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All About Wave Equations
Published in Bahman Zohuri, Patrick J. McDaniel, Electrical Brain Stimulation for the Treatment of Neurological Disorders, 2019
Bahman Zohuri, Patrick J. McDaniel
Interferometers are widely used in science and industry for the measurement of small displacements, refractive index changes and surface irregularities. In an interferometer, light from a single source is split into two beams that travel different optical paths, then combined again to produce interference. The resulting interference fringes give information about the difference in optical path length. In analytical science, interferometers are used to measure lengths and the shape of optical components with nanometer precision; they are the highest precision length measuring instruments existing. In Fourier transform spectroscopy they are used to analyze light containing features of absorption or emission associated with a substance or mixture. An astronomical interferometer consists of two or more separate telescopes that combine their signals, offering a resolution equivalent to that of a telescope of diameter equal to the largest separation between its individual elements.
From interferometry to color holography
Published in Stefano Discetti, Andrea Ianiro, Experimental Aerodynamics, 2017
Finally, when the two beams of the interferometer are entirely separated and when one of the two beams crosses through the test section, the interferometer is of conventional type and it is called “separated beams interferometer.” With this type of apparatus, the optical thickness E can be directly measured in the test section. The optical thickness determines the phase delay of the light passing through a medium with index of refraction n, and it is also referred to as optical path length. If the thickness e of the test section is known, E = (n − 1) ∙ e and the following Gladstone–Dale relationship (reported in Chapter 7 and here recalled) relates ρ to n: n−1ρ/ρs=K where ρs is a reference value of the gas densityK is the Gladstone–Dale constant depending on the gas Under standard conditions (0°C, 1 atm), ρs = 1.2928 kg/m3 and K = 293 × 10−6 for dry air.
Fundamentals of Optical Concentration
Published in Roberto Ramirez-Iniguez, Sevia M. Idrus, Ziran Sun, Optical Wireless Communications, 2008
Roberto Ramirez-Iniguez, Sevia M. Idrus, Ziran Sun
Two other fundamental concepts widely used in the design of optical concentrators are Fermat’s principle and the optical path length. The Fermat principle states that the time required by light to travel from one point to another (in an optical system) is the minimum when compared to the time required from neighboring paths (a more accurate definition is given in Section 4.3). This is particularly relevant in GO analysis and when producing ray-tracing algorithms. The optical path length refers to the geometrical length of the path followed by light when traveling through an optical system.
Pulsed laser heating of diesel engine and turbojet combustor soot: Changes in nanostructure and implications
Published in Aerosol Science and Technology, 2023
Randy L. Vander Wal, Madhu Singh, William Bachalo, Greg Payne, Julien Manin, Robert Howard
The cavity attenuated phase shift (CAPS) measurement technique (Yu et al. 2011; Kebabian, Robinson, and Freedman 2007) is similar in nature to cavity ringdown. It relies on the use of a sample cell employing high reflectivity mirrors wherein square wave modulated red light (∼630 nm) from a light emitting diode (LED) is directed through one mirror and into the sample cell. The distortion in the square wave caused by the effective optical path length within the cavity (∼1 km) is measured as a phase shift in the signal and detected by a vacuum photodiode located behind a second mirror. Aerosol particles result in a phase shift (θ) change, which is related to total extinction, and mass concentration using either an assumed mass specific absorption, or by direct mass calibration as was done in this study. Further details have been described previously (Massoli et al. 2010).
Transparent nanofluids with high thermal conductivity for improved convective thermal management of optoelectronic devices
Published in Experimental Heat Transfer, 2022
Hao Xu, Chao Chang, Jingyi Zhang, Jiale Xu, Huanbei Chen, Huaixin Guo, Benwei Fu, Chengyi Song, Wen Shang, Peng Tao, Tao Deng
where and are the transmitted and incident light intensity, is the radius of NPs, is the volume fraction of NPs, is the optical path length, is the wavelength of incident light, and and are the refractive index of Au NPs and the base fluid, respectively. The transmittance of the nanofluids exponentially decreases with the cubic of the radius of Au NPs and the volumetric loading of Au NPs.
Contactless measurement of fabric thickness using optical coherence tomography
Published in The Journal of The Textile Institute, 2025
Another OCT measurement that can be used to infer the fabric thickness would be to directly shine the light beam on to a location without scanning it. This corresponds to OCT-A measurements and was used successfully to measure inlay thickness in dentistry (Turk et al., 2018). The light beam from the OCT source was projected onto the fabric from above. The signal arising form the backscattered light then carries the information relating to the optical path it traversed. In order to infer the fabric thickness, the backreflection signal is saved and then examined in the Fourier Domain by applying a Fourier Transform (FT) (Figure 3). Because photons reflected from different sample depths produces interference patterns with the different frequency components, the FT data gives precise information about light scattered from the fabric. A strong back reflection due to an abrupt refractive index change results in the first peak in the depth profile when the light beam encounters the fabric top surface after having traversed in air. After this point, the photons enter and start traveling in the fabric, which results in a gradual attenuation of the signal due to scattering. The signal attenuation continues as photons travel through the fabric until they reach the end. Because the backreflection signal amplitude is caused by index of reflection differences, the signal level rises again when the light beam arrives at the fabric-air interface causing a second peak. The distance between these two peaks corresponds to the optical path length through the fabric. In order to infer the actual fabric thickness, the optical path length should be divided to the refractive index of the fabric, which is about 1.53 for the samples we used. Please note that the refractive index will vary slightly between 1.5 and 1.55 depending on the fabrics composition and the wavelength. It is known for example that the refractive index for cotton is 1.52, whereas wool has a refractive index of 1.54. All the fabric specimens had a flat frequency response meaning that their refractive index remained unchanged within the 100 nm bandwidth of the infrared light source. By taking 5 different measurements and averaging the results we found an estimate of the thickness. This procedure also gives an alternative way to infer fabric thickness from the optical path length. The conversion of optical path length to thickness is explained in the literature (Sabuncu & Akdoğan, 2014).