Using tiff is fine for me too. But there is no built in way to use the tiff map as light source directly?
I mean without inverting stuff…
Just load the map in a color channel somewhere and it casts light?
If you use the UE, then the light sphere is already setup for the most part and you can then just load your HDR tif into the light color channel and set the color to 255,255,255.
If you make your own surface light using the UberAreaLight, then you need to make sure that the surface normal is pointed towards your subject and you can also load a HDR tif map into the same channel for that surface. The example of using a primitive sphere and turning it inside out and turning it into a UberAreaLight is just another way of making your own UE.