Hey Alba, haven’t you answered your own question? You need to increase the resolution of the detector.

Hey Alba,

Contrast is (Imax-Imin)/(Imax+Imin) as a function of spatial frequency.

Imagine stray light that was a constant background level. That will reduce the contrast because Imin will increase everywhere. Now give the stray light some structure, so it’s brighter in some places than others. Imin will now have a spatial frequency component that interferes with the regular MTF. I don’t think there is any analytic way to examine the structure, so OS simply ‘takes what it’s given’ and computes the MTF with whatever the light distribution is.

Does that help? I don’t feel like I have answered the question “I don’t really know how to get any useful information on the loss of contrast from veiling glare in the image from this figure” other than to play with it and see. You could save a ZRD of the direct, intended signal and save a ZRD of the stray, and then add them together in varying ratios and see the effect of the stray on MTF as you turn up the intensity of the stray light.

Hope that helps, but please continue the discussion. I think its a really interesting topic

• Mark

Thanks Mike! But just to be clear, that assumes the stray light is a constant over the detector, right?

You guys may also want to take a look at the book Statistical Optics, 2nd Ed. by Goodman -- specifically, Chapter 8 (Imaging through Randomly Inhomogeneous Media), Section 8.1 (Effects of Thin Random Screens on Image Quality).  Not that a thin scattering screen is the best model for veiling glare in a lens system, but the analysis does lay a basic foundation that could be used for testing a first-pass model in OpticStudio before extending the model to the veiling glare case. 

I’ll see if I can distill the salient points here.  First, the geometry of the problem is illustrated below, with the scattering surface located in the pupil of the imaging system:

Goodman begins by showing that the OTF for this case is given by a product of the OTF for the ideal imaging system times an OTF that captures the impact of the random screen:

Of most interest is a random phase-only screen:

Assume the phase at any (x,y) location is a Gaussian random variable, with the “strength” of the scattering being characterized by the phase variance.

Plugging 8.2-1 into 8.1-8, and going through some statistical analysis, Goodman shows how the autocorrelation function of the screen (8.1-8), and hence the OTF of the screen, depends on both the phase variance (which is just a number) and the normalized autocorrelation function of the random phase distribution.  The result is:

That’s the math part.  Here are some example results. 

Therefore, we see that the imaging system OTF in the presence of scattering is reduced by a multiplicative function (i.e., the OTF of the screen) that can dramatically suppress higher spatial frequencies, with the effect becoming more pronounced as the phase variance increases (i.e., as the degree of scattering goes up). 

The PSF is the inverse Fourier transform of the system OTF.  Here are some example PSFs:

Sorry if you think I’ve gone a little too far into the weeds, but this is an interesting application of statistical optics.  It would be cool if similar results could be obtained with a model in OpticStudio.

Regards,

Jeff

And if we’re already writing scripts, to account for diffraction, you can turn on the Huygens PSF flag, get the diffraction PSF, take the FFT and get the diffraction optical MTF.  

It would be nice if non-sequential mode had an option to define an F/# for multiplying the Geometric MTF by the diffraction limit (to align it with the sequential Geometric MTF) and to provide the Huygens MTF if the Huygens PSF is already calculated.