A method for determining a mean cell volume for a blood sample, the method comprising: 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; characterized in that: for each one of the plurality of cells, determining the cell volume based on the integrated optical densities and based on the cell volumes corresponding to each of the plurality of illumination wavelengths, wherein, for each one of the plurality of cells, determining the cell volume corresponding to each of the plurality of illumination wavelengths comprises: for each of the plurality of illumination wavelengths, determining a mean optical density and a maximum optical density for the cell; and for each of the plurality of illumination wavelengths, determining the volume of the cell based on a ratio of the mean optical density to the maximum optical density corresponding to the wavelength; and determining the mean cell volume for the blood sample from the cell volumes for each one of the plurality of cells. 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; characterized in that: for each one of the plurality of cells, determining the cell volume based on the integrated optical densities and based on the cell volumes corresponding to each of the plurality of illumination wavelengths, wherein, for each one of the plurality of cells, determining the cell volume corresponding to each of the plurality of illumination wavelengths comprises: for each of the plurality of illumination wavelengths, determining a mean optical density and a maximum optical density for the cell; and for each of the plurality of illumination wavelengths, determining the volume of the cell based on a ratio of the mean optical density to the maximum optical density corresponding to the wavelength; and determining the mean cell volume for the blood sample from the cell volumes for each one of the plurality of cells. for each one of the plurality of cells, determining the cell volume based on the integrated optical densities and based on the cell volumes corresponding to each of the plurality of illumination wavelengths, wherein, for each one of the plurality of cells, determining the cell volume corresponding to each of the plurality of illumination wavelengths comprises: for each of the plurality of illumination wavelengths, determining a mean optical density and a maximum optical density for the cell; and for each of the plurality of illumination wavelengths, determining the volume of the cell based on a ratio of the mean optical density to the maximum optical density corresponding to the wavelength; and for each of the plurality of illumination wavelengths, determining a mean optical density and a maximum optical density for the cell; and for each of the plurality of illumination wavelengths, determining the volume of the cell based on a ratio of the mean optical density to the maximum optical density corresponding to the wavelength; and determining the mean cell volume for the blood sample from the cell volumes for each one of the plurality of cells.