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It sort of depends on where you position your light in the 'bator. For example, if it is in a place where the chicks will run smack into it after hatching, you block it to prevent that from happening. Another reason some block the light is that the light itself is a "focused" heat. To test this, put your hand over a light bulb and see just how close you can get to it before it's too darn hot. By putting a shield in front, preferably made from some sort of metal (i.e. aluminum foil), you create a heat deflector shield, allowing that "focused" heat of the bulb to be deflected more evenly through the incubator.