We calculate the Schmidt number for a two-dimensional model of the nonfactorable spatiotemporal wave-function
of biphotons produced in type-I spontaneous parametric down-conversion with degenerate and collinear phase-
matching taking into consideration a major part of the broad spectral and angular bandwidth of the down-
converted light. We derive an analytical expression for the Schmidt number as a function of the filter bandwidth
in the limit of spectrally narrow pump.