Background correction. (a) The test slide has an array of 4 by 12 blocks consisting of 2 pairs of positive controls (PKA) and random yeast proteins in the remaining spots in each block. (b) The autophosphorylation experiment showed typical bleeding problems in positive control spots. (c) Signal for one spot is measured as foreground minus local background intensity; therefore, artifacts in background add noise to the signal intensity. (d) Comparison of signal distributions of PKA spots before and after background corrections. The median of PKA signals is enhanced by 53% and the variance among the PKA spots is decreased by 41%.