An operator independent image cytometer having a method for image segmentation. Image segmentation comprises the steps of filtering a digital image of a cellular specimen and thresholding the resultant image. In addition, the thresholding may include the sorting of features extracted from the filtered image. The present invention also includes a method for cytometer autofocus that combines the benefits of sharpening and contrast metrics. The present invention further includes an arc lamp stabilization and intensity control system. The image cytometer has broad applications in determining DNA content and other cellular measurements on as many as 10.sup.5 individual cells, including specimens of living cells. Image segmentation applications include PAP smear analysis and particle recognition.