How Much Light Loss...

just curious to know how much light is lost: ...........If the top of A cab is black, the sides and base are painted flat white, and lights are mounted on the back, how much light will be absorbed into the top? How much gain would you get from painting the top white? And what would the difference be from painting the top of the cab white to getting a reflecor.... lets say a 400w bulb for instance

The amount of light lost is a direct percentage of the area that is black vs the rest white... In a cube, it would be 1/6 lost or ~ 16%

Having a reflector will give you even more light, it will be directed toward the plants instead of bouncing off the top and sides of the box... Essentially twice as much light will be directed down. A reflector is definately a good investment.