An optical device includes a first sub-assembly having an input lens for collimating illuminating light and having an optical axis. The first sub-assembly also has an output lens for focusing collimated light received from a sample, the output lens having an optical axis which is offset and substantially parallel with the optical axis of the input lens, and further includes a first support piece which houses and supports the input lens and the output lens. The optical device also includes a second sub-assembly having an input filter for filtering the collimated illuminating light, an output filter for filtering the collimated light received from the sample, and a second support piece which houses and supports the input filter and the output filter. The first and second support pieces are joined together by a liquid-tight joint.