In this paper we consider the problem of complementary DNA (cDNA) microarray image segmentation for the purpose of relative spot intensity measurement. Our aim is to find reliable background and target areas for the measurement. In contrast to global and local thresholding techniques, we investigate methods insensitive to changes in intensity scale. We combine characteristics of binary hit-or-miss transform and assumption of ordinal scale measurements, thus leading to a rank-based shape detector. We also provide a segmentation scheme based on the detector and a training approach and goodness criteria for the detector.