The photoelectric effect

University of Waikato science researcher Dr Adrian Dorrington explains the photoelectric effect. He then describes how camera sensors can be designed on the basis of this effect to enable light energy to be converted into electric potential energy. Each sensor is split up into an array of pixels so each pixel has a storage area.

Transcript

DR ADRIAN DORRINGTON
We can think of light in several ways, but if we think of light as a particle – that is, a photon – if it strikes a piece of metal with sufficient energy then that metal releases an electron, so that’s the photoelectric effect. In traditional image sensors, it’s a very similar approach but there’s a semiconductor, and the semiconductor conducts electricity – that is, there a flow of electrons – and it does it in a way where you have electrons and whole pairs. You have negative and positive charges, and they can flow and they can also recombine.

So when a photon strikes a semiconductor, it creates an electron whole pair, so it creates a positive and a negative charge, and if we apply an electric field across that, then we can attract the negative charge in one direction and the positive charge in the other direction. So we’re converting light into the flow of electricity.

So that flow of electricity we then accumulate in a charge well, or effectively in a capacitor, so for each photon that comes in, we’ll get an electron, and then another photon comes and we’ll get an electron, and we’ll accumulate these up and that creates a voltage, and then we can read that voltage out with an analogue to digital converter.

In a camera sensor, we have light striking the sensor, and the sensor is split up into an array of pixels, so each pixel has a storage area, so we get a certain amount of light falling on a pixel and the electrons generated are stored for that. What we then do is is we interrogate each pixel and see how much light has been stored there. Some pixels may have stored more light than others and will show up as a brighter pixel.