Real camera simulation:
I know how to simulate the optics part given the camera calibration model and parameters.
I would like to know how to characterize the grey level values given X amount of light, which I assume is more the CCD part.
Suggestions?

Find the minimum brightness needed to max out the sensor and the maximum brightness needed to zero out the sensor, and use that as your operating range? You could also measure brightness vs sensor output at different levels and establish a response curve that you can interpolate and use to give proper outputs.

Substitute brightness for radiance/irradiance/whatever quantity you're working on

@RememberMe Suppose I can measure the radiance from some light source, and record the corresponding grey levels. Can I translate it (roughly) to photons per area?
I was thinking about implementing it in Unity with ray tracing, so I would need to know what amount of light causes some level of grey....