Here's a link to 3M crystalline film that blocks heat but lets in most of the light (doesn't seem to say exactly how much but it is pretty clear). I wonder how it would test with a light transmission meter.

Crystalline Automotive Films - Clear automotive window film for cars, trucks - Crystalline - 3M US

This film looks like it would be great for front side windows, but I would be reluctant to put any film on a whole windshield because it will still reduce visibility a bit. The adhesive layer in any film diffuses light, and cleaning glass over the years will result in fogging the film thru small scratches.

Most states seem to allow a tinted strip at the top of a windshield. Michigan allows 4" but many states allow tint down to the "as1" line and arrow which you will see on every windshield. Summary of laws (which I can't guarantee are all right): Tint Laws

Regards,
George