Optoelectronic tweezers (OET) allow the control of micron-sized particles suspended in a liquid by controlling the dielectrophoretic force with the selective illumination of a photoconductor. High liquid conductivities give a negative force (away from illuminated areas) so that particles are trapped by illuminating a ring around them. This paper explores the force profile of these traps and shows that high stiffness traps can be created (8times10-7 Nm-1), with a linear profile, at low optical powers (8.29 muW).