Must be only one detector that senses both the leading and trailing edge of the object, then given the length of the projectile the speed can be calculated. I never saw one like that. I suspect it to be inherently inaccurate when compared to 2 or more detectors. Seems like differences in leading andtrailing edge geometrywould be a real problem. With 2 or more windows/detectors the length of the object is irrelevant and the shadow detected to both start and stop the timer are identical.