11,096 research outputs found
Light scattering from cold rolled aluminum surfaces
We present experimental light scattering measurements from aluminum surfaces
obtained by cold rolling. We show that our results are consistent with a scale
invariant description of the roughness of these surfaces. The roughness
parameters that we obtain from the light scattering experiment are consistent
with those obtained from Atomic Force Microscopy measurements
Towards absolute calibration of optical tweezers
Aiming at absolute force calibration of optical tweezers, following a
critical review of proposed theoretical models, we present and test the results
of MDSA (Mie-Debye-Spherical Aberration) theory, an extension of a previous
(MD) model, taking account of spherical aberration at the glass/water
interface. This first-principles theory is formulated entirely in terms of
experimentally accessible parameters (none adjustable). Careful experimental
tests of the MDSA theory, undertaken at two laboratories, with very different
setups, are described. A detailed description is given of the procedures
employed to measure laser beam waist, local beam power at the transparent
microspheres trapped by the tweezers, microsphere radius and the trap
transverse stiffness, as a function of radius and height in the (inverted
microscope) sample chamber. We find generally very good agreement with MDSA
theory predictions, for a wide size range, from the Rayleigh domain to large
radii, including the values most often employed in practice, and at different
chamber heights, both with objective overfilling and underfilling. The results
asymptotically approach geometrical optics in the mean over size intervals, as
they should, and this already happens for size parameters not much larger than
unity. MDSA predictions for the trapping threshold, position of stiffness peak,
stiffness variation with height, multiple equilibrium points and `hopping'
effects among them are verified. Remaining discrepancies are ascribed to focus
degradation, possibly arising from objective aberrations in the infrared, not
yet included in MDSA theory.Comment: 15 pages, 20 figure
The observation of Extensive Air Showers from an Earth-Orbiting Satellite
In this paper we review the main issues that are relevant for the detection
of Extensive Air Showers (EAS) from space. EAS are produced by the interaction
of Ultra-High Energy Cosmic Particles (UHECP) with the atmosphere and can be
observed from an orbiting telescope by detecting air fluorescence UV light. We
define the requirements and provide the main formulas and plots needed to
design and optimize a suitable telescope. We finally estimate its expected
performances in ideal conditions.Comment: 24 pages, 10 figures; submitted to Astroparticle Physics 27 pages, 14
figures; major revision; added new figures and sections; typos fixed. arXiv
admin note: substantial text overlap with arXiv:0810.571
Methods to calibrate and scale axial distances in confocal microscopy as a function of refractive index
Accurate distance measurement in 3D confocal microscopy is important for
quantitative analysis, volume visualization and image restoration. However,
axial distances can be distorted by both the point spread function and by a
refractive-index mismatch between the sample and immersion liquid, which are
difficult to separate. Additionally, accurate calibration of the axial
distances in confocal microscopy remains cumbersome, although several high-end
methods exist. In this paper we present two methods to calibrate axial
distances in 3D confocal microscopy that are both accurate and easily
implemented. With these methods, we measured axial scaling factors as a
function of refractive-index mismatch for high-aperture confocal microscopy
imaging. We found that our scaling factors are almost completely linearly
dependent on refractive index and that they were in good agreement with
theoretical predictions that take the full vectorial properties of light into
account. There was however a strong deviation with the theoretical predictions
using (high-angle) geometrical optics, which predict much lower scaling
factors. As an illustration, we measured the point-spread-function of a
point-scanning confocal microscope and showed that an index-matched,
micron-sized spherical object is still significantly elongated due to this PSF,
which confirms that single micron-sized spheres are not well suited to
determine accurate axial calibration nor axial scaling.Comment: 8 pages, 5 figure
Scattering of sound by atmospheric turbulence predictions in a refractive shadow zone
According to ray theory, regions exist in an upward refracting atmosphere where no sound should be present. Experiments show, however, that appreciable sound levels penetrate these so-called shadow zones. Two mechanisms contribute to sound in the shadow zone: diffraction and turbulent scattering of sound. Diffractive effects can be pronounced at lower frequencies but are small at high frequencies. In the short wavelength limit, then, scattering due to turbulence should be the predominant mechanism involved in producing the sound levels measured in shadow zones. No existing analytical method includes turbulence effects in the prediction of sound pressure levels in upward refractive shadow zones. In order to obtain quantitative average sound pressure level predictions, a numerical simulation of the effect of atmospheric turbulence on sound propagation is performed. The simulation is based on scattering from randomly distributed scattering centers ('turbules'). Sound pressure levels are computed for many realizations of a turbulent atmosphere. Predictions from the numerical simulation are compared with existing theories and experimental data
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