The Scanner Unit
The Scanner is installed in fish ladders, pools, traps or similar places where all the fish in the river have to pass. It consists of two scanner plates (20 x 60 cm) inside a frame, the distance between them being from 10 to 45 cm. Inside the scanner, light diodes send infra-red light beams to receivers on the other side.
When a fish swims through the net of light beams, the resulting silhouette image is used to count and estimate the size of each fish. Each individual image is memorised in the control unit so that the counting can be verified afterwards.