The fundamental issue surrounding bacterial culture growth measurements is that an absorbance spectrophotometer is being asked to determine light scattering caused by particulates in suspension. In this case, transmittance is not related to absorbance in the classical sense. Under normal true absorbance conditions, spectrophotometers can be comparable to one another because the sample actually absorbs electromagnetic energy. In the case of a reduction of transmittance caused by light scattering, readings are very dependent on the optics of a specific spectrophotometer as well as the cell type in suspension

we recommend calibrating OD using serial dilution of silica microspheres, which readily produces highly precise calibration (95.5% of teams having residuals less than 1.2-fold), is easily assessed for quality control, and as a side effect also assesses the effective linear range of an instrument.

to America or the Colonies