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Solid State Testing of Inhaled Formulations
Published in Anthony J. Hickey, Sandro R.P. da Rocha, Pharmaceutical Inhalation Aerosol Technology, 2019
Philip Chi Lip Kwok, Hak-Kim Chan
White-light interferometry is a non-contact, optical profilometry technique that can quantify roughness by measuring the interference pattern of split light beams scanning a surface (Adi et al. 2008). A white light beam is split into two beams, one travels to the sample and the other to a reference mirror inside the interferometer. The two beams are then relayed to the image sensor to form a pixelated interference pattern because the pathlengths travelled by the two beams are unequal. The constructive interference fringes (degree of coherence) at each pixel indicate the height at that point (Adi et al. 2008). The advantage of this technique is that it is fast and does not damage/change the sample surface. Its height and lateral resolutions are 0.1 nm and 150 nm, respectively. The working distance can be up to 10 mm so large areas with many particles can be covered in one scan. The three-dimensional images and Rq values of micron-sized particles and carrier particles measured by white-light interferometry and AFM have been shown to be comparable (Adi et al. 2008).
Advanced Measurement Techniques in Surface Metrology
Published in Salah H. R. Ali, Automotive Engine Metrology, 2017
A low-magnification objective can be used to look at large areas but the resolution is controlled by the resolution of the detector. Higher resolution images need higher-magnification objectives but a smaller area has to be measured. The current lateral resolution limit for white light interferometry is about 0.5 μm because diffraction effects limit the maximum possible resolution. Another consideration when choosing an objective is the numerical aperture. The numerical aperture (NA) is related to the angle of the light that is collected by the objective. The higher NA then the greater the angle can be measured. Normally the higher-magnification objectives have a higher NA. Problems in white light interferometry can arise from the presence of thin films which can generate a second set of interference fringes. The two sets of fringes can cause errors in the analysis. In addition, materials with dissimilar optical properties can give an error in the measurement [72]. Accordingly, the Zygo white light interferometric profilometer offers fast, non-contact, high accuracy 3D metrology of surface features for a wide variety of samples. The software provides graphic images and highresolution numerical analysis to characterize the surface structure of materials at magnifications up to 2000x. The maximum vertical range is 20 mm with a resolution of 0.1 nm [75].
Image texture analysis to evaluate the microtexture of coarse aggregates for pavement surface courses
Published in International Journal of Pavement Engineering, 2022
Nabanita Roy, Kranthi Kumar Kuna
As a more direct indicator of the aggregate surface texture, the surface roughness parameters were considered. These parameters were derived from the surface profile data collected using Bruker-made Contour GT 3D-Optical surface profilometer (3D OSP) that works on the principle of white light interferometry. The photograph of the 3D OSP used in the present study can be seen in Figure 2(a). The principle of a 3D OSP is to use the wave characteristics of light for comparing the optical path difference between a test surface and a reference surface. An optical profiler considers splitting of a light beam, half of which gets reflected from the test surface passing through the microscope, and the other half gets reflected from the reference mirror. The fragmented beams get united if the distance between the beam splitter and the reference mirror is the same as the distance between the beam splitter and test surface. Based on the variation in length, the combined beam creates lighter and darker bands, known as constructive and destructive interference fringes, respectively. The transition from light to dark signifies one-half a wavelength of difference from the reference surface to the test surface. Now for a known wavelength, the differences in height are determined across a surface, and a 3D surface map is obtained from the calculated height differences. In this study, the profile data were obtained before and after polishing the aggregate samples. To capture the 3D surface profile data before polishing, the angular side of the selected aggregate particles were embedded into deformable clay, and the comparatively flatter side of the aggregate was kept exposed for testing as shown in Figure 2(b). For collecting the data after polishing, the entire curved APM specimen was placed on the sample stage. Therefore, two locations were targeted from the central region of the test specimens and the laser beam was spotted on the aggregate as presented in Figure 2(c). Profilometry data were obtained at 10x magnification for acquiring the surface map having dimension of 477 × 357 µm; which captures the surface profile which lies within the range of wavelength containing aggregate microtexture information, similar to that seen in Figure 2(d).