For monochromatic light, the PSF can be calculated by Fourier Transformation. For a white light imaging system, the PSF is a weighted integration of the monochromatic PSFs. The weighting coefficient is the product of the sun-light spectrum, spectral reflection factor of the object, spectral transmission factor of the optical system, and the spectral response of the image detector. Because different object has a different spectral reflection factor, to be practical and simple, we use an average spectral reflection factor of typical ground scenes instead.
It will induce error in the PSF estimations by using the average reflection factor instead of the real one. This error is analyzed in the paper which shows that the error is acceptable.