Incidence angle, optical sensors

The evanescent wave depends on the angle of incidence and the incident wavelength. This phenomenon has been widely exploited to construct different types of optical sensors for biomedical applications. Because of the short penetration depth and the exponential decay of the intensity, the evanescent wave is absorbed mainly by absorbing compounds very close to the surface. In the case of particularly weak absorbing analytes, sensitivity can be enhanced by combining the evanescent wave principle with multiple internal reflections along the sides of an unclad portion of a fiber optic tip. 

Recent studies of micro ATR have defined the best experimental conditions for establishing optical contact between the ATR crystal and the sample [25]. This experimental approach has been applied to the analysis of the failure surfaces of adhesively bonded joints. ATR-microscopic measurements have been used for direct measurement and identification of raw materials in textiles coated and impregnated substances on paper [26]. An ATR microscopic probe has been developed which allows one to examine the sample optically through the probe in the microscope. The hemispheric ATR crystal is mounted at the focus of the Cassegrain objective, below the secondary mirror. One can position the crystal in contact with the sample, and run the spectra [27]. In the survey mode, visible light at nearly normal incidence is selected to locate the area of measurement. In the contact mode, low incident angle visible is used to detect contact of the sample to the ATR crystal surface. In the measurement mode, the ATR crystal is slid into position and the incident beam is optimized for total internal reflection. In the Spectra Tech version, all of the available crystals, i.e. ZnSe, Diamond, Silicon, and Germanium can be used. However, Ge and Si are opaque and cannot be used in the survey or contact mode. An optical contact sensor can be used. 

The background problem can be further overcome when using a surface-confined fluorescence excitation and detection scheme at a certain angle of incident light, total internal reflection (TIR) occurs at the interface of a dense (e.g. quartz) and less dense (e.g. water) medium. An evanescent wave is generated which penetrates into the less dense medium and decays exponentially. Optical detection of the binding event is restricted to the penetration depth of the evanescent field and thus to the surface-bound molecules. Fluorescence from unbound molecules in the bulk solution is not detected. In contrast to standard fluorescence scanners, which detect the fluorescence after hybridization, evanescent wave technology allows the measurement of real-time kinetics (www.zeptosens.com, www.affinity-sensors.com). 

The LT is a measuring instrument that tracks the movement of a reflector and calculates its position in spherical coordinates. The distance to the reflector (d) can be measured by an interferometer (IF) or by an absolute distance meter (ADM), while the inclination angles (cp) and azimuthal (6) are measured by two angular encoders. The reflector returns the laser beam, where the beam strikes a position sensor (PSD) that detects any change in position causing the movement of the axes of LT so that the laser beam is incident on the optical center of the reflector. Thus the LT head constantly monitors the position of the reflector. 

---