(i) Fluorescence was though very popular earlier, but has been shown to be difficult to interpret. The method depended on fluorescence of NADPH at 460nm, when cells are irradiated with UV (340nm). Fluorescence was found to be proportionate to cell concentration but becomes less sensitive at high concentration. Improvements are being made by using fluorescence spectra over a wide range of wavelengths: (ii) Light absorbance / scattering can also be used for measurement of cell concentration. Most commercial instrument are designed to measure high microbial cell-density (100gl-1) and are less sensitive at low concentration. More recently, instruments sensitive for low concentration, have been designed, using either light absorbance or light scattering as a parameter, (iii) An image analysis system with a CCD camara can also be used for estimation and monitoring of cell concentration. The cells can be viewed on the screen and therefore real-time imaging system may lead to breakthrough in bioprocess monitoring. Commercial image analysis system to work with bioreactors should become available in the near future. (iv) Real-time particle size or cell concentration analysis can be conducted with the help of a laser beam used in some commercial equipments. These equipments are being used in microbial-bioprocess monitoring and may be used for cell culture-bioprocess monitor in future.
