Methods and apparatus, including computer program products, implementing and using techniques for camera pan vector estimation, are disclosed. The interchangeable fan blade system is a set of support arms attached to the rotating portion of the fan and having one of a variety of connectors for removably attaching one end of the fan blades to the support arm. of less than about 8,000 cp, a yield stress of less than about than 80 psi and a creep performance for a bond made through strand coating of less than about 15%. The data output unit is used for providing the data output signal. The width of the first side wall is smaller than the width of the second side wall, and the second side wall includes the second spacer 103b containing a higher concentration of hydrogen than the first spacer 103a.