I was thinking of an array of phototransistors which have a directional response characteristic so that the one pointed most directly at the sun would have the greatest response. These phototransistors would correspond to a similar array of smart film segments on the sun visor, and the transistor receiving the maximum brightness would make the corresponding smart film segment opaque. The trick as you indicated is how to calibrate the transistor array to match the position of the driver's eyes!Perhaps, but how would you locate where the sun is in relation to the drivers eyes, which is what you need to do?