"This work we investigate a two-photon state in space and frequency using a Gaussian pump beam, for SPDC bulk in non-linear crystals, considering a collinear geometry, and restricting the spectral bandwidth to the narrowband case. We derive analytic expressions for two- and single photons spectral brightness. However, an analytical expression for the single-photon collection has not been (to the best of our knowledge) reported, in order to verify that our results are correct, we calculate the heralding efficiency and compare our results with previous studies. For both collection probabilities our findings are in excellent agreement."