when i select photons , pause the game, press D and hold left klick for a few sec or longer, the number of parts is increasing all the time... for example: there are now 20.000 parts of photons at a 4x4pixel field , but when i save the simulation and reopen it, there only 50-100 photons .... so., why they're deleted while saving?
This is because saves only store one particle per pixel. So if several particles are in the same place, like photons are in this case, not all of them will be saved.