PHENIX has measured low momentum direct photons at 200 GeV via their external conversions to di-electron pairs on a specific detector material. The advantage of this method along with the selection of this detector material is a purity to large extent in photon identification. We present the results of the measurements of low momentum direct photons at 39 GeV and 62.4 GeV based on the same external conversion method. These results, together with those obtained at 200 GeV, can help in constraining the sources of low momentum direct photons. Besides, we report on the observed scaling behavior of direct photons in terms of the photon integrated yield vs. charged-particle multiplicity by combining the data from various collision systems at four center-of-mass energies. The direct photon scaling properties show that the strength of the thermal radiation source grows faster than the charged-particle multiplicity.