Do you mean a heat lamp or a light that doesn't put much heat of and is for viewing purposes?
UTH's only heat the surface that they are in direct contact with, and not the air inside the cage. This means that if the room that the cage is in drops below 75 degrees a heat lamp will be necessary to maintain the air temperature inside the cage above 75 degrees (preferably around 80 F)
As for viewing lights such as LED or fluorescent bulbs they aren't necessary but can be a nice especially if your cage only has one side that is glass like mine do:
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