitude and phase of the wavefront will affect MTF.


- Mark

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Hi Mark


Good to hear from you. You are right about the coating and MTF but this occurs without coating, too and the basic question is why is the MTF different for 'all fields' mode as opposed to 'field 1' mode when there is one field only.


Kind regards, Dusan

Mark.Nicholson
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Userlevel 7
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Oh I see, sorry. I suggest you take the coating off (as it's not needed to demonstrate the issue) and post a ZAR of your file or send it to support@zemax.com. That's the easiest way to see what's happening.


My guess without seeing the file is that the sampling is changing in the two cases. I'd increase the sampling and see what happens to the difference. But as to whether that's a bug or normal operation, the support team will need to see a file that shows the issue. I had a look ata couple of files and ,needless to say, could not see any variance :-)


 

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Support just confirmed there is a discrepancy between 'all fields' and 'Field 1' MTF values. However, not in real MTF but in diffractive MTF, which I forgot to mention. I apologize for that.


Polarisation has to be ON, which I mentioned.


Thanks, Dusan

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