Thirdly, to keep fabrication costs as low as possible, we have used simple glass slides as beam-splitters. This design choice is energetically wasteful as the glass slides reflect only about four percent of the incident light into the object and reference channels. However, the wasted light does not typically present a problem from the perspective of analytical performance since -in the absence of strong sample absorption -the peak signals from the two amplified photodiodes are close to the 1 V limit of the analogue input pins of the Teensy 4.0. For highly absorbing samples where it is necessary to maximise the intensity of light passing through the sample, we recommend a modified optical set-up using a plane mirror in place of one of the glass slides, which directs approximately 96 % of the beam intensity through the optical cell (see Fig. ).