Reference cell (8) for use with a fibre optic probe (1) comprising a base wall (11), upstanding walls (12) each having an optical window (13, 15), and a top wall (14) having a further optical window (16) adapted to allow light to pass to and from a fibre optic probe. The base wall (11) comprises a reflector (19) which reflects incident light from the fibre optic probe back to said probe (1). The top wall (14) has an attachment for receiving an emitting and a receiving end of a fibre optic probe (1). The cell (8) allows use of standard fibre optic probe (1) with other optical equipment when the path length and the probe optical characteristics require validation to international standards. The calibration of the probe (1) is analogous to calibration of laboratory spectrophotometers and can therefore be validated. The device (8) enables probes (1) to be used for applications where precision and accuracy are essential.