A method for determining a mean cell volume for a blood sample includes: illuminating the sample with incident light at a plurality of illumination wavelengths and obtaining a two-dimensional image of the sample at each of the plurality of illumination wavelengths； identifying a plurality of cells that appear in each of the images； for each one of the plurality of cells, determining an integrated optical density corresponding to each of the plurality of illumination wavelengths； for each one of the plurality of cells, determining a cell volume based on the integrated optical densities corresponding to each of the plurality of illumination wavelengths； and determining the mean cell volume for the blood sample from the cell volumes for each one of the plurality of cells.
