A method is provided for measuring a response to a stimulus of a plurality of samples spots of a sample using a measuring system having a measurement range to generate an image of the sample in digital space. The method includes, for each sample, while measuring the response, varying the stimulus to include at least one stimulus value where the measured response corresponds to a value in an intermediate portion of the measuring range, and storing a value of the measured response that corresponds to a value in the intermediate portion of the measurement range, and the stimulus value that produced that value of the measured response.