  A single photon source with high repeatability and low uncertainties is the
key element for few-photon metrology based on photon numbers. While low photon
number fluctuations and high repeatability are important figures for
qualification as a standard light source, these characteristics are limited in
single photon emitters by some malicious phenomena like blinking or internal
relaxations to varying degrees in different materials. This study seeks to
characterize photon number fluctuations and repeatability for radiometry
applications at room temperature. For generality in this study, we collected
photon statistics data with various single photon emitters of $g^{(2)}(0) < 1$
at low excitation power and room temperature in three material platforms:
silicon vacancy in diamond, defects in GaN, and vacancy in hBN. We found common
factors related with the relaxation times of the internal states that
indirectly affect photon number stability. We observed a high stability of
photon number with defects in GaN due to faster relaxations compared with
vacancies in hBN, which on the other hand produced high rates ($> 10^6$) of
photons per second. Finally, we demonstrate repeatable radiant flux
measurements of a bright hBN single photon emitter for a wide radiant flux
range from a few tens of femtowatts to one picowatt.
