  In this paper, we report on the photon emission of Silicon Photomultipliers
(SiPMs) from avalanche pulses generated in dark condition, with the main
objective of better understanding the associated systematics for
next-generation, large area, SiPM-based physics experiments. A new apparatus
for spectral and imaging analysis was developed at TRIUMF and used to measure
the light emitted by the two SiPMs considered as photo-sensor candidates for
the nEXO neutrinoless double-beta decay experiment: one Fondazione Bruno
Kessler (FBK) VUV-HD Low Field (LF) Low After Pulse (Low AP) (VUV-HD3) SiPM,
and one Hamamatsu Photonics K.K. (HPK) VUV4 Multi-Pixel Photon Counter (MPPC).
Spectral measurements of their light emission were taken with varying
over-voltage in the wavelength range of 450--1020\,nm. For the FBK VUV-HD3, at
an over-voltage of $12.1\pm1.0$\,V, we measure a secondary photon yield (number
of photons ($\gamma$) emitted per charge carrier ($e^{-}$)) of
$(4.04\pm0.02)\times 10^{-6}$ $\gamma/e^{-}$. The emission spectrum of the FBK
VUV-HD3 contains an interference pattern consistent with thin-film
interference. Additionally, emission microscopy images (EMMIs) of the FBK
VUV-HD3 show a small number of highly localized regions with increased light
intensity (hotspots) randomly distributed over the SiPM surface area. For the
HPK VUV4 MPPC, at an over-voltage of $10.7\pm1.0$\,V, we measure a secondary
photon yield of $(8.71\pm0.04)\times 10^{-6}$ $\gamma/e^{-}$. In contrast to
the FBK VUV-HD3, the emission spectra of the HPK VUV4 don't show an
interference pattern -- most likely due to a thinner surface coating. EMMIs of
the HPK VUV4 also reveal a larger number of hotspots compared to the FBK
VUV-HD3, especially in one of the corners of the device.
