The photon-counting system seems to be the best solution for extremely low optical power measurements. The Hamamatsu HC135 photon counting module has a built-in high-voltage power supply amplifier, discriminator, micro-controller with an RS232 serial output. It requires only a +5V supply voltage and an IBM PC or compatible computer to run. The system is supplied with an application software. This talk is about the testing of the device.
Gyorgy Andor, Gyorgy Andor,
"Characterization of a new computer-ready photon counting system", Proc. SPIE 3573, OPTIKA '98: 5th Congress on Modern Optics, (19 August 1998); doi: 10.1117/12.321019; https://doi.org/10.1117/12.321019