At low angles, total internal reflection results when light propogating within a dense medium (e.g. quartz) reaches an interface with a less dense medium (e.g. aqueous solution). Although the light is fully reflected, an evanescent field is generated that extends beyond the interface and into the aqueous solution. Typically, the penetration depth (or the thickness of evanescent field) is in the range of half the wavelength of the light. The evanescent field provides the surface selectivity of TIRF. Only fluorophores adsorbed, adhered, or bound to the surface will be excited and therefore fluoresce. Conversely, fluorophores in bulk solution will not be excited. Therefore, if the surface is made biologically active so that one may 'trap' fluorescently-labelled compounds of interest, one can detect analytes within complex sample solutions. Because the excitation light is totally reflected away from the detection, one can easily discriminate the fluorescence signal from the excitation light and achieve very low sensitivities and detection limits. TIRF systems provide measurement of real time kinetics of a bioanalyte's binding to a surface immobilized sensor molecule. TIRF is fast, non-destructive, sensitive and versatile technique, that is well suited for monitoring biomolecular interactions. TIRF allows monitoring conformational changes, orientation changes, and lateral mobility of biomolecules. The sensitivity of TIRF is more than 10,000 times better than that of biosensor systems based on surface plasmon resonance.