The reliable generation of true single-photon wavepackets with well-defined modal structure is a crucial ingredient for the realization of linear optical quantum computing. In this paper we present experimental results indicating the efficient generation of conditionally prepared single photons from parametric downconversion in a KTP nonlinear waveguide. In addition we present, theoretically and experimentally, a novel criterion for the assessment of conditional single photon sources which takes into account the contributions of heralded vacuum due to optical losses, of higher photon numbers and of the binary response of typical single photon detectors. Utilizing this criterion, we show that our KTP nonlinear waveguide represents a high fidelity source of conditionally prepared single photons.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.