I don't have hot spots with the bulb. I do a very scientific test to make sure...I put my contact frame on an oversized cutting board that is white. And every time I set up I pull out the Pentax Digital Spot and meter the board. I put the frame in the light that is even enough to not change the meter.
Although I was just at a workshop in which we made a zone V exposure of an enlarger's cold light source with no negative, just the lens stopped down, and you could see the zig zag pattern of a cold light pretty clearly outlined in zigs and zags of more or less density on the paper. Basically, the way the cold light was rotated in its housing, it left two corners of the paper significantly lighter than the middle.