which the ligand has been immobilized (e.g., an antigen). The first step m an SPR experiment is to record the background R(O)-curve for the immobt- lized antigen, before the interaction, under continuous flow of the pure buffer in the flow channel, O,,( 1) in Fig. 2B. An analyte pulse containing an anttbody directed against the rmmobrlized antigen IS then Injected over the sensor surface, and an antigen-antibody complex IS formed. A new R(O) curve IS recorded after a given contact time with the antibody solu- tion and after rinsing with buffer O,,(2). The SPR signal, e.g., as a result of the formation of a stable antigen-antibody complex, can now be determined as the change in O,, between the second and first minima A@,, = O,,(2) -O,,(l), as shown in Fig. 2B. However, the setup in Fig. 2A displays a number of serious limitations. The most important limitation is related to the fact that one has to scan both the source and detector with respect to the prism to obtain the R(O) curve. Thrs scanning procedure is normally quite time- consummg, making tt difficult to study fast association and dissociatton phenomena between the interacting molecules. One way to overcome this problem IS to employ optics, without any movable parts. Fig. 2C schemati- cally shows an experimental setup based on so-called fan-shaped optics (IO). The “scanning” ltght source m Fig. 2A is replaced by a focused beam, which, within certain lrmrts, provtdes a continuum of angles of incidence. The single-element detector is replaced by a photodiode array. The angle of incidence at which resonance occurs, represented by dark lines in Fig. 2C, can be observed as a minimum of the reflected intensity for a given pixel on the photo diode array. The short readout time for photodiode arrays enables us to follow the resonance mmimum O,, in real-time. Thus, infor- mation about the kinetics of the interaction can be obtained with this setup by following the change m resonance angle O,, with time, the so-called sensorgram, Fig. 2D.