I've dealt with monochromatic sources for years, but having to deal with non-ideal waves has proven to be an issue for me. As an example, let's say a quasi-monochromatic, spatially incoherent light source illuminates a transparency with an intensity transmittance function t(x,y) at the front focal plane of a lens. How can we find the intensity distribution at the back focal plane of the lens? Typically a lens would perform a Fourier transform of the transmittance function with a monochromatic source, but how does this change with a quasi-monochromatic, spatially incoherent light source? Is it as simple as considering the intensity of the incoherent light at the transparency and propagating that forward rather than the field like with the coherent case or am I thinking about this the wrong way?