The present invention provides in one aspect a method for determining a production level of a polypeptide of interest by a cell or cell colony, comprising obtaining an image of one or more cells or cell colonies; and analyzing the image to detect a signal associated with the polypeptide of interest; wherein a signal level is determined for each cell or cell colony based on signal values from a predefined area surrounding the cell or cell colony, the signal level being indicative of the production level of the polypeptide of interest by the cell or cell colony.