An image sensor includes multiple photoactive pixels and multiple dark reference pixels arranged in rows and columns to form a pixel array. A dark signal is read out of one or more dark reference pixels in each column and used to determine a column offset for one or more columns in the pixel array. An offset window is used for each column in the pixel array to define an acceptable maximum dark signal and an acceptable minimum dark signal for each column. The dark signals from each column are analyzed to determine if there are any dark signals outside the offset window. If any of the dark signals are outside the offset window, the dark signal or signals can be compensated for or discarded.