The resistance of a polycrystalline photoconductive CdSe film upon a piezoelectric acoustic surface wave delay line is investigated. During the acoustic pulse propagates in the line, a resistance decrease by a factor up to ten is revealed. The relationship between the resistance change and the acoustic input voltage is strongly nonlinear and this property can be used for nonlinear signal processing. The effect is explained by the intercrystalline barrier modulation by the high electric fields associated with the acoustic surface wave on a piezoelectric substrate. Since the resistance change depends on light intensity, the method can be used, in a limited scale, for acoustical scanning of optical images.
Membrane waves at ultrasonic frequencies can be excited in thin stretched foils. In this paper the effects of surrounding air load on the propagation and attenuation properties of membrane waves are discussed. The air-mass loading makes the membrane waves slightly dispersive. Attenuation, in the case where the membrane wave does not radiate to the air, is mainly due to the viscous and thermal-conductivity losses in a thin boundary layer on the membrane surface. The electrostatic-excitation method is discussed in detail and includes the derivation of the voltage-transfer function. Theoretical results are compared with the measurements.
Continuosly variable analogue delay is demonstrated by using reflections from a laser-light-induced impedence discontinuity on a photoconductive layer evaporated on an LiNbO3 delay line at 75 MHz. −30dB reflection loss is obtained with a delay range of 20 μs.
The objective of the study was to assess the usability of a near-infrared spectroscopy (NIRS) device in multimodal measurements. We combined NIRS with electroencephalography (EEG) to record hemodynamic responses and evoked potentials simultaneously, and with transcranial magnetic stimulation (TMS) to investigate hemodynamic responses to repetitive TMS (rTMS). Hemodynamic responses and visual evoked potentials (VEPs) to 3, 6, and 12 s stimuli consisting of pattern-reversing checkerboards were successfully recorded in the NIRS/EEG measurement, and ipsi- and contralateral hemodynamic responses to 0.5, 1, and 2 Hz rTMS in the NIRS/TMS measurement. In the NIRS/EEG measurements, the amplitudes of the hemodynamic responses increased from 3- to 6-s stimulus, but not from 6- to 12-s stimulus, and the VEPs showed peaks N75, P100, and N135. In the NIRS/TMS measurements, the 2-Hz stimulus produced the strongest hemodynamic responses compared to the 0.5- and 1-Hz stimuli. In two subjects oxyhemoglobin concentration decreased and in one increased as a consequence of the 2-Hz rTMS. To locate the origin of the measured NIRS responses, methods have to be developed to investigate TMS-induced scalp muscle contractions. In the future, multimodal measurements may prove useful in monitoring or treating diseases such as stroke or Alzheimer's disease.
For the first time a silicon IR-source and CO/sub 2/-chamber system for measurement of CO/sub 2/ concentration is presented. This new miniaturized infrared sensor is specially designed for the measurement of respiratory gases present in patient airways during anaesthesia or intensive care. The IR-sensor assembly consists of an IR-source chip with two pairs of diagonally arranged IR-sources for the generation of two switched "sample" and reference beams. The gas filter chip is arranged with two CO/sub 2/ chambers directly beneath the two reference sources. These chambers "pre-absorb" the reference beam at the CO/sub 2/ wavelength band around 4.26 /spl mu/m. The reference beam is needed for long term stability and for compensation against cuvette window contamination. The electrically modulated IR-sources consist of incandescent polysilicon filaments coated with silicon nitride across a 220 /spl mu/m deep cavity. The CO/sub 2/-chambers with a length of 1 mm are fabricated by silicon fusion and anodic bonding at a chosen CO/sub 2/ pressure for optimal filter characteristics. Test measurements of the infrared sensor system show high CO/sub 2/-sensitivity meaning that the stringent requirements for this respirator application can be reached.
Near-infrared spectroscopy (NIRS) is a method for noninvasive estimation of cerebral hemodynamic changes. Principal component analysis (PCA) and independent component analysis (ICA) can be used for decomposing a set of signals to underlying components. Our objective is to determine whether PCA or ICA is more efficient in identifying and removing scalp blood flow interference from multichannel NIRS signals. Concentration changes of oxygenated (HbO2) and deoxygenated (HbR) hemoglobin are measured on the forehead with multichannel NIRS during hyper- and hypocapnia. PCA and ICA are used separately to identify and remove signal contribution from extracerebral tissue, and the resulting estimates of cerebral responses are compared to the expected cerebral responses. Both methods were able to reduce extracerebral contribution to the signals, but PCA typically performs equal to or better than ICA. The improvement in 3-cm signal quality achieved with both methods is comparable to increasing the source-detector separation from 3 to 5 cm. Especially PCA appears to be well suited for use in NIRS applications where the cerebral activation is diffuse, such as monitoring of global cerebral oxygenation and hemodynamics. Performance differences between PCA and ICA could be attributed primarily to different criteria for identifying the surface effect.
