Growth of cultures can be monitored by Spectrophotometry. The number of photons scattered is proportional to the mass of the cells in a sample (except for very concentrated cultures, as discussed below) or, for particular growth conditions, to the cell concentration.
However, the geometry of each spectrophotometer determines how much scattered light falls onto a photodetector. Therefore,a calibration curve that relates cell concentration and absorbance must be made for each Spectrophotometer. This is done by measuring the absorbance of suspensions of cells at different concentrations and concomitantly determining the cell concentration by direct counting or by plating on agar and measuring the number of colonies formed.