It depends on the type of cham, the wattage, how cold the room is and how big the cage is.
Generally speaking, a cage with a minimum heigth of around 2 ft. and a room temperature of around 65 to 68, a 75 watt bulb would be good. If you have a Jackson, maybe a lower wattage. In this case, the heat lamp could be placed on top of the screen.