How can I simulate PSF of incoherent light?

  1. Hi There,

    Recently, I am working on simulating of Point spread function of high numerical aperture objective lens, according to the Richard and Wolf's mathematics representation, I can do the calculation of PSF, like transverse size or longitudinal size without any difficulty, but this formula is based on diffraction theory, not working with incoherent light. How can I calculate transverse size and longitudinal size of incoherent light PSF ?

    Does anyone tell me an appreciate formula or method ?

    Best regards.

  2. jcsd
  3. DrDu

    DrDu 4,636
    Science Advisor

    You can simply average the intensity patterns of light with two orthogonal polarizations.
  4. Thanks for your reply.
    As your suggestion. I've done the simulation for orthogonal polarized light. but there is still side lope, I am wondering, if the incident light is incoherent, I don't think there is side lope emerged, because there is no interference between incoming rays.
    If I set random phase for each rays, in focal region, there is no PSF formed, the instead is homogeneous intensity distribution everywhere, which is not obviously wrong for light focusing. I am thinking maybe for the incoherent light focusing, the wolf's diffraction formula(based on debye approximation) no longer working.

    maybe orthogonal polarized light is suitable for simulation of unpolarized light(or random polarized light).

    the attachment is the PSF from orthogonal polarized light. I just add the intensity patterns of x-linear and y-linear. I also enclosed one paper which I use for simulate PSF.

    Maybe I didn't catch your idea. could you explain to me in detail ?

    thank you very much

    Attached Files:

  5. Andy Resnick

    Andy Resnick 6,142
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    In what sense do you mean 'incoherent'?- spatial? temporal? both?

    What's the Richard and Wolf reference? I tend to use Min Gu's "Advanced Optical Imaging", with high NA lenses covered in Chapter 6.
  6. Hi Andy Resnick,

    I think the both, because in my case, i just use white light source with vary narrow band pass filter. the light is focused by NA=0.6 objective.

    In second post, I've enclosed one relevant paper which I use to simulate the PSF.
    I think I gonna read the book 'Advanced optical imaging theory'.

  7. Andy Resnick

    Andy Resnick 6,142
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    Sorry- didn't see that attachment. I have a better understanding of what you are trying to do. I assume you are trying to understand equations (2) and (3)?

    The authors gloss over a few minor details- their eq. (1) is called the 'Sine condition', and is not the only apodization condition. There are also the Herschel (h = 2f sin(θ/2)), Helmoltz (r = f tan(θ)), and Uniform projection (r = fθ), and so it is important to know which condition the lens obeys (AFAIK, Leica objectives obey the sine condition, for example).

    Of more consequence is the effect of dielectric interfaces (for example, a coverslip): here, the 'high NA rays' will be grossly effected due to the different s- and p- polarization coefficients of reflectivity, resulting in significant changes to the PSF.

    Both of these are handled in vectoral Debeye theory- the results are straightforward, but the expressions are rather long (Gu's book has them on 6.5.9-6.5.13); 6.5.18 is the intensity.

    A few more references:

    P. Török, P. Varga, Z. Laczik, and G. Booker, "Electromagnetic diffraction of light focused through a planar interface between materials of mismatched refractive indices: an integral representation," J. Opt. Soc. Am. A 12, 325-332 (1995).

    P. Török and P. Varga, "Electromagnetic diffraction of light focused through a stratified medium," Appl. Opt. 36, 2305-2312 (1997).
  8. DrDu

    DrDu 4,636
    Science Advisor

  9. thanks, I also repeated this two references's results before without any difficulty, using the given mathematics formula, which based on Debey approximation. Now the problem is that for the incoherent light source, I think there is no interaction between each ray, it should be only sum of intensity in focal region. but once I used (Itotal=E1E1*+.....+EiEi*+......EalphaE*alpha) instead of Itotal=(E1+......Ei+......Ealpha)(E1+......Ei+......Ealpha)*, where alpha is maximum open angle of objective lens, alpha=asin(NA/n), I couldn't get any intensity pattern, the instead is the uniform intensity distribution everywhere in focal plane, I think maybe it is because each ray point to focal region as spherical shape, so if do square for only one ray it should be get uniform amplitude everywhere(quiet personal view). I am so confused for this. do you have any experience for this ?

    I do appreciate your timely help

    Best regards
  10. Andy Resnick

    Andy Resnick 6,142
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    I had a hard time understanding this post. In cylindrical coordinates, the field E(r,ψ,z) propogating in direction 'k' at focus is:

    E(r,ψ,z) = iπ/λ[(I_0+ cos(2ψ)I_2)i +sin(2ψ)I_2 j + 2icos(ψ)I_1 k,
    I_0 = ∫P(θ)sinθ(1+cosθ)J_0(krsinθ)exp(-ikzcosθ)dθ
    I_1 = ∫P(θ)sin^2(θ) J_1(krsinθ)exp(-ikzcosθ)dθ
    I_2 = ∫P(θ)sinθ(1-cosθ)J_2(krsinθ)exp(-ikzcosθ)dθ

    where i,j,k are unit vectors and there should not be confusion about the other i = √(-1) or other k = 2π/λ, J_0 etc are Bessel functions, θ runs from 0 to sin^-1(NA/n), P(θ) the apodization function, etc. In the paraxial approximation, I_1 = I_2 = 0, and the usual result is obtained.

    The intensity is |E|^2:

    I(r, ψ, z) = C[|I_0|^2 + 4|I_1|^2 cos^2(ψ)+|I_2|^2 +2cos(2ψ)Re(I_0 I_2*)]

    Does this help?
  11. Sorry for the making you so confused.

    Thank for your timely replay.

    I am not familiar for typing math here. so I enclosed pdf format document to here. please find the attachment.

    Thanks again for your help.


    Attached Files:

  12. Andy Resnick

    Andy Resnick 6,142
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    Thanks for the document, that does help me understand a lot better. I don't think you can simplify [4] the way you hope, and even worse I suspect that as your NA gets large, there may be numerical instabilities similar to Mie scattering results when the phase term oscillates very rapidly, if you discretize too coarsely.

    Also, your [2] looks vaguely familiar- where did that come from?

    Are you able to recover 'known' results if your NA is small?
  13. Hi Andy Resnick,
    Thanks for your replay.

    1. for the formula [2], there is one paper from Lars Egil Helseth: "Focusing of atoms with strongly confined light potentials" explained it in detail.

    2. normally, the mathematics representation based on vector debye approximation is more suitable for tight focusing where NA>0.6, for the case of extremely low NA, it does not give correct results, because polarization(vector property of light) no longer plays dominant role, the instead is using formula based on scalar theory.

    do you think the mathematics representation still valid in the case of incoherent ? I don't know how can i change the formula when the incident light is incoherent.

  14. Andy Resnick

    Andy Resnick 6,142
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    If I read that paper correctly, Helseth did most of your work already: all you need are the results [8-12] and [15-18]. Since radial and tangential polarization states are orthogonal, the (incoherent) intensity is simply I = 1/2[I_r + I_t], where I_r is the intensity from radial polarization I_r = |E_r|^2 and I_t the intensity from tangential polarization. The components of E_r are given in [8-12] and E_t in [15-18].

    Thanks for the reference- there's some good stuff in there.
  15. thanks for your replay.

    If I understand clearly, your meaning is that doing simply add the two intensity patterns of radial and tangential polarization? which means that in the case of incoherent, it is equivalent to the Superposition of PSF from radial polarized beam and PSF from tangential polarized beam. yes, there is no interaction between these two orthogonal polarized lights, we can assume that it is same like incoherent. if it works, I am wandering maybe for incoherent, superposition of PSF from x-linear and y-linear polarized light also can do incoherent work.

    Could you tell me further information about the assumption ? do you have any reference for that ?

    thank you very much for your help.

  16. Andy Resnick

    Andy Resnick 6,142
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    I'm not sure what you mean by 'assumption'...? For example, using the Stokes vector (S0, S1, S2, S3) to describe the polarization state, the total intensity is S0. You can choose any basis states for polarization: x and y, left- and right-circular, radial and tangential, etc.
  17. DrDu

    DrDu 4,636
    Science Advisor

    After having proposed to average over polarizations myself first, I now rather think that you have to average over waves entering under slightly different angles or from different points (depending on the characteristics of the incoherent light source!)
    Generally, I find this thread hard to follow: E.g. the article you cite does not mention point spread function.
    Maybe you could just write down the expression you found for coherent light and explain it?
  18. Hi Andy Resnick,


    Here I've enclosed one attachment, which is according to your suggestion.
    I = 1/2[I_r + I_t].

    I was confused the difference between incoherent light and unpolarized light. I think I = 1/2[I_r + I_t] is more suitable for unpolarized light. do you think <<1/2[I_r + I_t]>> can reflect the incoherent but linear polarized light ? (such as the light from white light source + filter+polarizer).

    thanks again


    Attached Files:

  19. DrDu

    DrDu 4,636
    Science Advisor

    I looked at the definition of the PSF and found that the answer to your original question is very simple: The point spread function defines the spread of light from a point source when passing through some optics. But the light from a point source is always completely spatially coherent and, if monochromatic, also temporally coherent.
  20. Hi

    Thank you very much for your post.

    I tried with small angle. please find the attachment. I also put the formula for PSF.

    Thanks again for your help,
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