During long-duration manned space-missions complex chemical and biological processes need to be managed accurately for the recycling of human wastes and to produce human consumables. As a result, there is an increasing interest in how the characteristics of microbes are influenced by micro-gravity. Compact optical instrumentation allows for real-time and non-invasive measurement of bacterial growth parameters during flight experiments. In close collaboration, the National Aerospace Laboratory NLR of the Netherlands and Bioclear Environmental Biotechnology developed and tested an on-line optical biomass sensor successfully. The sensor concept is based on a turbidity measurement technique operating in the VIS-blue part of the light spectrum with use of blue LED sources. A diagnostic tool has been developed using compact spectrometers and optical fibres to characterise bacterial cultures. As a result a few sensor applications operating at different colours and sensor layouts are discussed in the paper.