An antenna switching apparatus which detects an antenna of good reception with high precision and suppresses control errors in antenna switching. Improved x-ray scatter control is provided through the alignment of the system with the focal line of an anti-scatter grid. An image categoriser determines what further processing of the image is appropriate in dependence on the variance. As a result, the system size is small, and the iris image can be easily rapidly precisely obtained. As the water, soap mix, and bubbles fill the tank, they fall from the tank via an overflow lip that creates a sheeting action, with the water, soap mix, andor bubbles sheeting onto a vehicle passing below. , a low-discrepancy sequence, to determine a plurality of test conditions usable to characterize the vision system, where each test condition corresponds to a respective set of conditions.