An imaging sensor is signaled to capture a digital image of a dark scene. For each of the pixel columns in the image, a respective column value is computed that represents at least some of the pixels in the column. For each of the pixel columns in the image, a respective comparison is made between the respective column value of the pixel column and a reference value. A respective column score is computed, for each of the pixel columns, based on the respective comparison. An indication that identifies one or more of the pixel columns as anomalous is stored, when the respective column score of the one or more the pixel columns does not meet a criterion. Other embodiments are also described and claimed.