This idea was shared with me on the electro-tech-online forums, and I made up some graphics to help myself and others understand it.

The first step involves choosing your illuminator LED – this diode will provide light for nearby sensors to ‘see’.

The second step is to reverse bias a nearby LED, preparing it to be the sensor. A diode in an adjacent row and column must be selected, to avoid electrical interference from the illuminator.

The third step is reading the voltage present on the cathode terminal of the diode, which indicates the light level the sensor detected.

The controller can then choose another nearby diode to use as a sensor, or move on to another pixel in the array, repeating the entire process.