The fact that electrical signals are suitable for carrying information (e.g. to a measuring device) was discussed (again) in the previous chapter. However, processes that are not accompanied by an electrical voltage and/or current, but by light phenomena, can also provide quantifiable signals. In such a case, optical sensors are required, also known as optodes (a linguistic shorthand for “optical electrodes”). These are based on optical fibres (analogous to fibre optic cables), the (physical) principle of which will not be further discussed here; if needed, Harris provides a brief summary of the theory required for this.