A computer controlled optical trapping system has been designed and built, with the aim of being as flexible, compact and user friendly as possible. The optical trap was formed with a 1064 nm TEMoo mode laser beam and a microscope objective with a numerical aperture of 1.3. The system is based on a Leica DM IRB inverted microscope and programmed using the Lab VIEW programming language from National Instruments. The optical trapping system was used to investigate factors that affect the transverse trapping force measurements of an optical trap, in particular when using the viscous drag force method to characterise the trap. Measurements were made of the minimum power required to trap, trapping efficiency, trap stiffuess, potential well and transverse trapping force. The particle or fluid displacement functions, position of the particle within the sample and particle concentration, were all examined. When a particle was trapped off-axis by 33 J.lm in the x direction, the efficiency of the trap was found to reduce by roughly 32 %, compared to a particle trapped onaXIS. The minimum power required to retain a particle in an optical trap was independent of particle concentration, for concentrations below 0.1 % solids by weight. For a 6 J.lm diameter polystyrene sphere, an increase in power of approximately 50 % was required to trap a particle when the amplitude of distance moved by the sample stage was increased from 20 J.lm to 60 J.lm. This effect was only present below an amplitude of 53 ± 6 J.lm, above this value the power required to trap a 6 J.lm diameter sphere was independent of amplitude of distance moved.