camera use will work if the distance readability caution above is observed - but you then have to manually go thru and read all the data...

I think a camera could easily measure sub-millimeter changes in ride height if it were properly set up. I would suggest low-speed, high resolution film, with an intense flash tripped by the passing of the car, with the camera mounted 3-10 feet away.

Cameras and cross hairs are used frequently for this type of measurement in crash tests and are commonly used in wind tunnels.

I'm not knocking the use of other instrumentation, but whatever methodology is used, care should be taken to eliminate artifact.

There is usually more than one way to skin a cat, this is just another way to do it.

You know another thing that might be tried is to put a laser level on the dash pointing to a calibrated strip in the back seat. You would have to get up to speed on a very flat surface, but if you could get the level to calm down enough, you might be able to get a reasonable idea of the amount of frontward/backward pitch there would be. Then you could add pounds at rest to to calibrate for pounds of downward force the spoiler would apply. You could even run the car without the a spoiler to compensate for the regular lift the car generates.

The relatively long distance of measurement would magnify the end result making it more accurate, but that is assuming the level calms down on the ride, and you are flat during the measurement.