if you have no energy source inside your cabinet, your fan will make your cabinet temp the same as your intake temp. any energy source (light of course, but also any other electrical or mechanical equipment doing work) will generate heat and raise the cabinet temp.
a very efficient ventilation system will maintain the cabinet temp close to the intake temp; the more efficient, the closer. the more energy you put in the cab (the hotter the lights), the more you will test the efficiency of the system.
you can optimize the system by making sure there is adequate intake venting, and hopefully that the path from the intakes to the exhaust will include the material to be cooled. in other words, make sure the fan can suck enough air in.
you can also make sure the exhaust is optimized by reducing back pressure as much as possible. the bernoulli principle reminds us the rate of flow is proportional to the length of the pipe. so the shorter and wider the pipe the better. any obstruction will impede outflow, increasing back pressure.
but in no case will you be able to reduce the temp below the intake temp unless you use something to extract energy from the incoming air. some form of airconditioner.
