Secondary photons in SiPMs are responsible for at least three processes: (i) internal cross-talk (ii) external cross-talk and (iii) optically-induced afterpulsing. While the internal crosstalk and afterpulsing involves photon transport within the SiPM, the external cross-talk photons escape from the surface of one SPAD and potentially: (i) reflect back into the SiPM at the surface coating interface and trigger avalanches in neighbouring SPADs, (ii) transmit through the SiPM surface coating. Since some of the future multi-ton dark matter and neutrinoless double beta decay experiments are choosing SiPMs as photosensors, the external crosstalk can be a significant background due to each SiPM's tendency to trigger a nearby one. This mechanism may cause detector background and reduce the accuracy of photo-electron resolution for high photo-electron events, leading to a degradation in the position and energy reconstruction.

To quantify the systematic effects which deteriorate the overall performance of such detectors, a study on SiPM secondary photon emission was conducted. The SiPMs tested were 1 $\text{cm}^2$ FBK NUV HD-cryo. It determined the absolute secondary photon yield equal to the number of photons emitted per charge carrier ($\gamma/e^-$) using spectroscopy. The photon yields were calculated at 163 K and 87 K to mimic the SiPM performance at liquid Xenon (LXe) and liquid Argon (LAr) temperatures. A summary of the spectroscopy technique and data analysis used to quantify the secondary photon yield at cryogenic temperatures is reported.