A system and method for portable instrumentation is proposed, comprising a server computer and a portable instrument that is wirelessly connectable to the server computer. Optical switches are provided for selectively coupling inputsoutputs of different branches to enable a routeing of optical signals through the unit. Various means are disclosed to provide for a radial distribution of the produced light wherein the produced light properly strikes the collimating reflector for concentration into the beam of light. One or several characteristic variables of the force-time curves are determined for each scanning point from the real-time evaluation results and the stored digitized force-time curves and an image of the sample surface is obtained from said characteristic variables of the scanning points. The method comprises determining the effect of a candidate compound on the binding of a probe compound which binds to a first binding site on the biological substrate in the presence of a first multibinding compound which binds to the first and second binding sites on the biological substrate.