A centrifuge device that is used to centrifuge a fluid sample, such as a blood sample, to separate the fluid sample into its various component layers, and which is capable of measuring the length of the component layers to calculate cell counts for each layer. The centrifuge device includes a rotor assembly for rotating and thus centrifuging the fluid sample, and a movable optical reader device for reading the cell layers in the centrifuged sample. The centrifuge device is capable of accurately controlling the reading of the centrifuged sample based on the orientation of the rotor, so that the rotor can continue to rotate the centrifuged sample while the readings are being taken.