What is the principle of photometric calibration? How to do it in my own project?

[qdLMF]

Thanks for this awesome work!
I was investigating how mask.png was generated and I found that mask.png was generated by quadcam_tools/simple_photometrics_calib.py. And I have the following questions:

1. What is the principle of this photometric calibration process? Is there any document I can learn it from?
2. What is the bag file used by quadcam_tools/simple_photometrics_calib.py?
3. How to generate my own mask.png for my own camera?