Is the intensity reading by pixel proportional to the photon count?

To be more precise, I would like to know for example if I double the integration time, will the reading by pixel will double as well or there is some kind of gamma correction or else processed on board?

The camera ISP applies gamma correction. It’s a normal step.

I understand. However, is it possible to access the raw data without the gamma correction? The ones that should be proportional to the photon count or the absolute luminosity?

Yes, use the RAW BAYER image.

Set the pic format to BAYER.

Note, on the OV5640 the raw bayer image is still processed by default. If you have a MT9V034 sensor then this will be unprocessed.

Thanks for the quick reply. I will look for the MT9V034 and test if the readings are linear with the exposure time for a constant light source.