1. The problem statement, all variables and given/known data For poisson's spot to be observed, how accurately circular must the stop be? How is the argument for the Poisson's spot changed if the light is either; white or spatially incoherent? 2. Relevant equations The Cornu spiral for the circular aperture. 3. The attempt at a solution Using the Cornu spiral I understand that the Poisson spot will be present, however I don't understand the diffraction theory enough to be able to deduce how the solution to the Fresnel integral is going to change as a result of the non perfect circularity of the obstacle. The way I tried solving the problem was to think about the Fresnel half period zones. From the Cornu spiral it is clear that when the obstacle covers and odd number of the half period zones the Poisson spot will disappear. So if the defect in the obstacle is periodic (suppose like a sine wave) which is centred on the boundary between the ultimate and penultimate covered half zones, the if the sin has the amplitude equal to the width of the half plate zone the result will be the disappearance of the Poisson's spot. The second part of the question gets a bit more complicated, namely; the width of the half period zones is wavelength dependent. However it seems to me that this should not prevent there existing a Poisson's spot for certain wavelengths for certain obstacle sizes and distances? However I don't know what would be the effect of the spatial incoherence? Is the above reasoning correct? is there a way to make the explanation a bit less hand-wavy without involving mathematics?