of the illuminating optical fibres of the optical fibre bundle 71 an initial fabry perot interferometer is provided within the path of the illuminating radiation beam between the radiation source and the optical fibre bundle 71 the second ends of the optical fibres are positioned close to the measurement target 20 so that the end of each optical fibre is close to a determined measurement position 21 a 21 t of the target the target of fig1 has 20 measurement positions such as samples and thus the optical fibre bundle has 20 optical fibres for illuminating each target position however it is also possible that two or more optical fibres are used to guide illumination to each target position the measurement system also has a bundle of optical fibres for receiving radiation from the sample positions the first ends 93 of the optical fibres are positioned close to the measurement target 20 so that the end of each optical fibre is close to a determined measurement position 21 a 21 t of the target there is thus at least one optical fibre for illumination of each measurement position to be measured simultaneously and at least one optical fibre for reception of radiation however.