If the two fixed polarisers are oriented with their polarisation axes exactly parallel and no optically active sample is present in the optical cell, the two photodiodes generate sinusoidally varying signals of similar magnitude and equal phase. Inserting an optically active sample in the optical cell opens up a phase difference 𝜙 between the two signals, where 𝜙 is equal to twice the optical rotation angle 𝜃 $%& . In practice, the polariser axes cannot be aligned perfectly, and a slight phase difference of a degree or so will typically exist between the object and reference signals even in the absence of an optically active sample. The measurement of the phase difference is therefore carried out twice, first without (𝜙 ' ) and then with (𝜙 ) ) an optically active sample present in the cell. The change Δ𝜙 = 𝜙 ) -𝜙 -in the measured phase difference is equal to twice the optical rotation angle 𝜃 $%& , i.e.