An alert reader pointed to am error in a previous entry. Although absorbance is one source of optical density, the situation in our case is different. To quote
A solution absorbing at 600 nm has a blue color, the cells do not absorb light at this wavelength. You actually measure the light scattering caused by the cells. The scattered light is not detected by the spectrometer, this is the reason that you measure OD >0.
An additional note regarding the spectrophotometer is that we compared the readings on ours to three others in the institute. To our surprise we discovered that one of them has the same reading as ours, another is slightly off, and the third is way off (almost half the value we measure). We are not sure what to make of this, but it is a point of concern.