Reflection optical sensors

Determination of Reflectance Optical Sensor Array Configuration Using 3-Layer... 

The use of optical sensor method in detecting and measuring the fetal oximetry or heart rate applications has been adopted by many researchers [2-5], There are a few available commercial optical sensors for fetal oximetry in the market but all inherit the same limitation the technique is invasive to mothers. In this case, the reflectance optical sensors need to be inserted through the uterine cervix before attaching it on the head of the baby [6], Besides that, it is reported that the sensor will leave mark on the skin of the baby and can cause uncomfortable to mother [7],... 

Therefore, this study is conducted with objective to determine the most suitable configuration of the reflectance optical sensor array based on the highest Irradiance value. To achieve the objective, the light propagation through the tissue model mimics the pregnant women s abdomen will be simulated. 

C. Reflectance Optical Sensor Model and Configuration of Sensor Array... 

Measurement by means of a wall optical sensor The thickness of a film and the slope of its free surface can also be measured by means of a wall optical sensor, as proposed by Ohba et al. (1984). This sensor consists of a cluster of seven optical fibers mounted flush with the wall (Fig. 3.33). A laser beam passed through the central fiber is reflected by the free surface onto the other fiber tips, which collect the light and transmit it to two photodiodes. The light intensities received by these two detectors enable the film thickness and the inclination angle to be determined. 

As the potential of optical fiber probes for pH measurements was rapidly recognized, several other articles appeared within a few years75 83. Most were reflectance-based, and Seitz reported the first fluorescent pH sensors84, 78. The article by Janata85 on whether pH optical sensors can really measure pH is another "must" in the early literature since it points to aspects hardly addressed in pH sensor work. 

Optical transduction modes applied in combination with enzyme based fiber-optic sensors include absorbance, reflectance, fluorescence,... 

Goddard, N. J. Hulme, J. Malins, C. Singh, K. Fielden, P. R., Asymmetric anti resonant reflecting optical waveguides (ARROW) as chemical sensors, Analyst 2002, 127, 378 382... 

The most often used detection method for the optical sensors are based on absorption, luminescence, reflectance, and Raman scattering measurements. The basic theory and instrumentation of most of these... 

From a general point of view, a chemical sensor is a device capable of continuously monitoring the concentration of an analyte. The two main classes are electrochemical sensors and optical chemical sensors. The latter are based on the measurement of changes in an optical quantity refractive index, light scattering, reflectance, absorbance, fluorescence, chemiluminescence, etc. For remote sensing, an optical fiber is used, and the optical sensor is then called an optode because of... 

In a modern dew-point instrument, a sample is equilibrated within the headspace of a sealed chamber containing a mirror, an optical sensor, an internal fan, and an infrared thermometer (Figure A2.2.2). At equilibrium, the relative humidity of the air in the chamber is the same as the water activity of the sample. A thermoelectric (Peltier) cooler precisely controls the mirror temperature. An optical reflectance sensor detects the exact point at which condensation first appears a beam of infrared light is directed onto the mirror and reflected back to a photodetector, which detects the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror accurately measures the dew-point temperature. The internal fan is for air circulation to reduce vapor equilibrium time and to control the boundary layer conductance of the mirror surface (Campbell and Lewis, 1998). Additionally, an infrared thermometer measures the sample surface temperature. Both the dew-point and sample temperatures are then used to determine the water activity. The range of a commercially available dew-point meter is 0.030 to 1.000 aw, with a resolution of 0.001 aw and accuracy of 0.003 aw. Measurement time is typically less than 5 min. The performance of the instrument should be routinely verified as described in the Support Protocol. 

The second (real) term accounts for the exponential decay of the electric field intensity in the direction normal to the interface. The reflected beam combines with the incident beam, forming a standing electromagnetic wave at the interface (Fig. 9.9). The electric field that penetrates to the optically rarer medium of refractive index n, the evanescent field, plays a critical role in many optical sensors based on the waveguiding principle. Its depth of penetration dv is defined as the distance at which the initial intensity Eq decays to 1/e of its value. Thus from (9.18), dv is... 

Related to the plasmon resonance physics is the micromirror optical sensor for hydrogen (Butler, 1991). Like gold and silver, palladium is a free-electron gas metal in which charge groupings such as phonons or plasmons are likely to occur. As we have seen already, palladium has a natural selectivity due to its sorption of monoatomic hydrogen. In that sensor, the reflectivity of the thin Pd film mirror mounted at the end of cladded optical fiber (Fig. 9.19) is modulated by absorption of hydrogen. 

The reflecting surface can be a mirror or a membrane with a light-scattering surface. In any case, the sensor has the appearance of a monolithic probe (i.e., a dip-stick probe). Optical sensors based on absorption, fluorescence, phosphorescence, and luminescence can employ such a configuration. Various highly optimized fiberoptic probes for UV-Vis, NIR, and IR ranges are now commercially available, and their designs are shown in Fig. 9.23. 

OF optical fibers, IOS integrated optical sensors, A absorbance, R reflectance, F fluorescence, ev evanescent wave, ISP isopropyl alcohol, DOS bis(2-ethylhexyl)sebacate, o - NPOE ortho-nitrophenyl octyl ether, TOP tris(2-ethylhexyl)phosphate... 

Most present-day fiber-optic sensors use linear diode arrays combined with optical gratings and measure the absorption, transmission, fluorescence, and reflection in UV, visible, and NIR regions (see Table 3.1). Light travels to the sampling probe via one fiber-optic cable and returns to the instrument via a second. Laser excitation permits long-distance transmission of excitation radiation to get a useful signal from the sample. 

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