Enhancement factors under SERS conditions are characterised for two SERS active substrates with different geometries using a combination of optical pumping and photobleaching. A rigorous mathematical method of the photobleaching dynamics under SERS conditions is developed to allow the average enhancement factor to be investigated whilst maximal enhancement factors are studied using optical pumping. We show that both average and maximal enhancements are correlated for the surfaces with average enhancements of approximately 10(8).