GaAs lens have a different refractive index that the glass lens’ The same focal length lens in a GaAs will be almost flat compared to it’s glass counterpart. I have to set them on a flat surface to figure out the curve of the lens. Russ loves his for thick cutting, and my initial usage agrees that with the flatter lens you are getting more ‘central power’ out of the lens and deeper / cleaner cutting in thick material. I have begun using the GaAs lens’ for thick cutting and am so far very impressed. BTW, I have an 80W machine, he uses his in anything from a 50W to a 70W. It’s not the wattage, but what the lens can do with it that seems to make the difference.