1. A method comprising the steps of providing a vending machine having a plurality of product dispensing locations from which products are moved to a product retrieval location, monitoring the intensity of light at an array of pixels, detecting a change in the intensity of light at pixels in the array of pixels upon movement of a dispensed product from a product dispensing location to a product retrieval location, and providing a control signal in response to detection of a change in the intensity of light at pixels in the array of pixels.