Final answer:
The product of the first filter and the second filter determines the total gain or brightness increase in the image intensifier system.
Step-by-step explanation:
In the context of image intensifiers or brightness gain, the product of the first filter and the second filter results in the total gain of the image intensifier or brightness gain.
For example, if the first filter has a gain of 2 and the second filter has a gain of 3, the total gain would be 2 x 3 = 6.
Therefore, the product of the two filters determines the overall gain or brightness increase in the system.