Separation of front ground from the background of the spot is the secondary step in microarray investigation. Microarray spots are useful in determining the differential gene expression of given sample. In this article, a new approach for spot segmentation of microarray images using global intensity based model is proposed. A global intensity based model is used to resolve the issues like curve peak producing, curve breaking and combining etc. Before applying proposed method on spot, circular mask is calculated and edges of each spots are identified. Initially orthogonal projection from every edge point to every other point is drawn so that region of interest of spot identified. Next using external force energy function is used to cover all border points. The presented work is found to be accurate when compared to the methods like Fuzzy C-means, Morphological segmentation.