The difference is how much infrared radiation the film reflects — that is what heats the cabin. At the same darkness, nano-ceramic NEX reflects 96% of IR, metallised ICY about 36%. Compare NEX 35 and ICY 35: light transmission is nearly identical, 33.5% against 34.7%, yet NEX blocks 56% of solar energy against 36%. In short: regular tint makes the glass darker, athermal film also keeps the heat out. More on the
athermal tinting page.